What’s Vulnerable › Vulnerabilities

CVE-2021-21321: Prefix escape

• Severity: critical (CVSS 10.0 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-02 · updated 2026-06-17
• Weakness: CWE-20

Description

fastify-reply-from is an npm package which is a fastify plugin to forward the current http request to another server. In fastify-reply-from before version 4.0.2, by crafting a specific URL, it is possible to escape the prefix of the proxied backend service. If the base url of the proxied server is "/pub/", a user expect that accessing "/priv" on the target service would not be possible. In affected versions, it is possible. This is fixed in version 4.0.2.

Affected products

• Fastify-Reply-From Project Fastify-Reply-From: before 4.0.2 — fixed in 4.0.2

How to fix

Upgrade to a fixed version: Fastify-Reply-From Project Fastify-Reply-From 4.0.2. Check the vendor advisory for mitigations.

References

• https://github.com/fastify/fastify-reply-from/commit/dea227dda606900cc01870d08541b4dcc69d3889
• https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-qmw8-3v4g-gwj4
• https://www.npmjs.com/package/fastify-reply-from
• https://github.com/fastify/fastify-reply-from/commit/dea227dda606900cc01870d08541b4dcc69d3889
• https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-qmw8-3v4g-gwj4
• https://www.npmjs.com/package/fastify-reply-from

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