diff --git a/src/capabilities/default.json b/src/capabilities/default.json
index 778bfb5..e895c6b 100644
--- a/src/capabilities/default.json
+++ b/src/capabilities/default.json
@@ -5,7 +5,6 @@
   "windows": ["main"],
   "permissions": [
     "core:default",
-    "opener:default",
     "dialog:default"
   ]
 }
diff --git a/src/src/commands/about.rs b/src/src/commands/about.rs
index 7ec9f56..b847a85 100644
--- a/src/src/commands/about.rs
+++ b/src/src/commands/about.rs
@@ -23,8 +23,10 @@ fn os_pretty_name() -> String {
     fs::read_to_string("/etc/os-release")
         .ok()
         .and_then(|s| {
-            s.lines()
-                .find_map(|l| l.strip_prefix("PRETTY_NAME=").map(|v| v.trim_matches('"').to_string()))
+            s.lines().find_map(|l| {
+                l.strip_prefix("PRETTY_NAME=")
+                    .map(|v| v.trim_matches('"').to_string())
+            })
         })
         .unwrap_or_else(|| "BOS".to_string())
 }
@@ -49,11 +51,15 @@ fn cpu() -> String {
     let model = fs::read_to_string("/proc/cpuinfo")
         .ok()
         .and_then(|s| {
-            s.lines()
-                .find_map(|l| l.strip_prefix("model name").map(|v| v.trim_start_matches([':', ' ', '\t']).to_string()))
+            s.lines().find_map(|l| {
+                l.strip_prefix("model name")
+                    .map(|v| v.trim_start_matches([':', ' ', '\t']).to_string())
+            })
         })
         .unwrap_or_else(|| "unknown".to_string());
-    let cores = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(0);
+    let cores = std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(0);
     if cores > 0 {
         format!("{model} ({cores} threads)")
     } else {
@@ -62,14 +68,12 @@ fn cpu() -> String {
 }
 
 fn memory() -> String {
-    let kb = fs::read_to_string("/proc/meminfo")
-        .ok()
-        .and_then(|s| {
-            s.lines()
-                .find(|l| l.starts_with("MemTotal:"))
-                .and_then(|l| l.split_whitespace().nth(1))
-                .and_then(|v| v.parse::<u64>().ok())
-        });
+    let kb = fs::read_to_string("/proc/meminfo").ok().and_then(|s| {
+        s.lines()
+            .find(|l| l.starts_with("MemTotal:"))
+            .and_then(|l| l.split_whitespace().nth(1))
+            .and_then(|v| v.parse::<u64>().ok())
+    });
     match kb {
         Some(kb) => format!("{:.1} GiB", kb as f64 / 1024.0 / 1024.0),
         None => "unknown".to_string(),
@@ -96,7 +100,11 @@ async fn gpu() -> String {
 }
 
 async fn disk_usage() -> String {
-    let Ok(output) = Command::new("df").args(["-h", "--output=used,size,pcent", "/"]).output().await else {
+    let Ok(output) = Command::new("df")
+        .args(["-h", "--output=used,size,pcent", "/"])
+        .output()
+        .await
+    else {
         return "unknown".to_string();
     };
     let text = String::from_utf8_lossy(&output.stdout);
@@ -136,11 +144,35 @@ pub async fn get_system_info() -> SystemInfo {
     }
 }
 
+/// RFC 1123 labels (digit start allowed), no leading `-`. Linux static
+/// hostnames are also capped at `HOST_NAME_MAX` (64).
+fn valid_hostname(name: &str) -> bool {
+    let name = name.trim();
+    if name.is_empty() || name.len() > 64 || name.starts_with('-') {
+        return false;
+    }
+    if name.contains('\n') || name.contains('\r') || name.contains('\0') {
+        return false;
+    }
+    name.split('.').all(valid_dns_label)
+}
+
+fn valid_dns_label(label: &str) -> bool {
+    let b = label.as_bytes();
+    if b.is_empty() || b.len() > 63 {
+        return false;
+    }
+    if !b[0].is_ascii_alphanumeric() || !b[b.len() - 1].is_ascii_alphanumeric() {
+        return false;
+    }
+    b.iter().all(|c| c.is_ascii_alphanumeric() || *c == b'-')
+}
+
 #[tauri::command]
 pub async fn set_hostname(name: String) -> Result<(), String> {
     let name = name.trim();
-    if name.is_empty() {
-        return Err("Hostname can't be empty".into());
+    if !valid_hostname(name) {
+        return Err("invalid hostname".into());
     }
     let output = Command::new("pkexec")
         .args(["hostnamectl", "set-hostname", name])
@@ -153,3 +185,24 @@ pub async fn set_hostname(name: String) -> Result<(), String> {
         Err(String::from_utf8_lossy(&output.stderr).trim().to_string())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn hostname_rfc1123() {
+        assert!(valid_hostname("bos"));
+        assert!(valid_hostname("bos.local"));
+        assert!(valid_hostname("a1-b"));
+        assert!(valid_hostname("1host"));
+        assert!(!valid_hostname(""));
+        assert!(!valid_hostname("-bos"));
+        assert!(!valid_hostname("bos-"));
+        assert!(!valid_hostname("-foo.bar"));
+        assert!(!valid_hostname("foo_bar"));
+        assert!(!valid_hostname("bos\n-set-hostname evil"));
+        assert!(!valid_hostname("--help"));
+        assert!(!valid_hostname(&"a".repeat(65)));
+    }
+}
diff --git a/src/src/commands/aur.rs b/src/src/commands/aur.rs
index 226af63..6c72b64 100644
--- a/src/src/commands/aur.rs
+++ b/src/src/commands/aur.rs
@@ -10,6 +10,8 @@
 
 use serde::Serialize;
 
