CVE-2026-81315
MCP DNS-rebinding origin check in AshAi is bypassed by a spoofed X-Forwarded-Proto header
- State
- Published
- Severity
- 7.4 High
- Published
- Aug 31, 2026
- Updated
- Aug 31, 2026
- Reserved
- Aug 30, 2026
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.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 7.4 High | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N | EEF |
Weaknesses
- CWE-346 Origin Validation Error
Affected Products
| Product | Versions |
|---|---|
ash_aiash-project |
|
ash_aiash-project |
|
References
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