CVE-2026-50721: Libreswan remote code execution (CVSS 8.1)

Description

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.

CVSS 3.1 breakdown

Attack vectorNetwork

Attack complexityHigh

Privileges requiredNone

User interactionNone

ScopeUnchanged

ConfidentialityHigh

IntegrityHigh

AvailabilityHigh

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected products

Libreswan< 5.3.1

Remediation

Fixed in

Libreswan≥ 5.3.1

Patch & vendor advisories

References

https://libreswan.org/security/CVE-2026-50721/Vendor Advisory
https://libreswan.org/security/CVE-2026-50721/CVE-2026-50721.txtVendor Advisory
https://libreswan.org/security/CVE-2026-50722/CVE-2026-50722.txtVendor Advisory
https://www.rfc-editor.org/rfc/rfc2313Technical Description

Related CVEs

Recent high-severity vulnerabilities sharing a weakness type.

Frequently asked

Is CVE-2026-50721 being actively exploited?

CVE-2026-50721 is not in the CISA KEV catalog. Its EPSS exploit probability is 0.39%.

How severe is CVE-2026-50721?

CVE-2026-50721 has a CVSS 3.1 base score of 8.1, rated high. EPSS estimates a 0.39% probability of exploitation.

Which products are affected by CVE-2026-50721?

Affected products include Libreswan.

How do I fix CVE-2026-50721?

Upgrade affected software to a fixed version, for example Libreswan 5.3.1 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-50721 against the real software inventory on every managed device, as each hourly inventory arrives. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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