mcp-sanitizer: URL sanitizer allows localhost/IPv6 loopback SSRF bypass via explicit port or bracketed IPv6

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

URL sanitizer allows localhost/IPv6 loopback SSRF bypass via explicit port or bracketed IPv6

Summary `mcp-sanitizer` v1.3.3 fails to block Server-Side Request Forgery (SSRF) targets when a `localhost` URL includes an explicit port (e.g., `http://localhost:8080/`) or when an IPv6 loopback/link-local address is expressed in bracketed notation (e.g., `http://[::1]:8080/`, `http://[fe80::1]:8080/`). Because the library is explicitly designed to gate MCP tool parameters such as `web_fetch` and `web_scraper` before outbound HTTP requests are made, a bypass here directly enables an attacker-controlled URL to reach internal services, leaking their responses.

Details The vulnerability exists in `src/utils/validation-utils.js` inside the `validateURLLocation()` function that is called by `_secureSanitizeURL()` in `src/sanitizer/mcp-sanitizer.js:698`. Bug 1 — Port-guarded localhost check (line 274–276): ```js // src/utils/validation-utils.js:274-276 if ((hostname === 'localhost'…

Published by the source

28 Oct 2026, 19:51 UTC

Last modified by the source

28 Oct 2026, 20:00 UTC

Source last checked

29 Oct 2026, 06:00 UTC

1. Revision 2 · amended · 29 Oct 2026

   Amended from osv record GHSA-4mfg-r38w-w8fg

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-4mfg-r38w-w8fg

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# Patch Next advisory brief: GHSA-4mfg-r38w-w8fg

Advisory: mcp-sanitizer: URL sanitizer allows localhost/IPv6 loopback SSRF bypass via explicit port or bracketed IPv6
IDs: GHSA-4mfg-r38w-w8fg
Record: https://patchnext.com/advisories/ghsa-4mfg-r38w-w8fg (revision 2, source checked 29 Oct 2026, 06:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp-sanitizer 1.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm mcp-sanitizer: affected < 1.4.0; fixed in 1.4.0

## Suggested inspection (read-only)
- `npm ls mcp-sanitizer --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## 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 mcp-sanitizer.
- 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://github.com/advisories/GHSA-4mfg-r38w-w8fg (GitHub-reviewed advisory GHSA-4mfg-r38w-w8fg lists affected ranges: npm mcp-sanitizer >=0 <1.4.0)
- Verified: https://osv.dev/vulnerability/GHSA-4mfg-r38w-w8fg (OSV record GHSA-4mfg-r38w-w8fg lists affected ranges: npm mcp-sanitizer >=0 <1.4.0 (fixed))

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