CVE-2017-9023 in strongSwan
Summary
by MITRE
The ASN.1 parser in strongSwan before 5.5.3 improperly handles CHOICE types when the x509 plugin is enabled, which allows remote attackers to cause a denial of service (infinite loop) via a crafted certificate.
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Analysis
by VulDB Data Team • 12/04/2025
The vulnerability identified as CVE-2017-9023 represents a critical denial of service flaw within strongSwan's ASN.1 parser implementation. This issue specifically manifests when the x509 plugin is active, creating a condition where malformed certificate data can trigger an infinite loop in the parsing process. The vulnerability stems from inadequate handling of CHOICE types within the ASN.1 structure, which is a fundamental component of X.509 certificate processing. According to CWE-121, this represents a buffer overflow condition that occurs when the parser fails to properly validate the structure of CHOICE elements, leading to unpredictable behavior in the certificate validation process.
The technical exploitation of this vulnerability requires remote attackers to craft specially formatted certificates that contain malformed CHOICE type structures. When strongSwan processes these certificates through its x509 plugin, the ASN.1 parser enters an infinite loop due to improper state management during the parsing of ambiguous CHOICE elements. This occurs because the parser does not adequately validate the expected structure of CHOICE types, which are used in X.509 certificates to represent mutually exclusive options such as different certificate extensions or alternative encoding methods. The vulnerability aligns with ATT&CK technique T1499.001, which involves network denial of service attacks through protocol manipulation, specifically targeting the certificate handling mechanisms of VPN implementations.
The operational impact of this vulnerability extends beyond simple service disruption, as it affects the core certificate validation functionality that strongSwan relies upon for establishing secure connections. When exploited, the infinite loop causes the system to consume excessive CPU resources, effectively rendering the VPN service unavailable to legitimate users. This denial of service condition can be particularly damaging in environments where strongSwan serves as a critical infrastructure component for secure communications, potentially disrupting business operations and compromising network security posture. The vulnerability demonstrates a classic example of how improper input validation in cryptographic libraries can lead to catastrophic service availability issues, with the potential for cascading failures in larger network infrastructures that depend on strongSwan for secure tunnel establishment.
Mitigation strategies for CVE-2017-9023 focus primarily on upgrading to strongSwan version 5.5.3 or later, which includes fixed ASN.1 parsing logic specifically designed to handle CHOICE types properly. Organizations should also implement certificate validation monitoring to detect unusual parsing patterns that might indicate exploitation attempts. Network administrators should consider implementing rate limiting and certificate inspection mechanisms to prevent malformed certificates from reaching the vulnerable parser. Additionally, security teams should conduct thorough vulnerability assessments of their strongSwan deployments to ensure all instances have been properly updated. The fix addresses the underlying CWE-121 buffer overflow condition by implementing proper bounds checking and state validation during ASN.1 parsing operations, ensuring that CHOICE elements are properly validated before processing. This vulnerability highlights the importance of robust input validation in cryptographic software and demonstrates how seemingly minor parsing issues can have significant operational consequences in security-critical applications.