Vulnerability · Published Sep 15, 2026 · Assigned by EEF
CVE-2026-77866SSRF protection bypass in safeurl via IPv6 addresses and unresolvable hosts
Critical 9.0
In plain English
A critical-severity server-side request forgery in safeurl from Slab. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 42% of all known vulnerabilities).
Official description
Server-Side Request Forgery (SSRF) vulnerability in Slab safeurl allows an attacker who controls a validated URL to reach internal network destinations the library is configured to block. Only IPv4 addresses are matched against the reserved ranges and the blocklist. Every other address is treated as matching nothing, so a destination that is rejected in its IPv4 form is accepted when written as an IPv6 address, IPv6 entries in the blocklist never match, and a host that resolves to no IPv4 address is accepted regardless of where it points. Deployments that rely on the allowlist instead are unaffected, because there an unmatched address is rejected. This issue affects safeurl: from 0.1.0 onward.
Affected products (as reported)
Weakness: CWE-918, CWE-636 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:L/VA:N/SC:H/SI:L/SA:N
References
- EEF CNA record for CVE-2026-77866 related, third-party-advisory
- OSV record EEF-CVE-2026-77866 related
- Introducing commit feabbd0 in slab/safeurl-elixir related