diff --git a/apps/sim/app/api/workflows/[id]/execute/route.async.test.ts b/apps/sim/app/api/workflows/[id]/execute/route.async.test.ts
index ebce3426622..87715f0b514 100644
--- a/apps/sim/app/api/workflows/[id]/execute/route.async.test.ts
+++ b/apps/sim/app/api/workflows/[id]/execute/route.async.test.ts
@@ -194,6 +194,26 @@ describe('workflow execute async route', () => {
     )
   })
 
+  it('rejects cross-origin session requests before authorization work', async () => {
+    const req = createMockRequest(
+      'POST',
+      { input: { hello: 'world' } },
+      {
+        'Content-Type': 'application/json',
+        'Sec-Fetch-Site': 'cross-site',
+      }
+    )
+    const params = Promise.resolve({ id: 'workflow-1' })
+
+    const response = await POST(req, { params })
+    const body = await response.json()
+
+    expect(response.status).toBe(403)
+    expect(body.error).toBe('Access denied')
+    expect(mockAuthorizeWorkflowByWorkspacePermission).not.toHaveBeenCalled()
+    expect(mockEnqueue).not.toHaveBeenCalled()
+  })
+
   it('rejects oversized request bodies before authorization work', async () => {
     const req = createMockRequest(
       'POST',
diff --git a/apps/sim/app/api/workflows/[id]/execute/route.ts b/apps/sim/app/api/workflows/[id]/execute/route.ts
index 34868195dc1..6b048ac144a 100644
--- a/apps/sim/app/api/workflows/[id]/execute/route.ts
+++ b/apps/sim/app/api/workflows/[id]/execute/route.ts
@@ -20,6 +20,7 @@ import {
   getTimeoutErrorMessage,
   isTimeoutError,
 } from '@/lib/core/execution-limits'
+import { isCrossOriginSessionRequest } from '@/lib/core/security/same-origin'
 import { generateRequestId } from '@/lib/core/utils/request'
 import { SSE_HEADERS } from '@/lib/core/utils/sse'
 import {
@@ -393,6 +394,17 @@ async function handleExecutePost(
 
   try {
     const auth = await checkHybridAuth(req, { requireWorkflowId: false })
+
+    // CSRF guard: reject session-cookie execution that is provably cross-origin
+    // (a different site driving the user's browser). Scoped to session auth —
+    // API-key / public-API / internal-JWT callers don't use cookies. This is not
+    // a defense against a non-browser client forging headers; that surface is
+    // covered by the credit and execution rate-limit gates.
+    if (auth.success && auth.authType === AuthType.SESSION && isCrossOriginSessionRequest(req)) {
+      reqLogger.warn('Rejected cross-origin session-authenticated execute request')
+      return NextResponse.json({ error: 'Access denied' }, { status: 403 })
+    }
+
     const isMcpBridgeRequest =
       auth.authType === AuthType.INTERNAL_JWT && req.headers.get(MCP_TOOL_BRIDGE_HEADER) === 'true'
     const useMcpBridgeAuthenticatedUserAsActor =
diff --git a/apps/sim/lib/core/security/same-origin.test.ts b/apps/sim/lib/core/security/same-origin.test.ts
new file mode 100644
index 00000000000..4a73e6050bf
--- /dev/null
+++ b/apps/sim/lib/core/security/same-origin.test.ts
@@ -0,0 +1,53 @@
+/**
+ * @vitest-environment node
+ */
+import type { NextRequest } from 'next/server'
+import { describe, expect, it } from 'vitest'
+import { isCrossOriginSessionRequest } from '@/lib/core/security/same-origin'
+import { getBaseUrl } from '@/lib/core/utils/urls'
+
+function makeRequest(headers: Record<string, string>): NextRequest {
+  return { headers: new Headers(headers) } as unknown as NextRequest
+}
+
+describe('isCrossOriginSessionRequest', () => {
+  it('allows a same-origin browser fetch', () => {
+    expect(isCrossOriginSessionRequest(makeRequest({ 'sec-fetch-site': 'same-origin' }))).toBe(
+      false
+    )
+  })
+
+  it('rejects same-site requests (sibling subdomains are not our origin)', () => {
+    expect(isCrossOriginSessionRequest(makeRequest({ 'sec-fetch-site': 'same-site' }))).toBe(true)
+  })
+
+  it('rejects cross-site requests', () => {
+    expect(isCrossOriginSessionRequest(makeRequest({ 'sec-fetch-site': 'cross-site' }))).toBe(true)
+  })
+
+  it('rejects navigations not initiated from our front-end', () => {
+    expect(isCrossOriginSessionRequest(makeRequest({ 'sec-fetch-site': 'none' }))).toBe(true)
+  })
+
+  it('falls back to the Origin header when Sec-Fetch-Site is absent (same-origin allowed)', () => {
+    const origin = new URL(getBaseUrl()).origin
+    expect(isCrossOriginSessionRequest(makeRequest({ origin }))).toBe(false)
+  })
+
+  it('rejects a foreign Origin when Sec-Fetch-Site is absent', () => {
+    expect(isCrossOriginSessionRequest(makeRequest({ origin: 'https://evil.example.com' }))).toBe(
+      true
+    )
+  })
+
+  it('allows requests where the origin cannot be determined (no Sec-Fetch-Site, no Origin)', () => {
+    expect(isCrossOriginSessionRequest(makeRequest({}))).toBe(false)
+  })
+
+  it('trusts the unforgeable Sec-Fetch-Site over a spoofed same-origin Origin', () => {
+    const origin = new URL(getBaseUrl()).origin
+    expect(
+      isCrossOriginSessionRequest(makeRequest({ 'sec-fetch-site': 'cross-site', origin }))
+    ).toBe(true)
+  })
+})
diff --git a/apps/sim/lib/core/security/same-origin.ts b/apps/sim/lib/core/security/same-origin.ts
new file mode 100644
index 00000000000..ab976f0ca51
--- /dev/null
+++ b/apps/sim/lib/core/security/same-origin.ts
@@ -0,0 +1,36 @@
+import type { NextRequest } from 'next/server'
+import { isSameOrigin } from '@/lib/core/utils/validation'
+
+/**
+ * Returns true when a request is provably cross-origin — a browser fetch driven
+ * from a different site than our own. Used to reject session-cookie CSRF on
+ * state-changing routes: a cross-site browser request always carries
+ * `Sec-Fetch-Site: cross-site` or a mismatched `Origin`, and neither header can
+ * be set by in-browser attacker JavaScript (both are forbidden headers).
+ *
+ * `Sec-Fetch-Site` is the primary signal; only `same-origin` is treated as our
+ * own front-end. The app is single-origin, so `same-site` (sibling subdomains),
+ * `cross-site`, and `none` are all rejected. When it is absent, fall back to an
+ * `Origin` same-origin check. When neither header is present the origin cannot
+ * be determined, so the request is allowed — a genuine cross-site browser attack
+ * cannot omit these headers.
+ *
+ * This is CSRF protection only. It does not defend against a non-browser client
+ * that forges headers directly (no header-based check can); that surface is
+ * covered by the credit and execution rate-limit gates.
+ */
+export function isCrossOriginSessionRequest(req: NextRequest): boolean {
+  const secFetchSite = req.headers.get('sec-fetch-site')
+  if (secFetchSite) {
+    return secFetchSite !== 'same-origin'
+  }
+
+  const origin = req.headers.get('origin')
+  if (!origin) return false
+
+  try {
+    return !isSameOrigin(origin)
+  } catch {
+    return false
+  }
+}