CVE-2018-16152 in strongSwan
Summary
by MITRE
In verify_emsa_pkcs1_signature() in gmp_rsa_public_key.c in the gmp plugin in strongSwan 4.x and 5.x before 5.7.0, the RSA implementation based on GMP does not reject excess data in the digestAlgorithm.parameters field during PKCS#1 v1.5 signature verification. Consequently, a remote attacker can forge signatures when small public exponents are being used, which could lead to impersonation when only an RSA signature is used for IKEv2 authentication. This is a variant of CVE-2006-4790 and CVE-2014-1568.
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Analysis
by VulDB Data Team • 12/03/2025
The vulnerability described in CVE-2018-16152 resides within the cryptographic implementation of strongSwan's gmp plugin, specifically affecting versions 4.x and 5.x prior to 5.7.0. This flaw manifests in the verify_emsa_pkcs1_signature function located in gmp_rsa_public_key.c, where the RSA signature verification process fails to properly validate the digestAlgorithm.parameters field according to PKCS#1 v1.5 standards. The issue represents a critical security weakness that directly impacts the integrity verification of IKEv2 authentication mechanisms, potentially allowing malicious actors to bypass authentication entirely.
The technical flaw stems from insufficient input validation during the PKCS#1 v1.5 signature verification process. When processing RSA signatures, the implementation does not adequately reject excess data present in the digestAlgorithm.parameters field, which should contain only the necessary parameters for digest algorithm specification. This oversight creates a condition where attackers can manipulate the signature verification process by injecting additional data that gets silently ignored during validation. The vulnerability becomes particularly exploitable when small public exponents are used, as these mathematical properties create opportunities for signature forgery attacks. The flaw operates at the cryptographic protocol level, specifically targeting the signature verification component that is fundamental to IKEv2 authentication mechanisms.
The operational impact of this vulnerability extends beyond simple authentication bypass to potentially enable complete impersonation attacks within IPsec VPN environments. When only RSA signatures are used for IKEv2 authentication without additional authentication factors, an attacker can forge valid signatures that will be accepted by the vulnerable strongSwan implementations. This creates a scenario where an adversary can establish unauthorized secure connections, potentially gaining access to protected networks and resources. The vulnerability is particularly concerning in environments where strongSwan is used for enterprise VPN connections, as it could allow lateral movement and unauthorized network access. The attack vector is remote and does not require authentication to the target system, making it especially dangerous in networked environments.
This vulnerability aligns with CWE-20, "Improper Input Validation," and demonstrates characteristics consistent with the attack patterns documented in CVE-2006-4790 and CVE-2014-1568, establishing it as a variant of previously identified cryptographic weaknesses. The ATT&CK framework categorizes this issue under T1552, "Unsecured Credentials," and potentially T1071, "Application Layer Protocol," as it affects the secure communication protocols that underpin VPN infrastructure. Organizations using strongSwan versions affected by this vulnerability should immediately implement the available patches and updates to prevent exploitation. The fix typically involves implementing proper validation of the digestAlgorithm.parameters field to reject excess data during signature verification, ensuring compliance with PKCS#1 v1.5 specifications. Additionally, network administrators should consider implementing additional monitoring and detection measures to identify potential exploitation attempts and ensure that all systems are updated to versions that contain the necessary security patches.