CVE-2026-33768: Astro Vercel Auth Bypass Vulnerability

CVE-2026-33768 Overview

CVE-2026-33768 is an authorization bypass vulnerability in the Astro web framework's @astrojs/vercel serverless adapter. The vulnerability exists in versions prior to 10.0.2, where the serverless entrypoint reads the x-astro-path header and x_astro_path query parameter to rewrite the internal request path without any authentication or validation. This allows attackers to completely bypass Vercel's platform-level path restrictions on deployments without Edge Middleware.

Critical Impact

Attackers can bypass firewall rules and access protected administrative endpoints by manipulating request paths, potentially leading to unauthorized access to sensitive functionality and data manipulation across all HTTP methods (GET, POST, PUT, DELETE).

Affected Products

• Astro @astrojs/vercel versions prior to 10.0.2
• Vercel deployments using Astro serverless adapter without Edge Middleware
• Applications relying on Vercel firewall rules for path-based access control

Discovery Timeline

• 2026-03-24 - CVE CVE-2026-33768 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-33768

Vulnerability Analysis

This vulnerability is classified as CWE-441 (Unintended Proxy or Intermediary), where the @astrojs/vercel adapter acts as an unintentional proxy by processing path rewrite instructions from untrusted user input. The serverless entrypoint accepts path override values from both HTTP headers (x-astro-path) and query parameters (x_astro_path), allowing external actors to dictate internal routing behavior.

The critical flaw lies in the complete absence of authentication or authorization checks when processing these path rewrite instructions. This architectural oversight means that platform-level security controls, such as Vercel Firewall rules configured to block access to sensitive paths like /admin/*, become entirely ineffective. An attacker can simply send a request to an allowed endpoint while specifying a blocked path in the rewrite parameter.

Importantly, the path override preserves the original HTTP method and request body, expanding the attack surface beyond simple GET requests. This means POST, PUT, and DELETE operations can be redirected to protected endpoints, enabling attackers to perform state-changing operations on restricted resources.

Root Cause

The root cause is the unconditional trust of user-supplied path rewrite parameters in the serverless entrypoint. The @astrojs/vercel adapter was designed to support internal path rewriting functionality but failed to implement any access control mechanism to validate whether the requesting entity is authorized to trigger such rewrites. The entrypoint processes the x-astro-path header and x_astro_path query parameter directly without verifying the request origin, authentication status, or whether the target path should be accessible to the requester.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker exploits this vulnerability by crafting HTTP requests that include the malicious path override in either the header or query parameter. For example, a firewall rule blocking /admin/* can be bypassed by sending a request to a permitted endpoint such as /api/health while including x_astro_path=/admin/delete-user as a query parameter. The server processes the request as if it were directed to the protected path, completely circumventing the firewall rules.

The attack flow involves: (1) identifying a Vercel-deployed Astro application without Edge Middleware, (2) discovering protected paths that should be blocked by firewall rules, (3) crafting requests to allowed endpoints with the path override parameter pointing to restricted paths, and (4) executing arbitrary operations on the protected endpoints.

For detailed technical information about the vulnerability mechanism, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-33768

Indicators of Compromise

• HTTP requests containing x-astro-path headers with unexpected administrative or sensitive paths
• Query strings containing x_astro_path parameter with path values different from the actual request URI
• Unusual access patterns to protected endpoints appearing to originate from allowed paths in logs
• Firewall bypass indicators where blocked path access occurs without corresponding block events

Detection Strategies

• Implement web application firewall rules to detect and alert on x-astro-path headers and x_astro_path query parameters in incoming requests
• Configure application logging to capture and flag discrepancies between the original request path and any internal path rewrites
• Deploy anomaly detection for administrative endpoint access patterns that may indicate exploitation attempts
• Monitor for requests where the logged access path differs from the originally requested URL

Monitoring Recommendations

• Enable verbose logging on Vercel deployments to capture full request headers and query parameters
• Set up alerts for any access to sensitive administrative paths that don't originate from expected sources
• Implement rate limiting and monitoring on endpoints commonly used as bypass vectors (health checks, public APIs)
• Review access logs for patterns indicating path manipulation attempts

How to Mitigate CVE-2026-33768

Immediate Actions Required

• Upgrade @astrojs/vercel to version 10.0.2 or later immediately
• Enable Vercel Edge Middleware to provide an additional layer of path validation before requests reach the serverless function
• Audit existing Vercel deployments to identify applications using vulnerable versions of the adapter
• Review access logs for signs of prior exploitation attempts targeting administrative endpoints

Patch Information

The Astro team has addressed this vulnerability in version 10.0.2 of the @astrojs/vercel adapter. The fix is available through the GitHub commit. Organizations should update their dependencies immediately. The GitHub Release contains the patched version. For additional context, review the GitHub Pull Request that implemented the fix.

Workarounds

• Deploy Edge Middleware on affected Vercel applications to implement path validation before requests reach the serverless adapter
• Implement application-level authentication and authorization checks that don't rely solely on path-based firewall rules
• Configure Vercel Firewall rules to strip or block requests containing x-astro-path headers and x_astro_path query parameters
• Add request validation middleware in the application layer to reject requests with unexpected path manipulation parameters

# Update @astrojs/vercel to patched version
npm update @astrojs/vercel@10.0.2

# Or using yarn
yarn upgrade @astrojs/vercel@10.0.2

# Verify installed version
npm list @astrojs/vercel