CVE-2026-33805

Name: CVE-2026-33805
Creator: NVD / NIST
Published: 2026-04-15T11:16:34.99+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9/10EPSS 0.33%

Last modified Jun 17, 2026

CVE-2026-33805 is a critical-severity vulnerability rated 9/10 on the CVSS scale. @fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. EPSS estimates a 0.33% chance of exploitation in the next 30 days.

Description

@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from. Upgrade to @fastify/reply-from v12.6.2 or @fastify/http-proxy v11.4.4 or later.

Metrics

CVSS 3.1

8.6/10

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

CVSS 4.0

9/10

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

EPSS Probability

0.33%

24.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-644

Affected Software

Vendor	Product	Versions
Fastify	Fastify\/Http-Proxy	< 11.4.4
Fastify	Reply-From	< 12.6.2

References

https://cna.openjsf.org/security-advisories.htmlVendor Advisory
https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37Exploit, Mitigation, Vendor Advisory
https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37Exploit, Mitigation, Vendor Advisory

Timeline

Published: Apr 15, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-33805?

How severe is CVE-2026-33805?

CVE-2026-33805 has a CVSS score of 9/10 (CRITICAL severity). The EPSS model estimates a 0.33% probability of exploitation in the next 30 days.

How do I fix CVE-2026-33805?

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-2026-33805?

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

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Source: NVD / NIST