Executive brief
Vercel Workflow is a platform for automating tasks and integrations. The vulnerability allows unauthenticated attackers to guess webhook tokens and inject malicious requests into workflow executions, potentially triggering unauthorized API calls, database modifications, or deployments. This could enable attackers to manipulate business-critical automation without authentication.
Technical details
The createWebhook() function in Vercel Workflow DevKit accepts a user-specified token parameter that serves as the authentication credential for the public webhook endpoint /.well-known/workflow/v1/webhook/{token}. Official documentation recommended predictable token patterns, allowing an unauthenticated network attacker to guess tokens without requiring authentication or user interaction. An attacker who successfully guesses a webhook token can invoke the webhook with arbitrary HTTP request bodies, resuming workflows with attacker-controlled payloads that may trigger downstream side effects such as API calls, database writes, or deployments. The vulnerability is classified as an improper authentication (CWE-287) issue. The fix in version 4.2.0-beta.64 removes the token option from createWebhook(), ensuring tokens are always randomly generated by the SDK instead of user-specified.
Affected products
- Vercel Workflow <= 4.1.0-beta.63
- Vercel @workflow/core <= 4.1.0-beta.63
Timeline
- 2026-03-06: disclosed: Advisory published as GHSA-9r75-g2cr-3h76
- 2026-03-05: patched: Fix released in version 4.2.0-beta.64