CVE-2022-40617: 
strongSwan vulnerability analysis and mitigation

strongSwan before version 5.9.8 contains a vulnerability (CVE-2022-40617) related to online certificate revocation checking that can lead to a denial-of-service attack. The vulnerability was discovered by Lahav Schlesinger and publicly disclosed on October 3, 2022. The issue affects the revocation plugin in strongSwan, which handles certificate validation and revocation checking (StrongSwan Blog).

The vulnerability exists in the credential manager component of strongSwan, which performs online certificate revocation checks inline while traversing the certificate chain. The issue occurs because the certificate chain might not be trusted when the revocation plugin accesses contained URIs. This allows attackers to send crafted end-entity and intermediate CA certificates with URIs pointing to servers under their control, forcing strongSwan to connect to these servers. The vulnerability has a CVSS v3.1 base score of 7.5 (High) with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (Ubuntu CVE).

The vulnerability can lead to denial-of-service attacks through two main vectors. First, if the attacker's server completes the TCP handshake but sends no data, it can block worker threads indefinitely due to the lack of an overall timeout. Multiple such connections can prevent the daemon from processing any further IKE messages or events. Second, if the server sends excessive data, it can potentially exhaust the host's memory since the fetched OCSP/CRL data is stored without upper limits. This could force the system's OOM killer to terminate the IKE daemon (StrongSwan Blog).

The vulnerability was fixed in strongSwan version 5.9.8 by implementing a new approach that creates a trusted certificate chain before starting online revocation checks. For older releases, the strongSwan project provides patches that fix the vulnerability for versions 5.1.0 and newer. Systems not using the revocation plugin are not directly vulnerable, though similar issues could exist if custom plugins implement the validate() method of the cert_validator_t interface (StrongSwan Blog).