Executive brief
Slab safeurl is a library used to validate and restrict HTTP requests to prevent attacks on internal network resources. A flaw in its validation logic allows attackers to bypass blocklists by using IPv6 addresses or alternative hostname formats, potentially reaching internal systems that should be blocked—such as cloud metadata endpoints or internal databases.
Technical details
The vulnerability is a Server-Side Request Forgery (SSRF) bypass in Slab safeurl's URL validation logic. The library only matches blocklist entries and reserved IP ranges against IPv4 addresses; IPv6 addresses and hostnames that resolve to no IPv4 address bypass the checks entirely. An attacker who controls a validated URL can craft requests using IPv6 notation or alternative address formats that resolve to internal destinations the blocklist is meant to prevent. This affects all deployments using blocklist-based filtering (allowlist deployments are unaffected since unmatched addresses are rejected). The vulnerability affects safeurl 0.1.0 and later; patch status is unknown.
Affected products
- Slab safeurl 0.1.0 and later
Timeline
- 2026-09-15: published