vulnerability / NVI-00008944 / Revision 2
mcp-sanitizer: URL sanitizer allows localhost/IPv6 loopback SSRF bypass via explicit port or bracketed IPv6
GitHub lists this reviewed advisory for mcp-sanitizer. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
8.2 high
GitHub source severity; exploitability and local consequence require separate evidence.
As of · GitHub Advisory Database
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
mcp-sanitizer
source listed< 1.4.0
## 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' || hostname === '127.0.0.1' || hostname === '::1') && !parsedUrl.port) { throw new Error('URL points to localhost without explicit port'); } ``` The `&& !parsedUrl.port` guard means that any lo
mcp-sanitizer
source listedSEMVER: introduced 0; fixed 1.4.0
## 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' || hostname === '127.0.0.1' || hostname === '::1') && !parsedUrl.port) { throw new Error('URL points to localhost without explicit port'); } ``` The `&& !parsedUrl.port` guard means that any lo
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database
- vendor patch
The original advisory lists fixed boundaries 1.4.0 for mcp-sanitizer. Confirm the publisher’s latest guidance.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database via OSV
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-4mfg-r38w-w8fg ↗
GitHub Advisory Database · vendor advisory
Observed . GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.
- GHSA-4mfg-r38w-w8fg ↗
GitHub Advisory Database via OSV · vendor advisory
Observed . GitHub-origin advisory, CC-BY-4.0; OSV is an aggregate copy, not independent corroboration.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 2 ·
Source facts changed; compare the preserved prior revision.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.