CVE-2026-81315 MCP DNS-rebinding origin check in AshAi is bypassed by a spoofed X-Forwarded-Proto header
Attackers can bypass DNS-rebinding protections in AshAi MCP servers by spoofing headers, enabling cross-site requests to local servers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-81315?
CVE-2026-81315 is a high-severity vulnerability in ash-project ash_ai, published on 31 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
Origin Validation Error vulnerability in ash-project ash_ai allows a malicious web page to bypass the MCP server's DNS-rebinding protection and issue cross-site requests to a user's local MCP server with that user's actor.
In AshAi.Mcp.Server, with the default allowed_origins: nil, origin_allowed?/3 accepts an origin when uri.host == conn.host and the forwarded scheme is https. Both values are attacker-controlled: conn.host comes from the Host header and the scheme is read from the raw x-forwarded-proto header with no trusted-proxy check. Under DNS rebinding the browser sends the attacker's origin and a matching host, and page JavaScript may set X-Forwarded-Proto: https, so the check passes with no TLS or proxy involved. The fix trusts only localhost origins by default; other origins require an explicit allowed_origins allowlist.
This issue affects ash_ai: from 0.8.0 before 1.0.0.
Weakness: CWE-346
Which versions are affected by CVE-2026-81315?
1 open source package ship an affected version, including ash_ai.
| Package | Affected | Fixed in |
|---|---|---|
ash_ai Hex | From 0.8.0 | 1.0.0 |
How do I fix CVE-2026-81315?
Upgrade ash_ai to 1.0.0 or later.
Is CVE-2026-81315 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-81315 exists.
Sources
Also known as EEF-CVE-2026-81315, GHSA-c92r-f3rr-q49h.