CVE-2026-18446: fast-uri Interpretation Conflict

Vulnerability summary

CVE-2026-18446 is a high (CVSS 7.5) Interpretation Conflict vulnerability in fast-uri. EPSS estimates a 0.22% chance of exploitation in the next 30 days, higher than 11.9% of scored vulnerabilities.

Timeline

1. 31-07-2026
   PublishedVulnerability first disclosed.
2. 31-07-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-436Interpretation ConflictProduct A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.