---
id: CVE-2026-77866
title: >-
  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 rese…
summary: >-
  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 rese…
severity: critical
cvss: 9
cvssVector: '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'
cwe:
  - CWE-636
  - CWE-918
vendor: Slab
product: safeurl
affected:
  - safeurl >= 0.1.0 < *
  - slab/safeurl-elixir >= feabbd0a13f83028ab24b71710526e9da9841f70 < *
published: '2026-09-15'
updated: '2026-09-21'
sourceUpdated: '2026-09-21T19:17:10.973'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77866'
references:
  - url: 'https://cna.erlef.org/cves/CVE-2026-77866.html'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/slab/safeurl-elixir/commit/feabbd0a13f83028ab24b71710526e9da9841f70
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-77866'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-09-21T18:44:46.837335Z'
cvssSource: cna
epss: 0.0048
epssPercentile: 0.38735
ingestedAt: '2026-09-15T16:40:03.389Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.