Libreswan, via the function RSA_authenticate_hash_signature_raw_rsa(), did not correctly verify the length of the authentication hash when the SIG payload of an IKEv1 packet was encoded using PKCS #1 RSA Encryption as per RFC 2313. A remote attacker can use a variation on the Bleichenbacher attack to forge the SIG payload when small public exponents are being used (e.g., e=3), which could lead to impersonation. Additionally, a remote attacker, by encoding a shorter than expected hash in the SIG payload, could trigger an assertion leading to denial-of-service. The daemon aborts and restarts; continued exploitation causes sustained denial of service. Remote code execution is not possible. X.509 certificate verifications of remote IKE peers are not affected.

The version of Libreswan installed on the remote host is prior to 5.3.1. It is, therefore, affected by multiple vulnerabilities:

  - Libreswan, via the function RSA_authenticate_hash_signature_raw_rsa(), did not correctly verify the length of the     authentication hash when the SIG payload of an IKEv1 packet was encoded using PKCS #1 RSA Encryption as per RFC     2313. A remote attacker can use a variation on the Bleichenbacher attack to forge the SIG payload when small public     exponents are being used (e.g., e=3), which could lead to impersonation. Additionally, a remote attacker, by     encoding a shorter than expected hash in the SIG payload, could trigger an assertion leading to     denial-of-service. (CVE-2026-50721)

  - Libreswan, via the function RSA_authenticate_hash_signature_pkcs1_1_5_rsa(), did not correctly verify the DER     encoding of the ASN.1 digest when the IKEv2 AUTH payload was encoded using RSASSA-PKCS1-v1_5 (RFC 8017). A remote     attacker can use a variation on the Bleichenbacher attack to forge the AUTH payload when small public exponents are     used (e.g., e=3), leading to impersonation. Additionally, a remote attacker, by encoding a shorter than expected     hash in the AUTH payload, could trigger an assertion leading to denial-of-service. (CVE-2026-50722)

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

The version of CBL-Mariner Releases installed on the remote Azure Linux 3.0 host is prior to tested version. It is, therefore, affected by a vulnerability as referenced in the CVE-2026-50721 advisory.

  - Libreswan, via the function RSA_authenticate_hash_signature_raw_rsa(), did not correctly verify the length     of the authentication hash when the SIG payload of an IKEv1 packet was encoded using PKCS #1 RSA     Encryption as per RFC 2313. A remote attacker can use a variation on the Bleichenbacher attack to forge     the SIG payload when small public exponents are being used (e.g., e=3), which could lead to impersonation.
    Additionally, a remote attacker, by encoding a shorter than expected hash in the SIG payload, could     trigger an assertion leading to denial-of-service. The daemon aborts and restarts; continued exploitation     causes sustained denial of service. Remote code execution is not possible. X.509 certificate verifications     of remote IKE peers are not affected. (CVE-2026-50721)

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.

  - Libreswan, via the function RSA_authenticate_hash_signature_raw_rsa(), did not correctly verify the length     of the authentication hash when the SIG payload of an IKEv1 packet was encoded using PKCS #1 RSA     Encryption as per RFC 2313. A remote attacker can use a variation on the Bleichenbacher attack to forge     the SIG payload when small public exponents are being used (e.g., e=3), which could lead to impersonation.
    Additionally, a remote attacker, by encoding a shorter than expected hash in the SIG payload, could     trigger an assertion leading to denial-of-service. The daemon aborts and restarts; continued exploitation     causes sustained denial of service. Remote code execution is not possible. X.509 certificate verifications     of remote IKE peers are not affected. (CVE-2026-50721)

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

ID	Name	Product	Family	Severity
327415	Libreswan < 5.3.1 Multiple Vulnerabilities	Nessus	Misc.	medium
326876	Azure Linux 3.0 Security Update: CBL-Mariner Releases (CVE-2026-50721)	Nessus	Azure Linux Local Security Checks	medium
324997	Linux Distros Unpatched Vulnerability : CVE-2026-50721	Nessus	Misc.	medium

Detections

Analytics

CVE-2026-50721

medium

Tenable Plugins

medium

medium

medium