libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.

The remote host is affected by the vulnerability described in GLSA-202608-17 (libssh2: Multiple Vulnerabilities)

    Multiple vulnerabilities have been discovered in libssh2. Please review the CVE identifiers referenced     below for details.

Tenable has extracted the preceding description block directly from the Gentoo 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.

An update of the libssh2 package has been released.

The remote SUSE Linux SLES12 host has packages installed that are affected by multiple vulnerabilities as referenced in the SUSE-SU-2026:3525-1 advisory.

    - CVE-2025-15661: out-of-bounds heap read vulnerability in the `sftp_symlink()` function in `src/sftp.c`     (bsc#1268546).
    - CVE-2026-7598: integer overflow in function `userauth_password` of file `src/userauth.c` (bsc#1263890).
    - CVE-2026-55199: pre-authentication DoS via the `SSH_MSG_EXT_INFO` handler (bsc#1268530).
    - CVE-2026-58050: heap buffer overflow due to missing bounds check in attribute count of publickey-     subsystem response       (bsc#1269568).
    - CVE-2026-58051: uninitialized pointer freed when malformed responses are sent by an SSH server     (bsc#1269567).
    - CVE-2026-66032: arbitrary code execution via double-free in SFTP session (bsc#1272737).
    - CVE-2026-66033: denial of service via integer underflow in AES-GCM cipher negotiation (bsc#1272736).
    - CVE-2026-66034: information disclosure and potential arbitrary code execution via heap out-of-bounds     read       (bsc#1272735).
    - CVE-2026-66035: arbitrary code execution via heap buffer overflow during SSH negotiation (bsc#1272734).

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 packages installed that are affected by a vulnerability as referenced in the SUSE- SU-2026:3082-1 advisory.

    This update for libssh2_org fixes the following issue

    - CVE-2026-55199: pre-Authentication DoS via SSH_MSG_EXT_INFO Handler (bsc#1268530).

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 SLED15 / SLED_SAP15 / SLES15 / SLES_SAP15 host has packages installed that are affected by a vulnerability as referenced in the SUSE-SU-2026:3076-1 advisory.

    This update for libssh2_org fixes the following issue

    - CVE-2026-55199: pre-Authentication DoS via SSH_MSG_EXT_INFO Handler (bsc#1268530).

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 / SLES_SAP15 host has packages installed that are affected by a vulnerability as referenced in the SUSE-SU-2026:3074-1 advisory.

    This update for libssh2_org fixes the following issue

    Security changes:

    - CVE-2026-55199: pre-Authentication DoS via SSH_MSG_EXT_INFO Handler (bsc#1268530).


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 Fedora 43 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-eed9e67393 advisory.

    This update addresses a few security issues, one of which could plausibly result in remote code execution.


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 Ubuntu 24.04 LTS / 25.10 / 26.04 LTS host has packages installed that are affected by multiple vulnerabilities as referenced in the USN-8486-1 advisory.

    It was discovered that libssh2 incorrectly handled the sftp_symlink() function. A malicious SSH server or     machine-in-the-middle attacker could possibly use this issue to obtain sensitive information or cause a     denial of service. (CVE-2025-15661)

    It was discovered that libssh2 had a pre-authentication denial of service vulnerability in the     SSH_MSG_EXT_INFO handler. A malicious SSH server could possibly use this issue to cause a client CPU     exhaustion loop, resulting in a denial of service. (CVE-2026-55199)

    It was discovered that libssh2 incorrectly handled packet length fields. A remote attacker could possibly     use this issue to execute arbitrary code. This issue only affected Ubuntu 25.10 and Ubuntu 26.04 LTS.
    (CVE-2026-55200)

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 remote openSUSE 16 host has packages installed that are affected by multiple vulnerabilities as referenced in the openSUSE-SU-2026:21057-1 advisory.

    This update for libssh2_org fixes the following issues

    - CVE-2026-55199: pre-Authentication DoS via SSH_MSG_EXT_INFO Handler (bsc#1268530).
    - CVE-2026-55200: out-of-Bounds write via Unchecked packet_length in transport.c (bsc#1268531).

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 Fedora 44 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-ca858b3ed8 advisory.

    This update addresses a few security issues, one of which could plausibly result in remote code execution.


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 multiple vulnerabilities as referenced in the dsa-6365 advisory.

    - -------------------------------------------------------------------------     Debian Security Advisory DSA-6365-1                   security@debian.org     https://www.debian.org/security/                       Moritz Muehlenhoff     June 25, 2026                         https://www.debian.org/security/faq
    - -------------------------------------------------------------------------

    Package        : libssh2     CVE ID         : CVE-2025-15661 CVE-2026-7598 CVE-2026-55199 CVE-2026-55200

    Multiple security vulnerabilities were discovered in libssh2, a     client-side C library implementing the SSH2 protocol which could result     in memory disclosure, denial of service or potentially the execution of     arbitrary code.

    For the stable distribution (trixie), these problems have been fixed in     version 1.11.1-1+deb13u1.

    We recommend that you upgrade your libssh2 packages.

    For the detailed security status of libssh2 please refer to     its security tracker page at:
    https://security-tracker.debian.org/tracker/libssh2

    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 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.

  - libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service     vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause     a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set     nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over     60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not     apply to CPU-bound loops. (CVE-2026-55199)

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

ID	Name	Product	Family	Type	Severity
338224	GLSA-202608-17 : libssh2: Multiple Vulnerabilities	Nessus	Gentoo Local Security Checks	Local	critical
333979	Photon OS 5.0: Libssh2 PHSA-2026-5.0-92-0912	Nessus	PhotonOS Local Security Checks	Local	high
333542	SUSE SLES12 Security Update : libssh2_org (SUSE-SU-2026:3525-1)	Nessus	SuSE Local Security Checks	Local	high
332411	Photon OS 5.0: Libssh2 PHSA-2026-5.0-91-0912	Nessus	PhotonOS Local Security Checks	Local	high
327745	SUSE SLES15 Security Update : libssh2_org (SUSE-SU-2026:3082-1)	Nessus	SuSE Local Security Checks	Local	high
327571	SUSE SLED15 / SLES15 Security Update : libssh2_org (SUSE-SU-2026:3076-1)	Nessus	SuSE Local Security Checks	Local	high
327559	SUSE SLES15 Security Update : libssh2_org (SUSE-SU-2026:3074-1)	Nessus	SuSE Local Security Checks	Local	high
326401	Fedora 43 : libssh2 (2026-eed9e67393)	Nessus	Fedora Local Security Checks	Local	critical
325093	Photon OS 5.0: Libssh2 PHSA-2026-5.0-0912	Nessus	PhotonOS Local Security Checks	Local	high
325003	Ubuntu 24.04 LTS / 25.10 / 26.04 LTS : libssh2 vulnerabilities (USN-8486-1)	Nessus	Ubuntu Local Security Checks	Local	critical
324070	openSUSE 16 Security Update : libssh2_org (openSUSE-SU-2026:21057-1)	Nessus	SuSE Local Security Checks	Local	critical
323626	Fedora 44 : libssh2 (2026-ca858b3ed8)	Nessus	Fedora Local Security Checks	Local	critical
323060	Debian dsa-6365 : libssh2-1-dev - security update	Nessus	Debian Local Security Checks	Local	critical
321662	Linux Distros Unpatched Vulnerability : CVE-2026-55199	Nessus	Misc.	Local	high

Detections

Analytics

CVE-2026-55199

high

Tenable Plugins

critical

high

high

high

high

high

high

critical

high

critical

critical

critical

critical

high