diff --git a/browse/src/server.ts b/browse/src/server.ts
index 3941d826..c2bd07f8 100644
--- a/browse/src/server.ts
+++ b/browse/src/server.ts
@@ -1581,8 +1581,13 @@ async function start() {
       // Welcome page — served when GStack Browser launches in headed mode
       if (url.pathname === '/welcome') {
         const welcomePath = (() => {
-          // Check project-local designs first, then global
-          const slug = process.env.GSTACK_SLUG || 'unknown';
+          // Gate GSTACK_SLUG on a strict regex BEFORE interpolating it into
+          // the filesystem path. Without this, a slug like "../../etc/passwd"
+          // would resolve to ~/.gstack/projects/../../etc/passwd/... — path
+          // traversal.  Not exploitable today (attacker needs local env-var
+          // access), but the gate is one regex and buys us defense-in-depth.
+          const rawSlug = process.env.GSTACK_SLUG || 'unknown';
+          const slug = /^[a-z0-9_-]+$/.test(rawSlug) ? rawSlug : 'unknown';
           const homeDir = process.env.HOME || process.env.USERPROFILE || '/tmp';
           const projectWelcome = `${homeDir}/.gstack/projects/${slug}/designs/welcome-page-20260331/finalized.html`;
           if (fs.existsSync(projectWelcome)) return projectWelcome;
diff --git a/browse/test/dual-listener.test.ts b/browse/test/dual-listener.test.ts
index aa9ab118..c14966bb 100644
--- a/browse/test/dual-listener.test.ts
+++ b/browse/test/dual-listener.test.ts
@@ -280,3 +280,17 @@ describe('Rate limit + denial log wiring', () => {
     expect(registrySrc).not.toMatch(/CONNECT_RATE_LIMIT\s*=\s*3\s*;/);
   });
 });
+
+describe('E3: /welcome GSTACK_SLUG path traversal gate', () => {
+  test('/welcome validates GSTACK_SLUG against ^[a-z0-9_-]+$ before interpolating into path', () => {
+    const welcomeBlock = sliceBetween(
+      SERVER_SRC,
+      "url.pathname === '/welcome'",
+      'if (fs.existsSync(projectWelcome)) return projectWelcome;'
+    );
+    // Must validate the slug before using it in a path
+    expect(welcomeBlock).toMatch(/\/\^\[a-z0-9_-\]\+\$\/\.test\(rawSlug\)/);
+    // Must fall back to a safe default when the slug fails validation
+    expect(welcomeBlock).toContain("'unknown'");
+  });
+});