+use super::util;
+
 #[derive(Serialize, Clone)]
 pub struct AurResult {
     name: String,
@@ -19,7 +21,11 @@ pub struct AurResult {
 
 #[tauri::command]
 pub async fn search_aur(query: String) -> Vec<AurResult> {
-    let Ok(output) = tokio::process::Command::new("yay").args(["-Ss", "--aur", &query]).output().await else {
+    let Ok(output) = tokio::process::Command::new("yay")
+        .args(["-Ss", "--aur", &query])
+        .output()
+        .await
+    else {
         return Vec::new();
     };
     let text = String::from_utf8_lossy(&output.stdout);
@@ -28,12 +34,18 @@ pub async fn search_aur(query: String) -> Vec<AurResult> {
     while let Some(header) = lines.next() {
         // "aur/name version (+votes score) [Orphaned]" — name/version are
         // always the first two whitespace-separated fields after "aur/".
-        let Some(rest) = header.strip_prefix("aur/") else { continue };
+        let Some(rest) = header.strip_prefix("aur/") else {
+            continue;
+        };
         let mut parts = rest.split_whitespace();
         let Some(name) = parts.next() else { continue };
         let version = parts.next().unwrap_or("").to_string();
         let description = lines.next().unwrap_or("").trim().to_string();
-        results.push(AurResult { name: name.to_string(), version, description });
+        results.push(AurResult {
+            name: name.to_string(),
+            version,
+            description,
+        });
         if results.len() >= 50 {
             break;
         }
@@ -42,6 +54,13 @@ pub async fn search_aur(query: String) -> Vec<AurResult> {
 }
 
 #[tauri::command]
-pub fn install_aur_package(pkg: String) {
-    let _ = std::process::Command::new("kitty").args(["-e", "yay", "-S", &pkg]).spawn();
+pub fn install_aur_package(pkg: String) -> Result<(), String> {
+    if !util::valid_pkg_name(&pkg) {
+        return Err(format!("refusing to install '{pkg}'"));
+    }
+    std::process::Command::new("kitty")
+        .args(["-e", "yay", "-S", &pkg])
+        .spawn()
+        .map_err(|e| e.to_string())?;
+    Ok(())
 }
diff --git a/src/src/commands/breadpad.rs b/src/src/commands/breadpad.rs
index 5f0fb5d..19db123 100644
--- a/src/src/commands/breadpad.rs
+++ b/src/src/commands/breadpad.rs
@@ -35,22 +35,35 @@ pub struct BreadpadConfig {
 pub fn get_breadpad_config() -> BreadpadConfig {
     let doc = config::load_doc(&config_path());
     BreadpadConfig {
-        default_type: config::get_str(&doc, &["settings", "default_type"]).unwrap_or_else(|| "note".into()),
+        default_type: config::get_str(&doc, &["settings", "default_type"])
+            .unwrap_or_else(|| "note".into()),
         workspace_tag: config::get_bool(&doc, &["settings", "workspace_tag"]).unwrap_or(true),
         snooze_options: config::get_str_list(&doc, &["settings", "snooze_options"]),
-        archive_after_days: config::get_i64(&doc, &["settings", "archive_after_days"]).unwrap_or(30),
+        archive_after_days: config::get_i64(&doc, &["settings", "archive_after_days"])
+            .unwrap_or(30),
         model_path: config::get_str(&doc, &["model", "path"]).unwrap_or_default(),
         tokenizer_path: config::get_str(&doc, &["model", "tokenizer"]).unwrap_or_default(),
         ollama_enabled: config::get_bool(&doc, &["model", "ollama", "enabled"]).unwrap_or(true),
-        ollama_endpoint: config::get_str(&doc, &["model", "ollama", "endpoint"]).unwrap_or_default(),
+        ollama_endpoint: config::get_str(&doc, &["model", "ollama", "endpoint"])
+            .unwrap_or_default(),
         ollama_model: config::get_str(&doc, &["model", "ollama", "model"]).unwrap_or_default(),
-        ollama_confidence_threshold: config::get_f64(&doc, &["model", "ollama", "confidence_threshold"]).unwrap_or(0.6),
-        reminders_default_morning: config::get_str(&doc, &["reminders", "default_morning"]).unwrap_or_else(|| "7:00".into()),
-        reminders_missed_grace_minutes: config::get_i64(&doc, &["reminders", "missed_grace_minutes"]).unwrap_or(60),
+        ollama_confidence_threshold: config::get_f64(
+            &doc,
+            &["model", "ollama", "confidence_threshold"],
+        )
+        .unwrap_or(0.6),
+        reminders_default_morning: config::get_str(&doc, &["reminders", "default_morning"])
+            .unwrap_or_else(|| "7:00".into()),
+        reminders_missed_grace_minutes: config::get_i64(
+            &doc,
+            &["reminders", "missed_grace_minutes"],
+        )
+        .unwrap_or(60),
         calendar_enabled: config::get_bool(&doc, &["calendar", "enabled"]).unwrap_or(false),
         calendar_url: config::get_str(&doc, &["calendar", "url"]).unwrap_or_default(),
         calendar_username: config::get_str(&doc, &["calendar", "username"]).unwrap_or_default(),
-        calendar_password: config::get_str(&doc, &["calendar", "password"]).unwrap_or_default(),
+        // Write-only to the webview, same as restic — never round-trip the secret.
+        calendar_password: String::new(),
     }
 }
 
