CVE-2026-80347 mcp-fetch through 1.6.3 Server-Side Request Forgery via Unstripped IPv6 Literal Brackets
An attacker can bypass SSRF protections in mcp-fetch to read internal network resources by using IPv6 literal brackets, exposing sensitive data to the model context.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-80347?
CVE-2026-80347 is a high-severity server-side request forgery vulnerability in kazuph mcp-fetch, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
mcp-fetch checks a fetch target against its SSRF guard without removing the brackets that surround an IPv6 literal. isSafeUrl reads the hostname from the parsed URL, which for a literal such as http://[::1]/ yields the bracketed string, and then tests it with net.isIP. That call returns zero for a bracketed value, so the branch holding the private-address checks is skipped entirely. The guard falls back to resolving the hostname, the bracketed string is not a resolvable name, no addresses are returned, and the target is reported safe. The HTTP client then strips the brackets and connects. Because the address may be given in IPv4-mapped form, the same path reaches any IPv4 target the loopback and private checks were meant to exclude, including link-local metadata endpoints. isPrivateIPv6 also has no case for the ::ffff: prefix, so the mapped form would still pass even if the brackets were removed. The fetch target is supplied as a tool argument, so an attacker who can influence what the model requests can read internal responses back into the model context.
Weakness: CWE-918
Which versions are affected by CVE-2026-80347?
How do I fix CVE-2026-80347?
No fix has been published for CVE-2026-80347 yet. Limit exposure of the affected software until one is available.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-80347 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-80347 exists.
Sources
Also known as GHSA-mqq2-4r9w-4h99.