An Astro Vercel adapter bug allowed any visitor to include an x_astro_path header in an unauthenticated GET request to the ISR endpoint, overriding the intended path resolution. The path override could expose routes normally guarded by Vercel edge path rules or split edge middleware. This flaw does not directly provide code execution but permits unauthorized rendering of content or services, potentially leaking sensitive data or enabling further attacks. The weakness is rooted in improper input validation and missing authorization checks, corresponding to CWE-441 and CWE-862.

Astro users who deployed the Vercel integration with versions ranging from 10.0.3 through 11.0.3. The issue resides in the packages/integrations/vercel/src/serverless/entrypoint.ts file of the Astro framework. Not all Astro deployments are affected; only those that rely on the legacy ISR function without additional safeguards.

The flaw has a CVSS score of 6.5, indicating a moderate severity and a clear exploitation path by sending crafted headers to the /_isr endpoint. No EPSS data is available, but the vulnerability is not listed in CISA KEV, suggesting it is not currently widely exploited. Nevertheless, the attack does not require special credentials; an attacker can easily trigger it with a simple HTTP request. The remedy is to upgrade to version 11.0.3 or later where the check is remedied.