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.

There are packages installed that are affected by a vulnerability referenced in the following CVE:

  - 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. (CVE-2025-57804)

The remote SUSE Linux SLES15 / SLES_SAP15 host has a package installed that is affected by a vulnerability as referenced in the SUSE-SU-2025:03273-1 advisory.

    - CVE-2025-57804: Fixed HTTP Request Smuggling due to illegal characters in headers (bsc#1248737)

Tenable has extracted the preceding description block directly from the SUSE security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

It is, therefore, affected by a vulnerability as referenced in the ALAS2023-2025-1181 advisory.

    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. (CVE-2025-57804)

Tenable has extracted the preceding description block directly from the tested product security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

The remote SUSE Linux SLED15 / SLED_SAP15 / SLES15 / SLES_SAP15 / openSUSE 15 host has a package installed that is affected by a vulnerability as referenced in the SUSE-SU-2025:03199-1 advisory.

    - CVE-2025-57804: Fixed HTTP Request Smuggling due to illegal characters in headers (bsc#1248737)

Tenable has extracted the preceding description block directly from the SUSE security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

The remote Debian 11 host has a package installed that is affected by a vulnerability as referenced in the dla-4290 advisory.

    -------------------------------------------------------------------------     Debian LTS Advisory DLA-4290-1                debian-lts@lists.debian.org     https://www.debian.org/lts/security/                       Daniel Leidert     September 02, 2025                            https://wiki.debian.org/LTS
    -------------------------------------------------------------------------

    Package        : python-h2     Version        : 4.0.0-3+deb11u1     CVE ID         : CVE-2025-57804     Debian Bug     : 1112348

    A vulnerability has been discovered in python-h2, a Python HTTP/2     protocol implementation.

    CVE-2025-57804

       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.

    For Debian 11 bullseye, this problem has been fixed in version     4.0.0-3+deb11u1.

    We recommend that you upgrade your python-h2 packages.

    For the detailed security status of python-h2 please refer to     its security tracker page at:
    https://security-tracker.debian.org/tracker/python-h2

    Further information about Debian LTS security advisories, how to apply     these updates to your system and frequently asked questions can be     found at: https://wiki.debian.org/LTS     Attachment:
    signature.asc     Description: This is a digitally signed message part

Tenable has extracted the preceding description block directly from the Debian security advisory.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - 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. (CVE-2025-57804)

Note that Nessus relies on the presence of the package as reported by the vendor.

ID	Name	Product	Family	Type	Severity
457414	Echo: h2: security update to 4.3.0	Tenable Cloud Security	Echo Local Security Checks	Local	medium
457414	Echo: h2: security update to 4.3.0	Tenable Self-Hosted Container Security	Echo Local Security Checks	Local	medium
265678	SUSE SLES15 Security Update : python-h2 (SUSE-SU-2025:03273-1)	Nessus	SuSE Local Security Checks	Local	medium
264833	Amazon Linux 2023 : python3-h2 (ALAS2023-2025-1181)	Nessus	Amazon Linux Local Security Checks	Local	medium
264670	SUSE SLED15 / SLES15 / openSUSE 15 Security Update : python-h2 (SUSE-SU-2025:03199-1)	Nessus	SuSE Local Security Checks	Local	medium
260924	Debian dla-4290 : python3-h2 - security update	Nessus	Debian Local Security Checks	Local	medium
260516	Linux Distros Unpatched Vulnerability : CVE-2025-57804	Nessus	Misc.	Local	medium
435091	SCA: security update for h2 (GHSA-847f-9342-265h)	Tenable Cloud Security	SCA Checks	Local	medium
435091	SCA: security update for h2 (GHSA-847f-9342-265h)	Tenable Self-Hosted Container Security	SCA Checks	Local	medium

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CVE-2025-57804

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