diff --git a/src/docker-manager.test.ts b/src/docker-manager.test.ts
index ccb7dbcd..02951148 100644
--- a/src/docker-manager.test.ts
+++ b/src/docker-manager.test.ts
@@ -1,4 +1,4 @@
-import { generateDockerCompose, subnetsOverlap, writeConfigs, startContainers, stopContainers, cleanup, runAgentCommand } from './docker-manager';
+import { generateDockerCompose, subnetsOverlap, writeConfigs, startContainers, stopContainers, cleanup, runAgentCommand, validateIdNotInSystemRange, getSafeHostUid, getSafeHostGid, MIN_REGULAR_UID } from './docker-manager';
 import { WrapperConfig } from './types';
 import * as fs from 'fs';
 import * as path from 'path';
@@ -47,6 +47,127 @@ describe('docker-manager', () => {
     });
   });
 
+  describe('validateIdNotInSystemRange', () => {
+    it('should return 1000 for system UIDs (0-999)', () => {
+      expect(validateIdNotInSystemRange(0)).toBe('1000');
+      expect(validateIdNotInSystemRange(1)).toBe('1000');
+      expect(validateIdNotInSystemRange(13)).toBe('1000'); // proxy user
+      expect(validateIdNotInSystemRange(999)).toBe('1000');
+    });
+
+    it('should return the UID as-is for regular users (>= 1000)', () => {
+      expect(validateIdNotInSystemRange(1000)).toBe('1000');
+      expect(validateIdNotInSystemRange(1001)).toBe('1001');
+      expect(validateIdNotInSystemRange(65534)).toBe('65534'); // nobody user on some systems
+    });
+  });
+
+  describe('getSafeHostUid', () => {
+    const originalGetuid = process.getuid;
+    const originalSudoUid = process.env.SUDO_UID;
+
+    afterEach(() => {
+      process.getuid = originalGetuid;
+      if (originalSudoUid !== undefined) {
+        process.env.SUDO_UID = originalSudoUid;
+      } else {
+        delete process.env.SUDO_UID;
+      }
+    });
+
+    it('should return 1000 when SUDO_UID is a system UID', () => {
+      process.getuid = () => 0; // Running as root
+      process.env.SUDO_UID = '13'; // proxy user
+      expect(getSafeHostUid()).toBe('1000');
+    });
+
+    it('should return SUDO_UID when it is a regular user UID', () => {
+      process.getuid = () => 0; // Running as root
+      process.env.SUDO_UID = '1001';
+      expect(getSafeHostUid()).toBe('1001');
+    });
+
+    it('should return 1000 when SUDO_UID is 0', () => {
+      process.getuid = () => 0; // Running as root
+      process.env.SUDO_UID = '0';
+      expect(getSafeHostUid()).toBe('1000');
+    });
+
+    it('should return 1000 when running as root without SUDO_UID', () => {
+      process.getuid = () => 0;
+      delete process.env.SUDO_UID;
+      expect(getSafeHostUid()).toBe('1000');
+    });
+
+    it('should return 1000 for non-root system UID', () => {
+      process.getuid = () => 13; // proxy user
+      delete process.env.SUDO_UID;
+      expect(getSafeHostUid()).toBe('1000');
+    });
+
+    it('should return the UID when running as regular user', () => {
+      process.getuid = () => 1001;
+      delete process.env.SUDO_UID;
+      expect(getSafeHostUid()).toBe('1001');
+    });
+  });
+
+  describe('getSafeHostGid', () => {
+    const originalGetgid = process.getgid;
+    const originalSudoGid = process.env.SUDO_GID;
+
+    afterEach(() => {
+      process.getgid = originalGetgid;
+      if (originalSudoGid !== undefined) {
+        process.env.SUDO_GID = originalSudoGid;
+      } else {
+        delete process.env.SUDO_GID;
+      }
+    });
+
+    it('should return 1000 when SUDO_GID is a system GID', () => {
+      process.getgid = () => 0; // Running as root
+      process.env.SUDO_GID = '13'; // proxy group
+      expect(getSafeHostGid()).toBe('1000');
+    });
+
+    it('should return SUDO_GID when it is a regular user GID', () => {
+      process.getgid = () => 0; // Running as root
+      process.env.SUDO_GID = '1001';
+      expect(getSafeHostGid()).toBe('1001');
+    });
+
+    it('should return 1000 when SUDO_GID is 0', () => {
+      process.getgid = () => 0; // Running as root
+      process.env.SUDO_GID = '0';
+      expect(getSafeHostGid()).toBe('1000');
+    });
+
+    it('should return 1000 when running as root without SUDO_GID', () => {
+      process.getgid = () => 0;
+      delete process.env.SUDO_GID;
+      expect(getSafeHostGid()).toBe('1000');
+    });
+
+    it('should return 1000 for non-root system GID', () => {
+      process.getgid = () => 13; // proxy group
+      delete process.env.SUDO_GID;
+      expect(getSafeHostGid()).toBe('1000');
+    });
+
+    it('should return the GID when running as regular user', () => {
+      process.getgid = () => 1001;
+      delete process.env.SUDO_GID;
+      expect(getSafeHostGid()).toBe('1001');
+    });
+  });
+
+  describe('MIN_REGULAR_UID constant', () => {
+    it('should be 1000 (standard Linux regular user UID threshold)', () => {
+      expect(MIN_REGULAR_UID).toBe(1000);
+    });
+  });
+
   describe('generateDockerCompose', () => {
     const mockConfig: WrapperConfig = {
       allowedDomains: ['github.com', 'npmjs.org'],
diff --git a/src/docker-manager.ts b/src/docker-manager.ts
index 5195570b..29595d63 100644
--- a/src/docker-manager.ts
+++ b/src/docker-manager.ts
@@ -12,41 +12,70 @@ const SQUID_PORT = 3128;
 const SQUID_INTERCEPT_PORT = 3129; // Port for transparently intercepted traffic
 
