CVE-2026-50721: Libreswan, via the function RSA_authenticate_hash_signature_raw_rsa(), did not correctly verify the length of

Q: How do I fix CVE-2026-50721?

libreswan: upgrade to 5.3.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-50721 being exploited?

CVE-2026-50721 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-50721 affect?

CVE-2026-50721 affects libreswan. Severity is high (CVSS 8.1). 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

Q: How urgent is CVE-2026-50721?

Priority 41/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 8.1REMOTEUPGRADE

CVE-2026-50721 is a high-severity flaw (CVSS 8.1) affecting libreswan. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	8.1 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 68%)
CISA exploited list	No
Weakness	CWE-347, CWE-617
Published	2026-07-02
Last modified	2026-10-25

Publisher (CNA): Libreswan Project · Vendor: libreswan

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-02 22:16 UTCPublished CVE record published
2026-10-25 21:10 UTCUpdated Record last modified

libreswan libreswan UPGRADE

Affected: < 5.3.1

Fixed in: 5.3.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy libreswan || rpm -q libreswan || libreswan --version

2. UPGRADE to 5.3.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libreswan

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libreswan

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-50721?

libreswan: upgrade to 5.3.1. Exact commands for each OS are on this page.

Is CVE-2026-50721 being exploited?

CVE-2026-50721 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-50721 affect?

CVE-2026-50721 affects libreswan. Severity is high (CVSS 8.1). 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

How urgent is CVE-2026-50721?

Priority 41/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-12413 HIGH 7.5 UPGRADE An invalidly formatted IKEv2 fragment causes the Libreswan pluto daemon to crash and restart
CVE-2026-50722 HIGH 8.1 UPGRADE Libreswan, via the function RSA_authenticate_hash_signature_pkcs1_1_5_rsa(), did not correctly verify the DER

References

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