@@ -60,19 +73,76 @@ pub fn save_breadpad_config(cfg: BreadpadConfig) -> Result<(), String> {
     let mut doc = config::load_doc(&path);
     config::set_str(&mut doc, &["settings", "default_type"], &cfg.default_type);
     config::set_bool(&mut doc, &["settings", "workspace_tag"], cfg.workspace_tag);
-    config::set_str_list(&mut doc, &["settings", "snooze_options"], &cfg.snooze_options);
-    config::set_i64(&mut doc, &["settings", "archive_after_days"], cfg.archive_after_days);
+    config::set_str_list(
+        &mut doc,
+        &["settings", "snooze_options"],
+        &cfg.snooze_options,
+    );
+    config::set_i64(
+        &mut doc,
+        &["settings", "archive_after_days"],
+        cfg.archive_after_days,
+    );
     config::set_str_or_remove(&mut doc, &["model", "path"], &cfg.model_path);
     config::set_str_or_remove(&mut doc, &["model", "tokenizer"], &cfg.tokenizer_path);
-    config::set_bool(&mut doc, &["model", "ollama", "enabled"], cfg.ollama_enabled);
-    config::set_str_or_remove(&mut doc, &["model", "ollama", "endpoint"], &cfg.ollama_endpoint);
+    config::set_bool(
+        &mut doc,
+        &["model", "ollama", "enabled"],
+        cfg.ollama_enabled,
+    );
+    config::set_str_or_remove(
+        &mut doc,
+        &["model", "ollama", "endpoint"],
+        &cfg.ollama_endpoint,
+    );
     config::set_str_or_remove(&mut doc, &["model", "ollama", "model"], &cfg.ollama_model);
-    config::set_f64(&mut doc, &["model", "ollama", "confidence_threshold"], cfg.ollama_confidence_threshold);
-    config::set_str_or_remove(&mut doc, &["reminders", "default_morning"], &cfg.reminders_default_morning);
-    config::set_i64(&mut doc, &["reminders", "missed_grace_minutes"], cfg.reminders_missed_grace_minutes);
+    config::set_f64(
+        &mut doc,
+        &["model", "ollama", "confidence_threshold"],
+        cfg.ollama_confidence_threshold,
+    );
+    config::set_str_or_remove(
+        &mut doc,
+        &["reminders", "default_morning"],
+        &cfg.reminders_default_morning,
+    );
+    config::set_i64(
+        &mut doc,
+        &["reminders", "missed_grace_minutes"],
+        cfg.reminders_missed_grace_minutes,
+    );
     config::set_bool(&mut doc, &["calendar", "enabled"], cfg.calendar_enabled);
     config::set_str_or_remove(&mut doc, &["calendar", "url"], &cfg.calendar_url);
     config::set_str_or_remove(&mut doc, &["calendar", "username"], &cfg.calendar_username);
-    config::set_str_or_remove(&mut doc, &["calendar", "password"], &cfg.calendar_password);
+    apply_calendar_password(&mut doc, &cfg.calendar_password);
     config::save_doc(&path, &doc).map_err(|e| e.to_string())
 }
+
+/// Empty incoming password keeps the existing secret (PasswordField is write-only).
+fn apply_calendar_password(doc: &mut toml_edit::DocumentMut, incoming: &str) {
+    if incoming.is_empty() {
+        return;
+    }
+    config::set_str(doc, &["calendar", "password"], incoming);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty_password_keeps_existing_secret() {
+        let mut doc: toml_edit::DocumentMut =
+            "[calendar]\npassword = \"secret\"\n".parse().unwrap();
+        apply_calendar_password(&mut doc, "");
+        assert_eq!(
+            config::get_str(&doc, &["calendar", "password"]).as_deref(),
+            Some("secret")
+        );
+        apply_calendar_password(&mut doc, "newpass");
+        assert_eq!(
+            config::get_str(&doc, &["calendar", "password"]).as_deref(),
+            Some("newpass")
+        );
+    }
+}
diff --git a/src/src/commands/datetime.rs b/src/src/commands/datetime.rs
index 2e8660b..5413cc2 100644
--- a/src/src/commands/datetime.rs
+++ b/src/src/commands/datetime.rs
@@ -26,7 +26,12 @@ async fn list_timezones() -> Vec<String> {
         .output()
         .await
         .ok()
-        .map(|o| String::from_utf8_lossy(&o.stdout).lines().map(str::to_string).collect())
+        .map(|o| {
+            String::from_utf8_lossy(&o.stdout)
+                .lines()
+                .map(str::to_string)
+                .collect()
+        })
         .unwrap_or_default()
 }
 