 /**
- * Gets the host user's UID, with fallback to 1000 if unavailable or root (0).
+ * Minimum UID/GID value for regular users.
+ * UIDs 0-999 are reserved for system users on most Linux distributions.
+ */
+export const MIN_REGULAR_UID = 1000;
+
+/**
+ * Validates that a UID/GID value is safe for use (not in system range).
+ * Returns the value if valid, or the default (1000) if in system range.
+ * @internal Exported for testing
+ */
+export function validateIdNotInSystemRange(id: number): string {
+  // Reject system UIDs/GIDs (0-999) - use default unprivileged user instead
+  if (id < MIN_REGULAR_UID) {
+    return MIN_REGULAR_UID.toString();
+  }
+  return id.toString();
+}
+
+/**
+ * Gets the host user's UID, with fallback to 1000 if unavailable, root (0),
+ * or in the system UID range (0-999).
  * When running with sudo, uses SUDO_UID to get the actual user's UID.
+ * @internal Exported for testing
  */
-function getSafeHostUid(): string {
+export function getSafeHostUid(): string {
   const uid = process.getuid?.();
   
   // When running as root (sudo), try to get the original user's UID
   if (!uid || uid === 0) {
     const sudoUid = process.env.SUDO_UID;
-    if (sudoUid && sudoUid !== '0') {
-      return sudoUid;
+    if (sudoUid) {
+      const parsedUid = parseInt(sudoUid, 10);
+      if (!isNaN(parsedUid)) {
+        return validateIdNotInSystemRange(parsedUid);
+      }
     }
-    return '1000';
+    return MIN_REGULAR_UID.toString();
   }
   
-  return uid.toString();
+  return validateIdNotInSystemRange(uid);
 }
 
 /**
- * Gets the host user's GID, with fallback to 1000 if unavailable or root (0).
+ * Gets the host user's GID, with fallback to 1000 if unavailable, root (0),
+ * or in the system GID range (0-999).
  * When running with sudo, uses SUDO_GID to get the actual user's GID.
+ * @internal Exported for testing
  */
-function getSafeHostGid(): string {
+export function getSafeHostGid(): string {
   const gid = process.getgid?.();
   
   // When running as root (sudo), try to get the original user's GID
   if (!gid || gid === 0) {
     const sudoGid = process.env.SUDO_GID;
-    if (sudoGid && sudoGid !== '0') {
-      return sudoGid;
+    if (sudoGid) {
+      const parsedGid = parseInt(sudoGid, 10);
+      if (!isNaN(parsedGid)) {
+        return validateIdNotInSystemRange(parsedGid);
+      }
     }
-    return '1000';
+    return MIN_REGULAR_UID.toString();
   }
   
-  return gid.toString();
+  return validateIdNotInSystemRange(gid);
 }
 
 /**