phpseclib is a PHP secure communications library. From 0.1.1 until 1.0.30, 2.0.55, and 3.0.54, when an application validates an untrusted X.509 certificate with phpseclib, X509::validateSignature() reads a URL out of that certificate's Authority Information Access (AIA) extension and connects to it. Attacker who supplies certificate fully controls host, port, and path of that connection. URL fetching is enabled by default, and no destination is blocked. An unauthenticated attacker can therefore make a validating server open connections to internal hosts and ports it should never reach, for example loopback 127.0.0.1, cloud metadata address 169.254.169.254, and internal-only services. This is a server-side request forgery (SSRF) caused by an insecure default. This vulnerability is fixed in 1.0.30, 2.0.55, and 3.0.54.

The remote Debian 11 host has a package installed that is affected by multiple vulnerabilities as referenced in the dla-4670 advisory.

    - -----------------------------------------------------------------------     Debian LTS Advisory DLA-4670-1              debian-lts@lists.debian.org     https://www.debian.org/lts/security/                      Utkarsh Gupta     July 05, 2026                               https://wiki.debian.org/LTS
    - -----------------------------------------------------------------------

    Package        : php-phpseclib     Version        : 2.0.30-2+deb11u3     CVE ID         : CVE-2023-52892 CVE-2026-32935 CVE-2026-40194                      CVE-2026-44167 CVE-2026-55599     Debian Bug     : 1131483

    Several vulnerabilities were discovered in phpseclib, a PHP secure     communications library, which could result in hostname validation     bypass, timing side-channel attacks, denial of service, and     server-side request forgery (SSRF).

    CVE-2023-52892

        X509.php did not properly escape regular expression special         characters in a certificate's subjectAltName, allowing a crafted         certificate to bypass hostname validation in validateURL().

    CVE-2026-32935

        The block cipher unpadding routine in Crypt/Base.php used a         short-circuiting comparison, creating a timing side channel that         could aid padding-oracle-style attacks.

    CVE-2026-40194

        The SSH2 implementation compared incoming packet HMACs using a         variable-time string comparison, creating a timing side channel         on cryptographic material.

    CVE-2026-44167

        The ASN.1 decoder's 4096-byte Object Identifier limit (mitigating         CVE-2024-27355) was still large enough to allow an OID         amplification denial of service via crafted ASN.1 structures.

    CVE-2026-55599

        File_X509 could automatically fetch a URL from a certificate's         Authority Information Access extension without validating the         destination, allowing SSRF via a crafted certificate.

    For Debian 11 bullseye, these problems have been fixed in version     2.0.30-2+deb11u3.

    We recommend that you upgrade your php-phpseclib packages.

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

    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

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.

  - phpseclib is a PHP secure communications library. From 0.1.1 until 1.0.30, 2.0.55, and 3.0.54, when an     application validates an untrusted X.509 certificate with phpseclib, X509::validateSignature() reads a URL     out of that certificate's Authority Information Access (AIA) extension and connects to it. Attacker who     supplies certificate fully controls host, port, and path of that connection. URL fetching is enabled by     default, and no destination is blocked. An unauthenticated attacker can therefore make a validating server     open connections to internal hosts and ports it should never reach, for example loopback 127.0.0.1, cloud     metadata address 169.254.169.254, and internal-only services. This is a server-side request forgery (SSRF)     caused by an insecure default. This vulnerability is fixed in 1.0.30, 2.0.55, and 3.0.54. (CVE-2026-55599)

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

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

  - phpseclib is a PHP secure communications library. From 0.1.1 until 1.0.30, 2.0.55, and 3.0.54, when an
    application validates an untrusted X.509 certificate with phpseclib, X509::validateSignature() reads a URL
    out of that certificate's Authority Information Access (AIA) extension and connects to it. Attacker who
    supplies certificate fully controls host, port, and path of that connection. URL fetching is enabled by
    default, and no destination is blocked. An unauthenticated attacker can therefore make a validating server
    open connections to internal hosts and ports it should never reach, for example loopback 127.0.0.1, cloud
    metadata address 169.254.169.254, and internal-only services. This is a server-side request forgery (SSRF)
    caused by an insecure default. This vulnerability is fixed in 1.0.30, 2.0.55, and 3.0.54. (CVE-2026-55599)

ID	Name	Product	Family	Type	Severity
325133	Debian dla-4670 : php-phpseclib - security update	Nessus	Debian Local Security Checks	Local	high
322228	Linux Distros Unpatched Vulnerability : CVE-2026-55599	Nessus	Misc.	Local	medium
443288	SCA: security update for phpseclib/phpseclib (GHSA-m557-wrgg-6rp4)	Tenable Cloud Security	SCA Checks	Local	medium
443288	SCA: security update for phpseclib/phpseclib (GHSA-m557-wrgg-6rp4)	Tenable Self-Hosted Container Security	SCA Checks	Local	medium

Detections

Analytics

CVE-2026-55599

medium

Tenable Plugins

high

medium

medium

medium