# CVE-2026-117518: @fastify/reply-from path traversal via backslash dot-segment escape

Source: https://junglewise.ai/threats/cve-2026-117518-fastify-reply-from-is-a-fastify-plugin-that-forwards-requests-to  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-02. Updated: 2026-11-02  
License: CC BY 4.0 with a link back

CVE-2026-117518 · Severity: high · CVSS 8.2 · Published 2026-11-02

@fastify/reply-from is a Fastify plugin that forwards HTTP requests to an upstream server. An authentication bypass vulnerability allows unauthenticated attackers to escape the intended request path prefix by using backslash characters in the URL, potentially reaching unauthorized upstream endpoints. This could lead to unauthorized access to sensitive data or unintended API endpoints on the upstream server.

Vendors: [OpenJS Foundation](https://junglewise.ai/threats/vendors/openjs-foundation).

## Quick answers

**Is CVE-2026-117518 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-117518 being exploited in the wild.

**How severe is CVE-2026-117518?** High severity, with a CVSS score of 8.2, published 2 November 2026.

**Which products and versions are affected?** OpenJS Foundation @Fastify/Reply-From up to and including 12.8.0.

**Is there a fix for CVE-2026-117518?** Yes: fixed in 12.8.1. Until then: No workaround is available; upgrade to @fastify/reply-from 12.8.1 or later where buildURL() now normalizes backslashes and rejects path segments that resolve to dot or double-dot, along with control characters and ambiguous whitespace, before URL construction. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-117518 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-117518?** Check whether you run OpenJS Foundation @Fastify/Reply-From at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 12.8.1

No workaround is available; upgrade to @fastify/reply-from 12.8.1 or later where buildURL() now normalizes backslashes and rejects path segments that resolve to dot or double-dot, along with control characters and ambiguous whitespace, before URL construction.

## Technical details

The vulnerability exists in the buildURL() function which validates proxy paths against forward-slash dot-segments only, then resolves the target with the WHATWG URL parser. The parser normalizes backslashes and their percent-encoded form (%5c) to forward slashes on http/https/ws/wss schemes, allowing backslash dot-segments (\..\ or %5c..%5c) to bypass traversal checks. An application scoping proxying to a prefix or using route guards can be bypassed by an unauthenticated remote attacker over the network to access unintended upstream endpoints.

## Affected products

- OpenJS Foundation @fastify/reply-from up to and including 12.8.0

## Timeline

- 2026-11-02: disclosed

## References

- [https://cna.openjsf.org/security-advisories.html](https://cna.openjsf.org/security-advisories.html)
- [https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-vp2r-3vgv-32g9](https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-vp2r-3vgv-32g9)

## Related threats

- [@fastify/reply-from improper HTTPS certificate validation](https://junglewise.ai/threats/cve-2026-107318-fastify-reply-from-is-a-fastify-plugin-that-forwards-requests-to)