CVE-2009-2661 in strongSwan
Summary
by MITRE
The asn1_length function in strongSwan 2.8 before 2.8.11, 4.2 before 4.2.17, and 4.3 before 4.3.3 does not properly handle X.509 certificates with crafted Relative Distinguished Names (RDNs), which allows remote attackers to cause a denial of service (pluto IKE daemon crash) via malformed ASN.1 data. NOTE: this is due to an incomplete fix for CVE-2009-2185.
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Analysis
by VulDB Data Team • 08/13/2021
The vulnerability described in CVE-2009-2661 represents a critical denial of service flaw within strongSwan VPN implementations that affects multiple version ranges including 2.8.x before 2.8.11, 4.2.x before 4.2.17, and 4.3.x before 4.3.3. This issue specifically targets the asn1_length function which is responsible for parsing ASN.1 encoded data structures commonly used in X.509 certificates. The flaw occurs when processing certificates containing crafted Relative Distinguished Names that manipulate the ASN.1 length field in unexpected ways, causing the pluto IKE daemon to crash and terminate its operation. This represents a classic buffer over-read or improper input validation scenario where the software fails to properly handle malformed data structures, leading to system instability and service disruption.
The technical implementation of this vulnerability stems from an incomplete fix for a previously identified issue CVE-2009-2185, indicating that the developers attempted to address similar ASN.1 parsing problems but failed to account for all possible malformed input scenarios. When strongSwan processes X.509 certificates with specially crafted RDNs, the asn1_length function does not properly validate the length fields within the ASN.1 structure, allowing attackers to construct malicious certificates that cause the parsing routine to read beyond allocated memory boundaries or interpret incorrect length values. This type of vulnerability falls under CWE-129, which describes improper validation of length fields, and specifically aligns with ATT&CK technique T1499.004 for network denial of service attacks. The vulnerability is particularly dangerous because it can be exploited remotely without requiring authentication, making it a significant threat to VPN infrastructure security.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the availability of critical network infrastructure that relies on strongSwan for secure communications. When the pluto IKE daemon crashes, all active IPsec connections are terminated and new connections cannot be established until the service is manually restarted, creating a window of vulnerability that attackers can exploit for further attacks. Organizations using affected strongSwan versions face significant risk of operational disruption, especially in environments where continuous network connectivity is essential for business operations. The vulnerability affects the core functionality of the IKE daemon which is responsible for establishing and maintaining secure tunnels, making it a critical component that must remain operational for network security to function properly. This type of denial of service attack can be particularly devastating in enterprise environments where VPN access is required for remote workforce connectivity, cloud infrastructure access, and secure communication between branch offices.
Mitigation strategies for CVE-2009-2661 require immediate patching of affected strongSwan installations to versions 2.8.11, 4.2.17, or 4.3.3 respectively, which contain the proper fix for the ASN.1 parsing issue. Organizations should also implement network monitoring to detect unusual certificate processing patterns that might indicate exploitation attempts, and consider deploying intrusion detection systems that can identify malformed ASN.1 structures in network traffic. Network administrators should verify certificate validation processes and ensure that certificate authorities are properly configured to prevent the acceptance of malformed certificates. Additionally, implementing proper input validation at multiple layers of the application stack can provide defense in depth against similar vulnerabilities. The fix addresses the root cause by properly validating ASN.1 length fields and implementing proper bounds checking when processing Relative Distinguished Names in X.509 certificates, thereby preventing the crash condition that occurs when malformed data is processed by the asn1_length function.