diff --git a/src/services/api-proxy-service.ts b/src/services/api-proxy-service.ts
index deecd5bc..780f76f5 100644
--- a/src/services/api-proxy-service.ts
+++ b/src/services/api-proxy-service.ts
@@ -12,6 +12,7 @@ import { pickEnvVars } from '../env-utils';
 import { COPILOT_PLACEHOLDER_TOKEN } from '../constants/placeholders';
 import { NetworkConfig, ImageBuildConfig } from './squid-service';
 import { applyHostPathPrefixToVolumes } from './host-path-prefix';
+import { buildContainerSecurityHardening } from './service-security';
 
 interface ApiProxyBuildResult {
   /** The api-proxy service definition to add to Docker Compose services. */
@@ -208,16 +209,8 @@ export function buildApiProxyService(params: ApiProxyServiceParams): ApiProxyBui
       retries: 15,
       start_period: '30s',
     },
-    // Security hardening: Drop all capabilities
-    cap_drop: ['ALL'],
-    security_opt: [
-      'no-new-privileges:true',
-    ],
-    // Resource limits to prevent DoS attacks
-    mem_limit: '512m',
-    memswap_limit: '512m',
-    pids_limit: 100,
-    cpu_shares: 512,
+    // Security hardening and resource limits to prevent DoS attacks
+    ...buildContainerSecurityHardening({ memLimit: '512m', pidsLimit: 100, cpuShares: 512 }),
     stop_grace_period: '2s',
   };
 
diff --git a/src/services/cli-proxy-service.ts b/src/services/cli-proxy-service.ts
index 445b9bc4..b5428b56 100644
--- a/src/services/cli-proxy-service.ts
+++ b/src/services/cli-proxy-service.ts
@@ -6,6 +6,7 @@ import { logger } from '../logger';
 import { WrapperConfig, CLI_PROXY_PORT } from '../types';
 import { NetworkConfig, ImageBuildConfig } from './squid-service';
 import { applyHostPathPrefixToVolumes } from './host-path-prefix';
+import { buildContainerSecurityHardening } from './service-security';
 
 interface CliProxyBuildResult {
   /** The cli-proxy service definition to add to Docker Compose services. */
@@ -91,12 +92,8 @@ export function buildCliProxyService(params: CliProxyServiceParams): CliProxyBui
         condition: 'service_healthy',
       },
     },
-    cap_drop: ['ALL'],
-    security_opt: ['no-new-privileges:true'],
-    mem_limit: '256m',
-    memswap_limit: '256m',
-    pids_limit: 50,
-    cpu_shares: 256,
+    // Security hardening and resource limits to prevent DoS attacks
+    ...buildContainerSecurityHardening({ memLimit: '256m', pidsLimit: 50, cpuShares: 256 }),
     stop_grace_period: '2s',
   };
 
diff --git a/src/services/doh-proxy-service.ts b/src/services/doh-proxy-service.ts
index 383e6b66..da5cb3c5 100644
--- a/src/services/doh-proxy-service.ts
+++ b/src/services/doh-proxy-service.ts
@@ -2,6 +2,7 @@ import { DOH_PROXY_CONTAINER_NAME } from '../constants';
 import { logger } from '../logger';
 import { WrapperConfig } from '../types';
 import { NetworkConfig } from './squid-service';
+import { buildContainerSecurityHardening } from './service-security';
 
 interface DohProxyServiceParams {
   config: WrapperConfig;
@@ -34,12 +35,8 @@ export function buildDohProxyService(params: DohProxyServiceParams): any {
       retries: 5,
       start_period: '2s',
     },
-    // Security hardening: Drop all capabilities
-    cap_drop: ['ALL'],
-    security_opt: ['no-new-privileges:true'],
-    mem_limit: '128m',
-    memswap_limit: '128m',
-    pids_limit: 50,
+    // Security hardening and resource limits to prevent DoS attacks
+    ...buildContainerSecurityHardening({ memLimit: '128m', pidsLimit: 50 }),
   };
 
   logger.info(`DNS-over-HTTPS proxy sidecar enabled - DNS queries encrypted via ${config.dnsOverHttps}`);
diff --git a/src/services/service-security.ts b/src/services/service-security.ts
new file mode 100644
index 00000000..6aa24c50
--- /dev/null
+++ b/src/services/service-security.ts
@@ -0,0 +1,52 @@
+/**
+ * Security-hardening helpers for unprivileged proxy sidecar containers.
+ *
+ * This module centralises the `cap_drop`, `security_opt`, and resource-limit
+ * fields shared by the lightweight proxy sidecars (api-proxy, cli-proxy,
+ * doh-proxy) that run without any Linux capabilities.
+ *
+ * Note: other services such as `squid-service` and `agent-service` have
+ * different hardening requirements (custom cap_drop sets, seccomp profiles,
+ * AppArmor options) and should NOT use this helper.
+ *
+ * Using a single helper means a future hardening change (e.g. adding
+ * `read_only: true`) propagates to all three proxy sidecars automatically.
+ */
+
+interface ContainerResourceLimits {
+  /** Maximum memory for the container (Docker memory format, e.g. '512m'). */
+  memLimit: string;
+  /** Maximum number of processes/threads the container may create. */
+  pidsLimit: number;
+  /**
+   * Relative CPU weight (cpu_shares).
+   * If omitted the field is not included in the output.
+   */
+  cpuShares?: number;
+}
+
+/**
+ * Returns the standard security-hardening fields for an unprivileged proxy
+ * sidecar (api-proxy, cli-proxy, doh-proxy).
+ *
+ * `cap_drop: ['ALL']` and `security_opt: ['no-new-privileges:true']` are
+ * fixed; resource limits are caller-supplied because they differ per service.
+ *
+ * @example
+ * ```ts
+ * const service = {
+ *   ...buildContainerSecurityHardening({ memLimit: '512m', pidsLimit: 100, cpuShares: 512 }),
+ *   // other service-specific fields
+ * };
+ * ```
+ */
+export function buildContainerSecurityHardening(limits: ContainerResourceLimits): Record<string, unknown> {
+  return {
+    cap_drop: ['ALL'],
+    security_opt: ['no-new-privileges:true'],
+    mem_limit: limits.memLimit,
+    memswap_limit: limits.memLimit,
+    pids_limit: limits.pidsLimit,
+    ...(limits.cpuShares !== undefined && { cpu_shares: limits.cpuShares }),
+  };
+}