An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6.

The remote Fedora 44 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-53196fc291 advisory.

    Upgrade to 3.0.23 (see https://www.haproxy.org/download/3.0/src/CHANGELOG for full upstream changelog)



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

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

The remote Fedora 42 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-d790d66a08 advisory.

    Upgrade to 3.0.23 (see https://www.haproxy.org/download/3.0/src/CHANGELOG for full upstream changelog)



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

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

The remote Fedora 43 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-164a1e3151 advisory.

    Upgrade to 3.0.23 (see https://www.haproxy.org/download/3.0/src/CHANGELOG for full upstream changelog)



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

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

The remote Debian 13 host has packages installed that are affected by a vulnerability as referenced in the dsa-6291 advisory.

    - -------------------------------------------------------------------------     Debian Security Advisory DSA-6291-1                   security@debian.org     https://www.debian.org/security/                     Salvatore Bonaccorso     May 22, 2026                          https://www.debian.org/security/faq
    - -------------------------------------------------------------------------

    Package        : haproxy     CVE ID         : CVE-2026-33555

    Martino Spagnuolo reported that the HTTP/3 parsing code in HAProxy, a     fast and reliable load balancing reverse proxy, does not properly     validate the received body size and the announced content-length header,     which may result in HTTP request smuggling.

    For the stable distribution (trixie), this problem has been fixed in     version 3.0.11-1+deb13u3.

    We recommend that you upgrade your haproxy packages.

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

    Further information about Debian Security Advisories, how to apply     these updates to your system and frequently asked questions can be     found at: https://www.debian.org/security/

    Mailing list: debian-security-announce@lists.debian.org

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 remote Ubuntu 24.04 LTS / 25.10 / 26.04 LTS host has packages installed that are affected by a vulnerability as referenced in the USN-8208-1 advisory.

    Martino Spagnuolo discovered that HAProxy did not check received body lengths in the HTTP/3 parser. A     remote attacker could possibly use this issue to perform a request smuggling attack and obtain sensitive     information.

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 openSUSE 16 host has a package installed that is affected by a vulnerability as referenced in the openSUSE- SU-2026:20618-1 advisory.

    Security issue:

    - CVE-2026-33555: Request smuggling via HTTP/3 parser desynchronization (bsc#1262103).
    - bug in SLZ compression (bsc#1261626).

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 SUSE Linux SLES15 host has a package installed that is affected by a vulnerability as referenced in the SUSE- SU-2026:1568-1 advisory.

    This update for haproxy fixes the following issue:

    - CVE-2026-33555: Request smuggling via HTTP/3 parser desynchronization (bsc#1262103).

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

  - An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body     length matches a previously announced content-length when the stream is closed via a frame with an empty     payload. This can cause desynchronization issues with the backend server and could be used for request     smuggling. The earliest affected version is 2.6. (CVE-2026-33555)

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

ID	Name	Product	Family	Severity
317023	Fedora 44 : haproxy (2026-53196fc291)	Nessus	Fedora Local Security Checks	medium
317016	Fedora 42 : haproxy (2026-d790d66a08)	Nessus	Fedora Local Security Checks	medium
317008	Fedora 43 : haproxy (2026-164a1e3151)	Nessus	Fedora Local Security Checks	medium
316552	Debian dsa-6291 : haproxy - security update	Nessus	Debian Local Security Checks	medium
310793	Ubuntu 24.04 LTS / 25.10 / 26.04 LTS : HAProxy vulnerability (USN-8208-1)	Nessus	Ubuntu Local Security Checks	medium
310202	openSUSE 16 Security Update : haproxy (openSUSE-SU-2026:20618-1)	Nessus	SuSE Local Security Checks	medium
310068	SUSE SLES15 Security Update : haproxy (SUSE-SU-2026:1568-1)	Nessus	SuSE Local Security Checks	medium
306243	Linux Distros Unpatched Vulnerability : CVE-2026-33555	Nessus	Misc.	medium

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CVE-2026-33555

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