createWebhook() in Vercel Workflow DevKit accepts a user-specified token parameter that serves as the credential for the public webhook endpoint /.well-known/workflow/v1/webhook/{token}. Official documentation recommended predictable token patterns, making it possible for an unauthenticated remote attacker to guess the token and inject arbitrary payloads into the workflow execution context.

Impact

An attacker who guesses a webhook token can resume the associated workflow with an attacker-controlled HTTP request body, potentially triggering downstream side effects such as API calls, database writes, or deployments.

Fix

• Upgrade to version 4.2.0-beta.64. The fix removes the token option from createWebhook() so that webhook tokens are always randomly generated by the SDK.
• Runs created with versions prior to 4.2.0-beta.64, that are 1) still active (i.e. running), and 2) have open hooks, are still susceptible to this vulnerability. If you suspect the hook tokens are predictable or leaked - consider cancelling those runs and restarting them on the latest patch.

Workarounds

In case a version upgrade is not possible, avoid passing predictable or guessable values to the token parameter of createWebhook(). Instead, you can either

• switch from createWebhook() to createHook() instead and programmatically resume hooks using resumeHook() instead of the public webhook endpoint, or
• use createWebhook() without passing a user-provided token, which uses a non-guessable random nanoid by default.