h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

The remote Redhat Enterprise Linux 9 host has a package installed that is affected by a vulnerability as referenced in the RHSA-2025:7535 advisory.

    Security Fix(es):

    * h11 accepts some malformed Chunked-Encoding bodies (CVE-2025-43859)

    For more details about the security issue(s), including the impact, a CVSS     score, acknowledgments, and other related information, refer to the CVE     page listed in the References section.

Tenable has extracted the preceding description block directly from the Red Hat Enterprise Linux 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 Redhat Enterprise Linux 9 host has a package installed that is affected by a vulnerability as referenced in the RHSA-2025:7536 advisory.

    Security Fix(es):

    * h11 accepts some malformed Chunked-Encoding bodies (CVE-2025-43859)

    For more details about the security issue(s), including the impact, a CVSS     score, acknowledgments, and other related information, refer to the CVE     page listed in the References section.

Tenable has extracted the preceding description block directly from the Red Hat Enterprise Linux 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 Fedora 40 host has a package installed that is affected by a vulnerability as referenced in the FEDORA-2025-bd59b39ab6 advisory.

    Backport upstream fix for CVE-2025-43859

Tenable has extracted the preceding description block directly from the Fedora 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 Fedora 41 host has a package installed that is affected by a vulnerability as referenced in the FEDORA-2025-2fd25cfb83 advisory.

    Backport upstream fix for CVE-2025-43859

Tenable has extracted the preceding description block directly from the Fedora 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 Ubuntu 24.04 LTS / 24.10 / 25.04 host has a package installed that is affected by a vulnerability as referenced in the USN-7503-1 advisory.

    Jeppe Bonde Weikop discovered that h11 incorrectly handled crafted HTTP requests. A remote attacker could     possibly use this issue to smuggle malicious HTTP requests, which could potentially lead to security     control bypass and information leakage.

Tenable has extracted the preceding description block directly from the Ubuntu 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-2025:1430-1 advisory.

    - CVE-2025-43859: leniency when parsing of line terminators in chunked-coding message bodies can lead to     request       smuggling. (bsc#1241872)

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 version of FreeBSD installed on the remote host is prior to tested version. It is, therefore, affected by a vulnerability as referenced in the df126e23-24fa-11f0-ab92-f02f7497ecda advisory.

    h11 reports:
    h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line     terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain     conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of     buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

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

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

ID	Name	Product	Family	Severity
235932	RHEL 9 : Red Hat OpenStack Platform 18.0 (python-h11) (RHSA-2025:7535)	Nessus	Red Hat Local Security Checks	critical
235931	RHEL 9 : Red Hat OpenStack Platform 17.1 (python-h11) (RHSA-2025:7536)	Nessus	Red Hat Local Security Checks	critical
235692	Fedora 40 : python-h11 (2025-bd59b39ab6)	Nessus	Fedora Local Security Checks	critical
235690	Fedora 41 : python-h11 (2025-2fd25cfb83)	Nessus	Fedora Local Security Checks	critical
235614	Ubuntu 24.04 LTS / 24.10 / 25.04 : h11 vulnerability (USN-7503-1)	Nessus	Ubuntu Local Security Checks	critical
235092	SUSE SLED15 / SLES15 / openSUSE 15 Security Update : python-h11 (SUSE-SU-2025:1430-1)	Nessus	SuSE Local Security Checks	critical
234953	FreeBSD : h11 accepts some malformed Chunked-Encoding bodies (df126e23-24fa-11f0-ab92-f02f7497ecda)	Nessus	FreeBSD Local Security Checks	critical

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CVE-2025-43859

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