@@ -60,10 +65,32 @@ pub async fn get_datetime_info() -> DateTimeInfo {
     }
 }
 
+/// Reject flags, path traversal, and newlines before we ever exec. Charset
+/// matches IANA names (`Area/City`, `UTC`, `Etc/GMT+6`).
+fn timezone_looks_safe(tz: &str) -> bool {
+    let tz = tz.trim();
+    if tz.is_empty() || tz.len() > 64 || tz.starts_with('-') {
+        return false;
+    }
+    if tz.contains('\n') || tz.contains('\r') || tz.contains('\0') || tz.contains("..") {
+        return false;
+    }
+    tz.chars()
+        .all(|c| c.is_ascii_alphanumeric() || matches!(c, '/' | '_' | '+' | '-'))
+}
+
 #[tauri::command]
 pub async fn set_timezone(tz: String) -> Result<(), String> {
+    let tz = tz.trim();
+    if !timezone_looks_safe(tz) {
+        return Err("invalid timezone".into());
+    }
+    let listed = list_timezones().await;
+    if !listed.is_empty() && !listed.iter().any(|t| t == tz) {
+        return Err("unknown timezone".into());
+    }
     let output = Command::new("pkexec")
-        .args(["timedatectl", "set-timezone", &tz])
+        .args(["timedatectl", "set-timezone", tz])
         .output()
         .await
         .map_err(|e| e.to_string())?;
@@ -77,6 +104,28 @@ pub async fn set_timezone(tz: String) -> Result<(), String> {
 #[tauri::command]
 pub async fn set_ntp_enabled(enabled: bool) -> Result<(), String> {
     let val = if enabled { "true" } else { "false" };
-    Command::new("pkexec").args(["timedatectl", "set-ntp", val]).status().await.map_err(|e| e.to_string())?;
+    Command::new("pkexec")
+        .args(["timedatectl", "set-ntp", val])
+        .status()
+        .await
+        .map_err(|e| e.to_string())?;
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn timezone_rejects_flags_and_traversal() {
+        assert!(timezone_looks_safe("UTC"));
+        assert!(timezone_looks_safe("America/New_York"));
+        assert!(timezone_looks_safe("Etc/GMT+6"));
+        assert!(!timezone_looks_safe(""));
+        assert!(!timezone_looks_safe("-UTC"));
+        assert!(!timezone_looks_safe("--help"));
+        assert!(!timezone_looks_safe("America/../UTC"));
+        assert!(!timezone_looks_safe("UTC\n--adjust"));
+        assert!(!timezone_looks_safe("UTC;reboot"));
+    }
+}
diff --git a/src/src/commands/firewall.rs b/src/src/commands/firewall.rs
index 0841d7b..bfc6471 100644
--- a/src/src/commands/firewall.rs
+++ b/src/src/commands/firewall.rs
@@ -32,7 +32,9 @@ pub async fn get_firewall_status() -> Result<FirewallStatus, String> {
         let stderr = String::from_utf8_lossy(&output.stderr).trim().to_string();
         return Err(if stderr.is_empty() {
             match output.status.code() {
-                Some(127) => "no polkit authentication agent is available in this session".to_string(),
+                Some(127) => {
+                    "no polkit authentication agent is available in this session".to_string()
+                }
                 Some(code) => format!("pkexec exited with status {code}"),
                 None => "pkexec was terminated by a signal".to_string(),
             }
@@ -41,7 +43,10 @@ pub async fn get_firewall_status() -> Result<FirewallStatus, String> {
         });
     }
     let text = String::from_utf8_lossy(&output.stdout);
-    let active = text.lines().next().is_some_and(|l| l.trim() == "Status: active");
+    let active = text
+        .lines()
+        .next()
+        .is_some_and(|l| l.trim() == "Status: active");
     let rules = text
         .lines()
         .filter_map(|l| {
@@ -51,7 +56,10 @@ pub async fn get_firewall_status() -> Result<FirewallStatus, String> {
             }
             let (num, rest) = l.split_once(']')?;
             let number = num.trim_start_matches('[').trim().to_string();
-            Some(FirewallRule { number, text: rest.trim().to_string() })
+            Some(FirewallRule {
+                number,
+                text: rest.trim().to_string(),
+            })
         })
         .collect();
     Ok(FirewallStatus { active, rules })
@@ -60,7 +68,11 @@ pub async fn get_firewall_status() -> Result<FirewallStatus, String> {
 #[tauri::command]
 pub async fn set_firewall_enabled(enabled: bool) -> Result<(), String> {
     let verb = if enabled { "enable" } else { "disable" };
-    let output = Command::new("pkexec").args(["ufw", "--force", verb]).output().await.map_err(|e| e.to_string())?;
+    let output = Command::new("pkexec")
+        .args(["ufw", "--force", verb])
+        .output()
+        .await
+        .map_err(|e| e.to_string())?;
     if output.status.success() {
         Ok(())
     } else {
@@ -68,9 +80,54 @@ pub async fn set_firewall_enabled(enabled: bool) -> Result<(), String> {
     }
 }
 
+/// Port, optional `/tcp`/`/udp`, or optional space-separated proto. No
+/// service names, IPs, or flags — those become extra `ufw allow` operands.
+fn valid_firewall_rule(rule: &str) -> bool {
+    let rule = rule.trim();
+    if rule.is_empty() || rule.len() > 16 || rule.starts_with('-') {
+        return false;
+    }
+    if rule.contains('\n') || rule.contains('\r') || rule.contains('\0') {
+        return false;
+    }
+    let (port, proto) = if let Some((p, rest)) = rule.split_once('/') {
+        (p, Some(rest))
+    } else if let Some((p, rest)) = rule.split_once(' ') {
+        (p, Some(rest.trim()))
+    } else {
+        (rule, None)
+    };
+    let Ok(n) = port.parse::<u16>() else {
+        return false;
+    };
+    if n == 0 {
+        return false;
+    }
+    match proto {
+        None => true,
+        Some(p) => p == "tcp" || p == "udp",
+    }
+}
+
+fn valid_rule_number(number: &str) -> bool {
+    let t = number.trim();
+    !t.is_empty()
+        && t.len() <= 8
+        && t.bytes().all(|b| b.is_ascii_digit())
+        && t.parse::<u32>().is_ok_and(|n| n > 0)
+}
+
 #[tauri::command]
 pub async fn add_firewall_rule(rule: String) -> Result<(), String> {
-    let output = Command::new("pkexec").args(["ufw", "allow", rule.trim()]).output().await.map_err(|e| e.to_string())?;
+    let rule = rule.trim();
+    if !valid_firewall_rule(rule) {
+        return Err("invalid firewall rule".into());
+    }
+    let output = Command::new("pkexec")
+        .args(["ufw", "allow", rule])
+        .output()
+        .await
+        .map_err(|e| e.to_string())?;
     if output.status.success() {
         Ok(())
     } else {
@@ -80,10 +137,48 @@ pub async fn add_firewall_rule(rule: String) -> Result<(), String> {
 
