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Medium, CVSS 6.8amended

agent-router: MCP session resume does not verify the embedded anti-hijacking subject; default Helm seed makes session IDs forgeable

GHSA-953h-535j-jg2v

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (github.com/envoyproxy/ai-gateway 1.2.0-rc1), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.8 · CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N · score source
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

Summary The MCP proxy's client-facing session ID embeds the authenticated caller's subject (JWT sub claim) specifically as an anti-hijacking discriminator — matching the Model Context Protocol specification's documented session-hijacking guidance (https://modelcontextprotocol.io/specification/2025-06-18/basic/securitybestpractices#session-hijacking). However, the session-resume path never checks that discriminator against the current caller's identity, so any client that obtains a session ID (its own or another user's) can resume that session and act with it — including on routes with per-user backend sessions. This is aggravated by the official Helm chart shipping a literal, publicly known default seed ("default-insecure-seed") for the key that encrypts these session IDs. Impact An attacker who obtains (or, under the default seed, forges) another user's Mcp-Session-Id value can resume that user's MCP session against the backend MCP server(s) bound to the route — reading…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-953h-535j-jg2v

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-953h-535j-jg2v

Agent brief

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# Patch Next advisory brief: GHSA-953h-535j-jg2v

Advisory: agent-router: MCP session resume does not verify the embedded anti-hijacking subject; default Helm seed makes session IDs forgeable
IDs: GHSA-953h-535j-jg2v
Record: https://patchnext.com/advisories/ghsa-953h-535j-jg2v (revision 3, source checked 26 Oct 2026, 15:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/envoyproxy/ai-gateway 1.2.0-rc1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/envoyproxy/ai-gateway: affected >= 0.4.0-rc1, < 1.2.0-rc1; fixed in 1.2.0-rc1

## Suggested inspection (read-only)
- Go github.com/envoyproxy/ai-gateway: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/envoyproxy/ai-gateway.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-953h-535j-jg2v (OSV record GHSA-953h-535j-jg2v lists affected ranges: Go github.com/envoyproxy/ai-gateway >=0.4.0-rc1 <1.2.0-rc1 (fixed))
- Verified: https://github.com/advisories/GHSA-953h-535j-jg2v (GitHub-reviewed advisory GHSA-953h-535j-jg2v lists affected ranges: Go github.com/envoyproxy/ai-gateway >=0.4.0-rc1 <1.2.0-rc1)

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