{"id":"EEF-CVE-2026-81315","summary":"MCP DNS-rebinding origin check in AshAi is bypassed by a spoofed X-Forwarded-Proto header","details":"## Summary\n\nOrigin 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.\n\nIn `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.\n\nThis issue affects ash_ai: from 0.8.0 before 1.0.0.","aliases":["CVE-2026-81315","GHSA-c92r-f3rr-q49h"],"modified":"2026-09-08T02:15:03.732553667Z","published":"2026-08-31T00:57:55.910Z","database_specific":{"capec_ids":["CAPEC-62"],"cpe_ids":["cpe:2.3:a:ash-project:ash_ai:*:*:*:*:*:*:*:*"],"cwe_ids":["CWE-346"]},"references":[{"type":"ADVISORY","url":"https://github.com/ash-project/ash_ai/security/advisories/GHSA-c92r-f3rr-q49h"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-81315.html"},{"type":"FIX","url":"https://github.com/ash-project/ash_ai/commit/28af68d73134df0b8fb3aa6ab03e8fd795b07c21"},{"type":"PACKAGE","url":"https://hex.pm/packages/ash_ai"}],"affected":[{"package":{"name":"ash_ai","ecosystem":"Hex","purl":"pkg:hex/ash_ai"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.8.0"},{"fixed":"1.0.0"}]}],"versions":["0.8.0","0.8.1","0.8.2"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-81315.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/ash-project/ash_ai","events":[{"introduced":"c94f0b17fbe252f68755ba512678586164ba6139"},{"fixed":"28af68d73134df0b8fb3aa6ab03e8fd795b07c21"}]}],"versions":["v0.8.2","v0.8.1","v0.8.0"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-81315.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}],"credits":[{"name":"Peter Ullrich","type":"FINDER"},{"name":"Peter Ullrich","type":"REPORTER"},{"name":"Zach Daniel / Ash Project","type":"REMEDIATION_DEVELOPER"},{"name":"Jonatan Männchen / EEF","type":"COORDINATOR"}]}