 #[tauri::command]
 pub async fn remove_firewall_rule(number: String) -> Result<(), String> {
-    let output = Command::new("pkexec").args(["ufw", "--force", "delete", &number]).output().await.map_err(|e| e.to_string())?;
+    if !valid_rule_number(&number) {
+        return Err("invalid rule number".into());
+    }
+    let output = Command::new("pkexec")
+        .args(["ufw", "--force", "delete", number.trim()])
+        .output()
+        .await
+        .map_err(|e| e.to_string())?;
     if output.status.success() {
         Ok(())
     } else {
         Err(String::from_utf8_lossy(&output.stderr).trim().to_string())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn firewall_rule_is_port_and_optional_proto() {
+        assert!(valid_firewall_rule("22"));
+        assert!(valid_firewall_rule("8080/tcp"));
+        assert!(valid_firewall_rule("53/udp"));
+        assert!(valid_firewall_rule("80 tcp"));
+        assert!(!valid_firewall_rule("OpenSSH"));
+        assert!(!valid_firewall_rule("-f"));
+        assert!(!valid_firewall_rule("22;id"));
+        assert!(!valid_firewall_rule("22/tcp\nallow 23"));
+        assert!(!valid_firewall_rule("0"));
+        assert!(!valid_firewall_rule("65536"));
+        assert!(!valid_firewall_rule("22/all"));
+    }
+
+    #[test]
+    fn firewall_delete_is_positive_int() {
+        assert!(valid_rule_number("1"));
+        assert!(valid_rule_number("12"));
+        assert!(!valid_rule_number("0"));
+        assert!(!valid_rule_number("-1"));
+        assert!(!valid_rule_number("1;2"));
+        assert!(!valid_rule_number("1\n2"));
+        assert!(!valid_rule_number(""));
+    }
+}
diff --git a/src/src/commands/power.rs b/src/src/commands/power.rs
index 3d82def..d1d6820 100644
--- a/src/src/commands/power.rs
+++ b/src/src/commands/power.rs
@@ -7,13 +7,22 @@
 use serde::Serialize;
 use tokio::process::Command;
 
+use super::util;
+
 async fn upower_device(kind: &str) -> Option<String> {
     let out = Command::new("upower").arg("-e").output().await.ok()?;
-    String::from_utf8_lossy(&out.stdout).lines().find(|l| l.to_lowercase().contains(kind)).map(str::to_string)
+    String::from_utf8_lossy(&out.stdout)
+        .lines()
+        .find(|l| l.to_lowercase().contains(kind))
+        .map(str::to_string)
 }
 
 async fn upower_field(device: &str, field: &str) -> Option<String> {
-    let out = Command::new("upower").args(["-i", device]).output().await.ok()?;
+    let out = Command::new("upower")
+        .args(["-i", device])
+        .output()
+        .await
+        .ok()?;
     let text = String::from_utf8_lossy(&out.stdout);
     text.lines()
         .find(|l| l.trim_start().starts_with(field))
@@ -39,12 +48,18 @@ async fn battery_summary() -> Vec<(String, String)> {
     if let Some(t) = t {
         rows.push(("Time remaining".to_string(), t));
     }
-    let full: Option<f64> = upower_field(&bat, "energy-full").await.and_then(|v| v.split_whitespace().next()?.parse().ok());
-    let design: Option<f64> =
-        upower_field(&bat, "energy-full-design").await.and_then(|v| v.split_whitespace().next()?.parse().ok());
+    let full: Option<f64> = upower_field(&bat, "energy-full")
+        .await
+        .and_then(|v| v.split_whitespace().next()?.parse().ok());
+    let design: Option<f64> = upower_field(&bat, "energy-full-design")
+        .await
+        .and_then(|v| v.split_whitespace().next()?.parse().ok());
     if let (Some(full), Some(design)) = (full, design) {
         if design > 0.0 {
-            rows.push(("Battery health".to_string(), format!("{:.0}% of design capacity", full / design * 100.0)));
+            rows.push((
+                "Battery health".to_string(),
+                format!("{:.0}% of design capacity", full / design * 100.0),
+            ));
         }
     }
     rows
@@ -64,12 +79,19 @@ async fn power_source() -> String {
 async fn tlp_profile() -> Option<String> {
     let out = Command::new("tlp-stat").arg("-s").output().await.ok()?;
     let text = String::from_utf8_lossy(&out.stdout);
-    text.lines().find(|l| l.trim_start().starts_with("TLP profile")).and_then(|l| l.split('=').nth(1)).map(|v| v.trim().to_string())
+    text.lines()
+        .find(|l| l.trim_start().starts_with("TLP profile"))
+        .and_then(|l| l.split('=').nth(1))
+        .map(|v| v.trim().to_string())
 }
 
