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.
" } ], "value": "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." } ], "affected": [ { "vendor": "The Libreswan Project", "product": "libreswan", "packageName": "libreswan", "collectionURL": "https://github.com/libreswan/libreswan", "repo": "https://github.com/libreswan/libreswan", "versions": [ { "version": "0", "status": "affected", "lessThanOrEqual": "5.3", "versionType": "semver" }, { "version": "5.3.1", "status": "unaffected", "versionType": "semver" } ], "defaultStatus": "unaffected", "programRoutines": [ { "name": "RSA_authenticate_hash_signature_raw_rsa" } ] } ], "problemTypes": [ { "descriptions": [ { "lang": "en", "type": "CWE", "cweId": "CWE-347", "description": "CWE-347: Improper Verification of Cryptographic Signature" } ] }, { "descriptions": [ { "lang": "en", "type": "CWE", "cweId": "CWE-617", "description": "CWE-617: Reachable Assertion" } ] } ], "impacts": [ { "capecId": "CAPEC-463", "descriptions": [ { "lang": "en", "value": "Denial of Service via assertion failure in pluto daemon when processing malformed RSA PKCS#1 v1.5 SIG payloads in IKEv1" } ] }, { "capecId": "CAPEC-473", "descriptions": [ { "lang": "en", "value": "Authentication bypass via Bleichenbacher-style signature forgery when weak RSA exponents (e.g., e=3) are in use" } ] } ], "metrics": [ { "format": "CVSS", "scenarios": [ { "lang": "en", "value": "GENERAL" } ], "cvssV3_1": { "version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH" } }, { "format": "CVSS", "scenarios": [ { "lang": "en", "value": "Weak RSA exponent (e=3) in use, enabling Bleichenbacher signature forgery" } ], "cvssV3_1": { "version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H", "baseScore": 8.1, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH" } }, { "other": { "type": "vendorSeverity", "content": { "value": "MEDIUM", "description": "Vendor-assessed severity: Medium. Authentication bypass requires weak RSA exponents (e=3) which have been disallowed by most cryptographic libraries for over a decade. DoS is mitigated by automatic daemon restart." } } } ], "workarounds": [ { "lang": "en", "supportingMedia": [ { "base64": false, "type": "text/html", "value": "IKEv1 only supports RSA-SHA1 (PKCS#1 Version 1.5) with public key authentication, so there is no way to disable the vulnerable code path within IKEv1. Migrate IKEv1 connections to IKEv2 where authby=ecdsa or authby=rsa-sha2 can be configured. For static tunnel configurations (not Remote Access VPN Client groups), authentication can be changed to use PSK via authby=secret after coordination with the remote peer.
Upgrade to libreswan 5.3.1 or later. Patches for libreswan 4.15 and 5.3 are available at https://libreswan.org/security/CVE-2026-50721/
" } ], "value": "Upgrade to libreswan 5.3.1 or later. Patches for libreswan 4.15 and 5.3 are available at https://libreswan.org/security/CVE-2026-50721/" } ], "exploits": [ { "lang": "en", "value": "No known exploitation in the wild. The authentication bypass requires the target to use RSA keys with weak exponents (e=3), which have been disallowed by most cryptographic libraries for at least a decade. The denial-of-service attack is exploitable against any IKEv1 configuration using the default authby=rsasig option." } ], "configurations": [ { "lang": "en", "value": "Any server or client that accepts RSA-based IKEv1 connections via the default authby=rsasig option is vulnerable to denial of service. Authentication bypass additionally requires the use of RSA keys with weak exponents (e=3). IKEv1 only supports RSA-SHA1 (PKCS#1 Version 1.5) for public key authentication, so the vulnerable code path cannot be disabled without migrating to IKEv2 or switching to PSK." } ], "credits": [ { "lang": "en", "type": "finder", "value": "Yeonghyeon Choi" }, { "lang": "en", "type": "finder", "value": "Duyeong Kim" }, { "lang": "en", "type": "analyst", "value": "Andrew Cagney (The Libreswan Team)" } ], "source": { "discovery": "EXTERNAL", "defects": [ "CVE-2026-50721" ] }, "references": [ { "name": "Libreswan Security Advisory CVE-2026-50721", "url": "https://libreswan.org/security/CVE-2026-50721/CVE-2026-50721.txt", "tags": [ "vendor-advisory" ] }, { "name": "Libreswan CVE-2026-50721 Patches", "url": "https://libreswan.org/security/CVE-2026-50721/", "tags": [ "patch" ] }, { "name": "Related: CVE-2026-50722 (IKEv2 variant)", "url": "https://libreswan.org/security/CVE-2026-50722/CVE-2026-50722.txt", "tags": [ "related" ] }, { "name": "RFC 2313 - PKCS #1: RSA Encryption Version 1.5", "url": "https://www.rfc-editor.org/rfc/rfc2313", "tags": [ "technical-description" ] } ], "timeline": [ { "time": "2026-03-24T00:00:00.000Z", "lang": "en", "value": "Libreswan notified of the issue via security@libreswan.org" }, { "time": "2026-06-16T00:00:00.000Z", "lang": "en", "value": "Advanced notice given to supported customers and distributions" }, { "time": "2026-06-24T00:00:00.000Z", "lang": "en", "value": "Public announcement and release of libreswan 5.3.1" } ], "taxonomyMappings": [ { "taxonomyName": "ATT&CK", "taxonomyVersion": "15.1", "taxonomyRelations": [ { "taxonomyId": "T1499.004", "relationshipName": "maps to", "relationshipValue": "Application or System Exploitation (DoS)" } ] } ], "x_generator": { "engine": "cvelib 1.8.0" } }, "adp": [ { "metrics": [ { "other": { "type": "ssvc", "content": { "timestamp": "2026-07-06T18:12:17.199605Z", "id": "CVE-2026-50721", "options": [ { "Exploitation": "none" }, { "Automatable": "yes" }, { "Technical Impact": "partial" } ], "role": "CISA Coordinator", "version": "2.0.3" } } } ], "title": "CISA ADP Vulnrichment", "providerMetadata": { "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2026-07-07T17:01:58.011Z" } } ] } }