CVE-2009-0770 in dkim-milter
Summary
by MITRE
dkim-milter 2.6.0 through 2.8.0 allows remote attackers to cause a denial of service (crash) by signing a message with a key that has been revoked in DNS, which triggers an assertion error.
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Analysis
by VulDB Data Team • 04/27/2025
The vulnerability identified as CVE-2009-0770 affects dkim-milter versions 2.6.0 through 2.8.0, representing a critical denial of service weakness that can be exploited remotely by attackers to crash the mail filtering service. This issue specifically manifests when the milter processes email messages that contain DKIM signatures using revoked DNS keys, creating a condition that triggers an assertion error within the software's processing logic. The flaw resides in the DKIM signature verification mechanism where the system fails to properly handle revoked key scenarios during the message filtering process, leading to an abrupt termination of the milter service.
The technical implementation of this vulnerability stems from inadequate error handling within the dkim-milter's DNS key validation routine. When processing signed messages, the software attempts to validate the DKIM signature by resolving the public key from DNS records, but fails to account for scenarios where the key has been revoked or expired. This oversight creates a path where the assertion check fails, causing the milter to terminate unexpectedly. The vulnerability directly maps to CWE-682, which encompasses incorrect arithmetic operations and logic errors in security-critical code paths, and aligns with ATT&CK technique T1499.004 for network denial of service attacks targeting mail services. The assertion error occurs during the signature validation phase, where the milter's internal state becomes inconsistent when encountering revoked key data, leading to an unhandled exception that crashes the entire service.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be leveraged by attackers to systematically target email infrastructure and cause widespread communication failures. Organizations relying on dkim-milter for email security may experience cascading failures where legitimate email traffic is blocked while malicious actors exploit the service crash to disrupt normal operations. The vulnerability affects systems where DKIM-based email authentication is implemented, particularly those using the milter framework for email filtering, making it a significant concern for enterprises and service providers maintaining email security infrastructure. Attackers can craft specific email messages containing revoked DKIM signatures to trigger the crash, potentially leading to persistent service unavailability that requires manual intervention to restore normal operations.
Mitigation strategies for CVE-2009-0770 focus primarily on upgrading to patched versions of dkim-milter where the assertion error handling has been corrected. System administrators should immediately update to versions beyond 2.8.0 that contain proper error handling for revoked DNS keys. Additional protective measures include implementing proper monitoring and alerting for milter service availability, deploying redundant mail filtering services, and configuring failover mechanisms to maintain email delivery during potential exploitation attempts. Network-level controls such as rate limiting and signature validation timeouts can help reduce the impact of exploitation attempts, while regular security assessments should verify that all email infrastructure components properly handle edge cases in cryptographic validation. The vulnerability highlights the importance of robust error handling in security-critical applications and demonstrates the necessity of comprehensive testing for edge cases involving cryptographic key management and DNS-based validation systems.