fast-uri vulnerable to host confusion via percent-encoded scheme normalization

Description

fast-uri is a URI parser for Node.js. During parsing it runs a legacy decoding pass over the scheme component and never re-escapes the result, and serialization writes the scheme back out verbatim, unlike the host component which is re-escaped. As a result an input whose scheme carries percent-encoded slashes parses as a scheme with no authority, so the parsed host and error are both undefined, yet resolving or normalizing that same input emits a network-path reference whose authority is attacker-chosen and re-parses to that host. An application that allowlists on the parsed host, or treats a reference with no authority as safe to resolve against its base, gets the opposite of what it checked, giving an off-site redirect, server-side request forgery, or address-policy bypass. The legacy decoder also expands non-standard escape forms, widening the issue past upstream filters, and control characters in the scheme can reach the output as raw carriage return and line feed. The affected versions are 2.3.1 up to but not including 2.4.5, 3.0.0 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which reject a scheme that is not valid after decoding. Users should upgrade to a patched version.

Scores

EPSS (exploitation in 30 days)

0.23% · top 86% of all CVEs (6 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: yes · Technical impact: partial

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.5.

Weakness

• CWE-177: Improper Handling of URL Encoding (Hex Encoding) (opens in a new tab)All about CWE-177 and its CVEs

Affected products

• fast-uri · fast-uri

  • affected 2.3.1 – < 2.4.5
  • unaffected 2.4.5
  • affected 3.0.0 – < 3.1.6
  • unaffected 3.1.6
  • affected 4.0.0 – < 4.1.3
  • unaffected 4.1.3

Affected packages

From reviewed advisories of the GitHub Advisory Database.

• npmfast-uriGHSA-jqff-g426-hqxp (opens in a new tab)

References (2)

• https://github.com/fastify/fast-uri/security/advisories/GHSA-jqff-g426-hqxp (opens in a new tab)
• https://cna.openjsf.org/security-advisories.html (opens in a new tab)

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisory GHSA-jqff-g426-hqxp (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-76172](https://trackcve.app/badge/CVE-2026-76172.svg)](https://trackcve.app/cve/CVE-2026-76172)

Record last updated 24 Aug 2026, 14:29 UTC.