 async fn brightness_device() -> Option<String> {
     let out = Command::new("brightnessctl").output().await.ok()?;
-    String::from_utf8_lossy(&out.stdout).lines().find(|l| l.starts_with("Device")).and_then(|l| l.split('\'').nth(1)).map(str::to_string)
+    String::from_utf8_lossy(&out.stdout)
+        .lines()
+        .find(|l| l.starts_with("Device"))
+        .and_then(|l| l.split('\'').nth(1))
+        .map(str::to_string)
 }
 
 async fn brightness_pct() -> Option<u32> {
@@ -98,7 +120,10 @@ fn charge_threshold_paths() -> Option<(std::path::PathBuf, std::path::PathBuf)>
 }
 
 fn read_threshold(path: &std::path::Path) -> i64 {
-    std::fs::read_to_string(path).ok().and_then(|s| s.trim().parse().ok()).unwrap_or(100)
+    std::fs::read_to_string(path)
+        .ok()
+        .and_then(|s| s.trim().parse().ok())
+        .unwrap_or(100)
 }
 
 #[derive(Serialize)]
@@ -130,16 +155,57 @@ pub async fn set_brightness(percent: i64) -> Result<(), String> {
         return Err("No controllable backlight found".into());
     };
     let pct = format!("{percent}%");
-    Command::new("brightnessctl").args(["--device", &device, "set", &pct]).status().await.map_err(|e| e.to_string())?;
+    Command::new("brightnessctl")
+        .args(["--device", &device, "set", &pct])
+        .status()
+        .await
+        .map_err(|e| e.to_string())?;
     Ok(())
 }
 
+fn charge_threshold_write(which: &str, percent: i64) -> Result<(String, i64), String> {
+    if which != "start" && which != "end" {
+        return Err("threshold must be start or end".into());
+    }
+    Ok((which.to_string(), percent.clamp(0, 100)))
+}
+
 #[tauri::command]
 pub async fn set_charge_threshold(which: String, percent: i64) -> Result<(), String> {
+    let (which, percent) = charge_threshold_write(&which, percent)?;
     let Some((start, end)) = charge_threshold_paths() else {
         return Err("No charge threshold support on this hardware".into());
     };
     let path = if which == "start" { start } else { end };
-    Command::new("pkexec").args(["tee", &path.display().to_string()]).arg(percent.to_string()).output().await.map_err(|e| e.to_string())?;
-    Ok(())
+    // GNU tee writes stdin to its path operands — the percent must be piped,
+    // not passed as a second path argument.
+    let input = format!("{percent}\n");
+    if util::run_with_stdin(&["pkexec", "tee", &path.display().to_string()], &input).await {
+        Ok(())
+    } else {
+        Err("Failed to set charge threshold".into())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn charge_threshold_clamps_and_restricts_which() {
+        assert_eq!(
+            charge_threshold_write("start", 80).unwrap(),
+            ("start".into(), 80)
+        );
+        assert_eq!(
+            charge_threshold_write("end", 150).unwrap(),
+            ("end".into(), 100)
+        );
+        assert_eq!(
+            charge_threshold_write("start", -5).unwrap(),
+            ("start".into(), 0)
+        );
+        assert!(charge_threshold_write("both", 50).is_err());
+        assert!(charge_threshold_write("-start", 50).is_err());
+    }
 }
diff --git a/src/src/commands/service.rs b/src/src/commands/service.rs
index c99fee4..17a1540 100644
--- a/src/src/commands/service.rs
+++ b/src/src/commands/service.rs
@@ -21,6 +21,19 @@ pub enum ServiceAction {
     Restart,
 }
 
+/// Units the frontend already hardcodes in ServiceControl call sites.
+const ALLOWED_UNITS: &[&str] = &[
+    "breadd.service",
+    "breadclipd.service",
+    "breadcrumbs.service",
+    "breadmill.service",
+    "breadbox-sync.service",
+];
+
+fn allowed_unit(unit: &str) -> bool {
+    ALLOWED_UNITS.contains(&unit)
+}
+
 async fn systemctl_active(unit: &str) -> bool {
     Command::new("systemctl")
         .args(["--user", "is-active", "--quiet", unit])
@@ -40,15 +53,21 @@ async fn systemctl_enabled(unit: &str) -> bool {
 }
 
 #[tauri::command]
-pub async fn get_service_status(unit: String) -> ServiceStatus {
-    ServiceStatus {
+pub async fn get_service_status(unit: String) -> Result<ServiceStatus, String> {
+    if !allowed_unit(&unit) {
+        return Err("unknown service".into());
+    }
+    Ok(ServiceStatus {
         active: systemctl_active(&unit).await,
         enabled: systemctl_enabled(&unit).await,
-    }
+    })
 }
 
