fast-uri <=4.1.2: Backslash URL Parsing Breaks Host Validation (SSRF)
CVE-2026-18446 Published on July 31, 2026
fast-uri vulnerable to host confusion via backslash authority introducer
fast-uri before 4.1.2, 3.1.5, and 2.4.4 requires a literal double forward slash to recognize a URI authority, so a reference that uses a backslash based introducer in place of it (backslash backslash, forward slash backslash, or backslash forward slash) is parsed with no authority and folds into the path. Node's native WHATWG URL parser instead treats a backslash as interchangeable with a forward slash for special schemes, so the two parsers extract different hosts from the same input. Applications that use fast-uri to enforce host based policy such as allowlists, SSRF filtering, or redirect validation before passing the same URL into Node's URL or fetch consumers can be steered to an unintended host. Upgrade to fast-uri 4.1.2, 3.1.5, or 2.4.4.
Weakness Type
Interpretation Conflict
Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state. This is generally found in proxies, firewalls, anti-virus software, and other intermediary devices that monitor, allow, deny, or modify traffic based on how the client or server is expected to behave.
Affected Versions
fast-uri:- Version 4.0.0 and below 4.1.2 is affected.
- Version 4.1.2 is unaffected.
- Version 3.0.0 and below 3.1.5 is affected.
- Version 3.1.5 is unaffected.
- Before 2.4.4 is affected.
- Version 2.4.4 is unaffected.