CVE-2026-77866
CVE-2026-77866 is a critical-severity vulnerability with a CVSS 4.0 base score of 9.0. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-636.
Key facts
- Severity: Critical (CVSS 4.0 base score 9.0)
- EPSS exploit prediction: 0% (39th percentile)
- Actively exploited: Not listed in CISA KEV
- Weakness: CWE-636
- Published:
- Last modified:
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.
Frequently asked questions
- What is CVE-2026-77866?
- 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.
- How severe is CVE-2026-77866?
- CVE-2026-77866 has a CVSS 4.0 base score of 9.0, rated critical severity.
- Is CVE-2026-77866 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (39th percentile), an estimate of the probability of exploitation in the next 30 days.
- How do I fix CVE-2026-77866?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
- When was CVE-2026-77866 published?
- CVE-2026-77866 was published on 2026-09-15 and last updated on 2026-09-21.
References
- https://cna.erlef.org/cves/CVE-2026-77866.html
- https://github.com/slab/safeurl-elixir/commit/feabbd0a13f83028ab24b71710526e9da9841f70
- https://osv.dev/vulnerability/EEF-CVE-2026-77866
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