 #[tauri::command]
 pub async fn service_action(unit: String, action: ServiceAction) -> Result<(), String> {
+    if !allowed_unit(&unit) {
+        return Err("unknown service".into());
+    }
     let verb = match action {
         ServiceAction::Start => "start",
         ServiceAction::Stop => "stop",
@@ -70,8 +89,31 @@ pub async fn service_action(unit: String, action: ServiceAction) -> Result<(), S
 /// panel, no reason to pull an open-ended `journalctl -f` tail into the
 /// webview.
 #[tauri::command]
-pub fn open_logs(unit: String) {
-    let _ = std::process::Command::new("kitty")
+pub fn open_logs(unit: String) -> Result<(), String> {
+    if !allowed_unit(&unit) {
+        return Err("unknown service".into());
+    }
+    std::process::Command::new("kitty")
         .args(["-e", "journalctl", "--user", "-u", &unit, "-f"])
-        .spawn();
+        .spawn()
+        .map_err(|e| e.to_string())?;
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn units_match_frontend_hardcoded_list() {
+        assert!(allowed_unit("breadd.service"));
+        assert!(allowed_unit("breadclipd.service"));
+        assert!(allowed_unit("breadcrumbs.service"));
+        assert!(allowed_unit("breadmill.service"));
+        assert!(allowed_unit("breadbox-sync.service"));
+        assert!(!allowed_unit("sshd.service"));
+        assert!(!allowed_unit("breadd.service;reboot"));
+        assert!(!allowed_unit("../sshd.service"));
+        assert!(!allowed_unit("-u sshd"));
+    }
 }
diff --git a/src/src/commands/users.rs b/src/src/commands/users.rs
index d26ff59..8b0db82 100644
--- a/src/src/commands/users.rs
+++ b/src/src/commands/users.rs
@@ -3,9 +3,10 @@
 //! `pkexec`.
 
 use serde::Serialize;
-use tokio::io::AsyncWriteExt;
 use tokio::process::Command;
 
+use super::util;
+
 #[derive(Serialize, Clone)]
 pub struct Account {
     username: String,
@@ -26,10 +27,16 @@ fn list_accounts() -> Vec<Account> {
             let shell = f[6];
             // Real human accounts: normal UID range, a real login shell
             // (excludes system/service accounts like greeter, avahi, etc).
-            if !(1000..60000).contains(&uid) || shell.ends_with("nologin") || shell.ends_with("/false") {
+            if !(1000..60000).contains(&uid)
+                || shell.ends_with("nologin")
+                || shell.ends_with("/false")
+            {
                 return None;
             }
-            Some(Account { username: f[0].to_string(), full_name: f[4].split(',').next().unwrap_or("").to_string() })
+            Some(Account {
+                username: f[0].to_string(),
+                full_name: f[4].split(',').next().unwrap_or("").to_string(),
+            })
         })
         .collect()
 }
@@ -42,29 +49,69 @@ pub struct UsersInfo {
 
 #[tauri::command]
 pub fn get_users_info() -> UsersInfo {
-    UsersInfo { accounts: list_accounts(), current_user: std::env::var("USER").unwrap_or_default() }
+    UsersInfo {
+        accounts: list_accounts(),
+        current_user: std::env::var("USER").unwrap_or_default(),
+    }
 }
 
-/// Runs a root command that needs a line of input on stdin (chpasswd's own
-/// "user:password" format). `pkexec` inherits the spawning process's stdin
-/// only when explicitly piped, so this pipes it through.
-async fn run_with_stdin(args: &[&str], input: String) -> bool {
-    let Ok(mut child) = Command::new(args[0]).args(&args[1..]).stdin(std::process::Stdio::piped()).stdout(std::process::Stdio::null()).stderr(std::process::Stdio::null()).spawn()
-    else {
+/// shadow-utils `USER_NAME_MAX` is 32; keep chpasswd/useradd operands inside it.
+const USERNAME_MAX: usize = 32;
+
+/// `[a-z_][a-z0-9_-]*`, length-capped, no leading `-`. Also rejects `:`,
+/// newlines, and other chpasswd field/line separators.
+fn valid_username(name: &str) -> bool {
+    let bytes = name.as_bytes();
+    if bytes.is_empty() || bytes.len() > USERNAME_MAX {
         return false;
-    };
-    if let Some(mut stdin) = child.stdin.take() {
-        if stdin.write_all(input.as_bytes()).await.is_err() {
-            return false;
-        }
     }
-    child.wait().await.map(|s| s.success()).unwrap_or(false)
+    let first = bytes[0];
+    if first != b'_' && !first.is_ascii_lowercase() {
+        return false;
+    }
+    bytes[1..]
+        .iter()
+        .all(|b| b.is_ascii_lowercase() || b.is_ascii_digit() || matches!(*b, b'_' | b'-'))
+}
+
+/// chpasswd reads `user:password` lines — a `:`, `\n`, or `\r` in either
+/// field injects extra passwd entries or shifts columns.
+fn valid_chpasswd_password(password: &str) -> bool {
+    !password.is_empty()
+        && password.len() <= 512
+        && !password.contains('\n')
+        && !password.contains('\r')
+        && !password.contains(':')
+        && !password.contains('\0')
+}
+
+fn chpasswd_input(username: &str, password: &str) -> Result<String, String> {
+    if !valid_username(username) {
+        return Err("invalid username".into());
+    }
+    if !valid_chpasswd_password(password) {
+        return Err("invalid password".into());
+    }
+    Ok(format!("{username}:{password}\n"))
+}
+
+fn may_delete_user(username: &str, current: &str) -> Result<(), String> {
+    if !valid_username(username) {
+        return Err("invalid username".into());
+    }
+    if username == "root" {
+        return Err("refusing to remove root".into());
+    }
+    if !current.is_empty() && username == current {
+        return Err("refusing to remove the current user".into());
+    }
+    Ok(())
 }
 
