diff --git a/src/backend/core/http/middleware/authProbe.test.ts b/src/backend/core/http/middleware/authProbe.test.ts
index ae586ea72..7589dd6f2 100644
--- a/src/backend/core/http/middleware/authProbe.test.ts
+++ b/src/backend/core/http/middleware/authProbe.test.ts
@@ -73,6 +73,7 @@ interface ReqInit {
     query?: Record<string, unknown>;
     handshakeQuery?: Record<string, unknown>;
     actor?: Actor;
+    protocol?: string;
 }
 
 const makeReq = (init: ReqInit = {}): Request => {
@@ -82,6 +83,7 @@ const makeReq = (init: ReqInit = {}): Request => {
         body: init.body,
         query: (init.query ?? {}) as Request['query'],
         headers: headers as unknown as Request['headers'],
+        protocol: init.protocol,
         header(name: string) {
             // Express's `req.header()` is case-insensitive; mirror that.
             return headers[name.toLowerCase()];
@@ -319,6 +321,114 @@ describe('createAuthProbe — cookie reading', () => {
         );
         expect(stub.seenTokens).toEqual([]);
     });
+
+    it('ignores session cookies on cross-origin browser requests', async () => {
+        const stub = makeStubAuth();
+        const probe = createAuthProbe({
+            authService: stub.service,
+            cookieName: 'puter_token',
+        });
+        const { req } = await runProbe(
+            probe,
+            makeReq({
+                protocol: 'https',
+                headers: {
+                    host: 'api.puter.test',
+                    origin: 'https://attacker.example',
+                },
+                cookieHeader: 'puter_token=session-abc',
+            }),
+        );
+
+        expect(stub.seenTokens).toEqual([]);
+        expect(req.actor).toBeUndefined();
+        expect(req.tokenAuthFailed).toBeUndefined();
+    });
+
+    it('still accepts bearer tokens on cross-origin browser requests', async () => {
+        const stub = makeStubAuth();
+        const probe = createAuthProbe({
+            authService: stub.service,
+            cookieName: 'puter_token',
+        });
+        await runProbe(
+            probe,
+            makeReq({
+                protocol: 'https',
+                headers: {
+                    authorization: 'Bearer header-tok',
+                    host: 'api.puter.test',
+                    origin: 'https://app.example',
+                },
+                cookieHeader: 'puter_token=session-abc',
+            }),
+        );
+
+        expect(stub.seenTokens).toEqual(['header-tok']);
+    });
+
+    it('keeps session cookies for same-origin browser requests', async () => {
+        const stub = makeStubAuth();
+        const probe = createAuthProbe({
+            authService: stub.service,
+            cookieName: 'puter_token',
+        });
+        await runProbe(
+            probe,
+            makeReq({
+                protocol: 'https',
+                headers: {
+                    host: 'api.puter.test',
+                    origin: 'https://api.puter.test',
+                },
+                cookieHeader: 'puter_token=session-abc',
+            }),
+        );
+
+        expect(stub.seenTokens).toEqual(['session-abc']);
+    });
+
+    it('normalizes default ports when comparing browser origins', async () => {
+        const stub = makeStubAuth();
+        const probe = createAuthProbe({
+            authService: stub.service,
+            cookieName: 'puter_token',
+        });
+        await runProbe(
+            probe,
+            makeReq({
+                protocol: 'https',
+                headers: {
+                    host: 'api.puter.test:443',
+                    origin: 'https://api.puter.test',
+                },
+                cookieHeader: 'puter_token=session-abc',
+            }),
+        );
+
+        expect(stub.seenTokens).toEqual(['session-abc']);
+    });
+
+    it('treats protocol mismatches as cross-origin for session cookies', async () => {
+        const stub = makeStubAuth();
+        const probe = createAuthProbe({
+            authService: stub.service,
+            cookieName: 'puter_token',
+        });
+        await runProbe(
+            probe,
+            makeReq({
+                protocol: 'https',
+                headers: {
+                    host: 'api.puter.test',
+                    origin: 'http://api.puter.test',
+                },
+                cookieHeader: 'puter_token=session-abc',
+            }),
+        );
+
+        expect(stub.seenTokens).toEqual([]);
+    });
 });
 
 // ── Behavior when AuthService responds ──────────────────────────────
diff --git a/src/backend/core/http/middleware/authProbe.ts b/src/backend/core/http/middleware/authProbe.ts
index 17693aa3b..c71b2f8c2 100644
--- a/src/backend/core/http/middleware/authProbe.ts
+++ b/src/backend/core/http/middleware/authProbe.ts
@@ -44,7 +44,7 @@ interface AuthProbeOptions {
  *   1. `req.body.auth_token`
  *   2. `Authorization: Bearer <token>` header
  *   3. `x-api-key` header — third-party SDK convention (Anthropic etc.)
- *   4. Session cookie
+ *   4. Session cookie, only when the browser request is same-origin
  *   5. `?auth_token=...` query param
  *   6. Socket handshake query (for ws upgrades that pass through HTTP first)
  */
@@ -126,10 +126,11 @@ const extractToken = (req: Request, cookieName?: string): string | null => {
         return stripBearer(xApiKey);
     }
 
-    // 4. Cookie (set by login flow for session tokens). We parse the
-    // Cookie header directly rather than depending on `cookie-parser`
-    // middleware — the probe only needs one named value.
-    if (cookieName) {
+    // 4. Cookie (set by login flow for session tokens). Do not let an
+    // arbitrary browser Origin spend an ambient session cookie against the
+    // credentialed API CORS surface; bearer/body/x-api-key tokens remain
+    // available for cross-origin SDK requests.
+    if (cookieName && !isCrossOriginBrowserRequest(req)) {
         const cookieToken = readCookie(req, cookieName);
         if (cookieToken) {
             return stripBearer(cookieToken);
@@ -157,6 +158,29 @@ const extractToken = (req: Request, cookieName?: string): string | null => {
 
 const stripBearer = (t: string): string => t.replace(/^Bearer\s+/i, '').trim();
 
+const isCrossOriginBrowserRequest = (req: Request): boolean => {
+    const origin =
+        typeof req.header === 'function' ? req.header('origin') : undefined;
+    if (!origin) return false;
+
+    const host =
+        typeof req.header === 'function' ? req.header('host') : undefined;
+    if (!host) return true;
+
+    const protocol =
+        typeof req.protocol === 'string' && req.protocol.length > 0
+            ? req.protocol
+            : undefined;
+    if (!protocol) return true;
+
+    try {
+        const requestOrigin = new URL(`${protocol}://${host.trim()}`).origin;
+        return new URL(origin).origin !== requestOrigin;
+    } catch {
+        return true;
+    }
+};
+
 /**
  * Minimal cookie reader. Avoids pulling in `cookie-parser` for the one
  * lookup the probe needs. Handles quoted values and URL-decodes the result.