CVE-2025-57804

Description

h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.

Is your site exposed to CVE-2025-57804?

Run a free security scan — no signup, results in seconds.

Weakness Type (CWE)

CWE-93 CWE-93

References

Other References

https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html

Frequently Asked Questions

What is CVE-2025-57804? +

h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.

How do I check if I'm vulnerable to CVE-2025-57804? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-49756

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in wojtekmach Req allows multipart parameter smuggling via attacker-influenced part metadata. Req.Utils.encode_form_part/2 …

CVE-2026-42257

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, several Net::IMAP …

CVE-2026-45067

### Description `Symfony\Component\Mime\Address` is the value-object every Symfony Mailer address (to/cc/bcc/from/reply-to) flows through; its constructor is documented as validating the …

CVE-2026-43969

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated …

CVE-2026-15429

A privilege escalation vulnerability exists in the HTTP authentication component in Archer VX1800v v1. Improper handling of user-controlled input may …

CVE-2026-50188

Kirby is an open-source content management system. Prior to 4.9.4 and 5.4.4, Kirby sites and plugins using the Kirby Http …