 #[tauri::command]
 pub async fn change_password(username: String, password: String) -> Result<(), String> {
-    let input = format!("{username}:{password}\n");
-    if run_with_stdin(&["pkexec", "chpasswd"], input).await {
+    let input = chpasswd_input(&username, &password)?;
+    if util::run_with_stdin(&["pkexec", "chpasswd"], &input).await {
         Ok(())
     } else {
         Err("Failed to change password".into())
@@ -73,7 +120,13 @@ pub async fn change_password(username: String, password: String) -> Result<(), S
 
 #[tauri::command]
 pub async fn remove_user(username: String) -> Result<(), String> {
-    let output = Command::new("pkexec").args(["userdel", "-r", &username]).output().await.map_err(|e| e.to_string())?;
+    let current = std::env::var("USER").unwrap_or_default();
+    may_delete_user(&username, &current)?;
+    let output = Command::new("pkexec")
+        .args(["userdel", "-r", &username])
+        .output()
+        .await
+        .map_err(|e| e.to_string())?;
     if output.status.success() {
         Ok(())
     } else {
@@ -83,22 +136,68 @@ pub async fn remove_user(username: String) -> Result<(), String> {
 
 #[tauri::command]
 pub async fn add_user(username: String, full_name: String, password: String) -> Result<(), String> {
-    let username = username.trim().to_string();
-    let mut useradd_args = vec!["pkexec".to_string(), "useradd".to_string(), "-m".to_string(), "-s".to_string(), "/bin/bash".to_string()];
+    let username = username.trim();
+    let input = chpasswd_input(username, &password)?;
+    let mut useradd_args = vec![
+        "pkexec".to_string(),
+        "useradd".to_string(),
+        "-m".to_string(),
+        "-s".to_string(),
+        "/bin/bash".to_string(),
+    ];
     if !full_name.trim().is_empty() {
         useradd_args.push("-c".to_string());
         useradd_args.push(full_name.trim().to_string());
     }
-    useradd_args.push(username.clone());
+    useradd_args.push(username.to_string());
     let args_ref: Vec<&str> = useradd_args.iter().map(String::as_str).collect();
-    let output = Command::new(args_ref[0]).args(&args_ref[1..]).output().await.map_err(|e| e.to_string())?;
+    let output = Command::new(args_ref[0])
+        .args(&args_ref[1..])
+        .output()
+        .await
+        .map_err(|e| e.to_string())?;
     if !output.status.success() {
         return Err(String::from_utf8_lossy(&output.stderr).trim().to_string());
     }
-    let input = format!("{username}:{password}\n");
-    if run_with_stdin(&["pkexec", "chpasswd"], input).await {
+    if util::run_with_stdin(&["pkexec", "chpasswd"], &input).await {
         Ok(())
     } else {
         Err("User created, but setting the password failed.".into())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn chpasswd_rejects_newline_injection() {
+        assert!(chpasswd_input("alice", "pw\nroot:evil").is_err());
+        assert!(chpasswd_input("alice\nroot", "pw").is_err());
+        assert!(chpasswd_input("alice\rroot", "pw").is_err());
+        assert!(chpasswd_input("alice", "pw\rroot:x").is_err());
+        assert!(chpasswd_input("al:ice", "pw").is_err());
+        assert!(chpasswd_input("alice", "p:w").is_err());
+        assert_eq!(chpasswd_input("alice", "secret").unwrap(), "alice:secret\n");
+    }
+
+    #[test]
+    fn username_grammar() {
+        assert!(valid_username("alice"));
+        assert!(valid_username("_svc"));
+        assert!(valid_username("a1-b_c"));
+        assert!(!valid_username(""));
+        assert!(!valid_username("-alice"));
+        assert!(!valid_username("Alice"));
+        assert!(!valid_username("root user"));
+        assert!(!valid_username(&"a".repeat(USERNAME_MAX + 1)));
+    }
+
+    #[test]
+    fn remove_user_refuses_root_and_self() {
+        assert!(may_delete_user("root", "alice").is_err());
+        assert!(may_delete_user("alice", "alice").is_err());
+        assert!(may_delete_user("root\n", "alice").is_err());
+        assert!(may_delete_user("bob", "alice").is_ok());
+    }
+}
diff --git a/src/src/commands/util.rs b/src/src/commands/util.rs
index 1acb283..d7a40d6 100644
--- a/src/src/commands/util.rs
+++ b/src/src/commands/util.rs
@@ -90,6 +90,30 @@ pub fn bos_settings_dir() -> PathBuf {
     config::config_dir().join("bos-settings")
 }
 
+/// Pipe `input` to a command's stdin (`pkexec` does not inherit a piped
+/// stdin unless we set it). Used by chpasswd and `pkexec tee`.
+pub async fn run_with_stdin(args: &[&str], input: &str) -> bool {
+    if args.is_empty() {
+        return false;
+    }
+    let Ok(mut child) = tokio::process::Command::new(args[0])
+        .args(&args[1..])
+        .stdin(std::process::Stdio::piped())
+        .stdout(std::process::Stdio::null())
+        .stderr(std::process::Stdio::null())
+        .spawn()
+    else {
+        return false;
+    };
+    if let Some(mut stdin) = child.stdin.take() {
+        use tokio::io::AsyncWriteExt;
+        if stdin.write_all(input.as_bytes()).await.is_err() {
+            return false;
+        }
+    }
+    child.wait().await.map(|s| s.success()).unwrap_or(false)
+}
+
 /// Atomic write with mode 0600 set on the new inode before/after replace,
 /// matching breadcrumbs' `networks.toml` care.
 pub fn write_secure(path: &Path, contents: &str) -> Result<(), String> {