CVE-2021-21321

Name: CVE-2021-21321
Creator: NVD / NIST
Published: 2021-03-02T04:15:12.487+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 10/10EPSS 1.82%

Last modified Jun 17, 2026

CVE-2021-21321 is a critical-severity vulnerability rated 10/10 on the CVSS scale. 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. EPSS estimates a 1.82% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

10/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

EPSS Probability

1.82%

76.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-20

Affected Software

Vendor	Product	Versions
Fastify-Reply-From Project	Fastify-Reply-From	< 4.0.2

References

https://github.com/fastify/fastify-reply-from/commit/dea227dda606900cc01870d08541b4dcc69d3889Patch, Third Party Advisory
https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-qmw8-3v4g-gwj4Third Party Advisory
https://www.npmjs.com/package/fastify-reply-fromProduct, Third Party Advisory
https://github.com/fastify/fastify-reply-from/commit/dea227dda606900cc01870d08541b4dcc69d3889Patch, Third Party Advisory
https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-qmw8-3v4g-gwj4Third Party Advisory
https://www.npmjs.com/package/fastify-reply-fromProduct, Third Party Advisory

Timeline

Published: Mar 2, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-21321?

How severe is CVE-2021-21321?

CVE-2021-21321 has a CVSS score of 10/10 (CRITICAL severity). The EPSS model estimates a 1.82% probability of exploitation in the next 30 days.

How do I fix CVE-2021-21321?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-21321?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST