HAProxy through 3.4.0, fixed in commit 5985276, contains an integer overflow vulnerability in the fcgi_conn structure's drl field that allows buffer misparse as new FCGI record headers. When contentLength is 65535 and paddingLength is 1 or more, the drl field wraps to 0, causing incorrect record consumption and allowing malicious FastCGI backends to desynchronize the FCGI framing parser, potentially causing request routing errors, response smuggling, or memory safety issues.

Multiple Vulnerabilities in HaProxy

The remote AlmaLinux 9 host has a package installed that is affected by multiple vulnerabilities as referenced in the ALSA-2026:55772 advisory.

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)
      * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

Tenable has extracted the preceding description block directly from the AlmaLinux 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 AlmaLinux 10 host has a package installed that is affected by multiple vulnerabilities as referenced in the ALSA-2026:55679 advisory.

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)
      * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

Tenable has extracted the preceding description block directly from the AlmaLinux 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 RockyLinux 10 host has packages installed that are affected by multiple vulnerabilities as referenced in the RLSA-2026:55679 advisory.

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

Tenable has extracted the preceding description block directly from the RockyLinux 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 Redhat Enterprise Linux 10 host has a package installed that is affected by multiple vulnerabilities as referenced in the RHSA-2026:55679 advisory.

    The haproxy packages provide a reliable, high-performance network load balancer for TCP and HTTP-based     applications.

    Security Fix(es):

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

    For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and     other related information, refer to the CVE page(s) 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 these issues 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 multiple vulnerabilities as referenced in the RHSA-2026:55772 advisory.

    The haproxy packages provide a reliable, high-performance network load balancer for TCP and HTTP-based     applications.

    Security Fix(es):

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

    For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and     other related information, refer to the CVE page(s) 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 these issues but has instead relied only on the application's self-reported version number.

The remote Oracle Linux 10 host has a package installed that is affected by multiple vulnerabilities as referenced in the ELSA-2026-55679 advisory.

    - Fix uint16_t overflow in FCGI demux record length (CVE-2026-55203)       Resolves: RHEL-211068

Tenable has extracted the preceding description block directly from the Oracle Linux 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 Oracle Linux 9 host has a package installed that is affected by multiple vulnerabilities as referenced in the ELSA-2026-55772 advisory.

    - Fix NULL pointer dereference in hpack_dht_insert() (CVE-2026-55204)       Resolves: RHEL-211082

Tenable has extracted the preceding description block directly from the Oracle Linux 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 RockyLinux 9 host has packages installed that are affected by multiple vulnerabilities as referenced in the RLSA-2026:55772 advisory.

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

Tenable has extracted the preceding description block directly from the RockyLinux 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 Redhat Enterprise Linux 9 host has a package installed that is affected by multiple vulnerabilities as referenced in the RHSA-2026:55802 advisory.

    The haproxy packages provide a reliable, high-performance network load balancer for TCP and HTTP-based     applications.

    Security Fix(es):

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

    For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and     other related information, refer to the CVE page(s) 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 these issues 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 multiple vulnerabilities as referenced in the RHSA-2026:55803 advisory.

    The haproxy packages provide a reliable, high-performance network load balancer for TCP and HTTP-based     applications.

    Security Fix(es):

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

    For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and     other related information, refer to the CVE page(s) 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 these issues but has instead relied only on the application's self-reported version number.

The remote Redhat Enterprise Linux 10 host has a package installed that is affected by multiple vulnerabilities as referenced in the RHSA-2026:55800 advisory.

    The haproxy packages provide a reliable, high-performance network load balancer for TCP and HTTP-based     applications.

    Security Fix(es):

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

    For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and     other related information, refer to the CVE page(s) 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 these issues 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 multiple vulnerabilities as referenced in the RHSA-2026:55801 advisory.

    The haproxy packages provide a reliable, high-performance network load balancer for TCP and HTTP-based     applications.

    Security Fix(es):

    * haproxy: HAProxy: Denial of Service via HPACK dynamic table insertions (CVE-2026-55204)

    * haproxy: HAProxy: Response smuggling due to integer overflow in FastCGI record length handling     (CVE-2026-55203)

    For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and     other related information, refer to the CVE page(s) 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 these issues 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 multiple vulnerabilities as referenced in the openSUSE-SU-2026:21245-1 advisory.

    This update for haproxy fixes the following issues

    - Update to version 3.2.21+git0.dbe43be37
    - CVE-2026-55203: integer overflow vulnerability in the fcgi_conn structure's drl field that allows buffer     misparse as new FCGI record headers (bsc#1268557).
    - CVE-2026-55204: null pointer dereference vulnerability in hpack_dht_insert() within src/hpack-tbl.c     (bsc#1268558).

Tenable has extracted the preceding description block directly from the SUSE 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 SUSE Linux SLES15 host has a package installed that is affected by multiple vulnerabilities as referenced in the SUSE-SU-2026:2652-1 advisory.

    This update for haproxy fixes the following issues

    - CVE-2026-55203: integer overflow vulnerability in the fcgi_conn structure's drl field that allows buffer     misparse as       new FCGI record headers (bsc#1268557).
    - CVE-2026-55204: null pointer dereference vulnerability in hpack_dht_insert() within src/hpack-tbl.c     (bsc#1268558).

Tenable has extracted the preceding description block directly from the SUSE 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 SUSE Linux SLES15 host has a package installed that is affected by multiple vulnerabilities as referenced in the SUSE-SU-2026:2651-1 advisory.

    This update for haproxy fixes the following issues

    - CVE-2026-55203: integer overflow vulnerability in the fcgi_conn structure's drl field that allows buffer     misparse as       new FCGI record headers (bsc#1268557).
    - CVE-2026-55204: null pointer dereference vulnerability in hpack_dht_insert() within src/hpack-tbl.c     (bsc#1268558).

Tenable has extracted the preceding description block directly from the SUSE 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 Ubuntu 22.04 LTS / 24.04 LTS / 25.10 / 26.04 LTS host has packages installed that are affected by multiple vulnerabilities as referenced in the USN-8459-1 advisory.

    It was discovered that HAProxy incorrectly handled the FCGI demultiplexer record length field. A remote     attacker could possibly use this issue to cause incorrect request routing, response smuggling, or other     memory safety issues. (CVE-2026-55203)

    It was discovered that HAProxy failed to validate the return value of the HPACK dynamic table     defragmentation function when memory was exhausted. A remote attacker could possibly use this issue to     cause HAProxy to crash, resulting in a denial of service. (CVE-2026-55204)

Tenable has extracted the preceding description block directly from the Ubuntu 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 Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - HAProxy through 3.4.0, fixed in commit 5985276, contains an integer overflow vulnerability in the     fcgi_conn structure's drl field that allows buffer misparse as new FCGI record headers. When contentLength     is 65535 and paddingLength is 1 or more, the drl field wraps to 0, causing incorrect record consumption     and allowing malicious FastCGI backends to desynchronize the FCGI framing parser, potentially causing     request routing errors, response smuggling, or memory safety issues. (CVE-2026-55203)

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

ID	Name	Product	Family	Type	Severity
337658	AlmaLinux 9 : haproxy (ALSA-2026:55772)	Nessus	Alma Linux Local Security Checks	Local	critical
337652	AlmaLinux 10 : haproxy (ALSA-2026:55679)	Nessus	Alma Linux Local Security Checks	Local	critical
337639	RockyLinux 10 : haproxy (RLSA-2026:55679)	Nessus	Rocky Linux Local Security Checks	Local	critical
337347	RHEL 10 : haproxy (RHSA-2026:55679)	Nessus	Red Hat Local Security Checks	Local	critical
337337	RHEL 9 : haproxy (RHSA-2026:55772)	Nessus	Red Hat Local Security Checks	Local	critical
337335	Oracle Linux 10 : haproxy (ELSA-2026-55679)	Nessus	Oracle Linux Local Security Checks	Local	critical
337252	Oracle Linux 9 : haproxy (ELSA-2026-55772)	Nessus	Oracle Linux Local Security Checks	Local	critical
337097	RockyLinux 9 : haproxy (RLSA-2026:55772)	Nessus	Rocky Linux Local Security Checks	Local	critical
336001	RHEL 9 : haproxy (RHSA-2026:55802)	Nessus	Red Hat Local Security Checks	Local	critical
335998	RHEL 9 : haproxy (RHSA-2026:55803)	Nessus	Red Hat Local Security Checks	Local	critical
335989	RHEL 10 : haproxy (RHSA-2026:55800)	Nessus	Red Hat Local Security Checks	Local	critical
335986	RHEL 9 : haproxy (RHSA-2026:55801)	Nessus	Red Hat Local Security Checks	Local	critical
326036	openSUSE 16 Security Update : haproxy (openSUSE-SU-2026:21245-1)	Nessus	SuSE Local Security Checks	Local	critical
323245	SUSE SLES15 Security Update : haproxy (SUSE-SU-2026:2652-1)	Nessus	SuSE Local Security Checks	Local	critical
323243	SUSE SLES15 Security Update : haproxy (SUSE-SU-2026:2651-1)	Nessus	SuSE Local Security Checks	Local	critical
322262	Ubuntu 22.04 LTS / 24.04 LTS / 25.10 / 26.04 LTS : HAProxy vulnerabilities (USN-8459-1)	Nessus	Ubuntu Local Security Checks	Local	critical
321685	Linux Distros Unpatched Vulnerability : CVE-2026-55203	Nessus	Misc.	Local	critical

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