CVE-2019-13050 in SKS Keyserver
Summary
by MITRE
Interaction between the sks-keyserver code through 1.2.0 of the SKS keyserver network, and GnuPG through 2.2.16, makes it risky to have a GnuPG keyserver configuration line referring to a host on the SKS keyserver network. Retrieving data from this network may cause a persistent denial of service, because of a Certificate Spamming Attack.
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Analysis
by VulDB Data Team • 10/09/2023
The vulnerability identified as CVE-2019-13050 represents a critical security flaw in the interaction between the SKS keyserver network and GnuPG software versions up to 2.2.16. This issue stems from the inherent design of the SKS keyserver network architecture and how it interacts with GnuPG's keyserver configuration mechanisms. The vulnerability manifests when GnuPG clients attempt to retrieve keys from hosts participating in the SKS keyserver network, creating a scenario where legitimate key retrieval operations can be exploited to cause persistent denial of service conditions.
The technical root cause of this vulnerability lies in the Certificate Spamming Attack pattern that affects the SKS keyserver network's certificate handling mechanisms. When GnuPG attempts to synchronize keys from SKS network hosts, the system processes certificates in a way that allows malicious actors to flood the network with spam certificates. This creates a condition where legitimate keyserver operations become overwhelmed by certificate processing overhead, leading to persistent denial of service. The vulnerability specifically affects the sks-keyserver code through version 1.2.0, which contains flawed certificate validation and processing logic that does not adequately distinguish between legitimate and malicious certificate submissions.
The operational impact of this vulnerability extends beyond simple service disruption to create a persistent threat that can affect multiple systems simultaneously. Organizations using GnuPG for key management and verification operations face the risk of their keyserver configurations becoming compromised through the SKS network. When a GnuPG client encounters a host on the SKS network that has been compromised by certificate spamming, the entire key retrieval process becomes non-functional, effectively preventing legitimate key operations. This vulnerability is particularly dangerous because it can be triggered through normal keyserver configuration practices, making it difficult for administrators to detect and prevent the attack vector.
The attack pattern aligns with several cybersecurity frameworks including CWE-400, which addresses Uncontrolled Resource Consumption, and follows the ATT&CK technique T1499.004 for Network Denial of Service. This vulnerability demonstrates how distributed network protocols can be exploited to create persistent service disruptions that are difficult to isolate and remediate. The certificate spamming attack leverages the trust relationships inherent in the SKS keyserver network, where each participating host validates certificates from other hosts without sufficient verification mechanisms.
Mitigation strategies for CVE-2019-13050 require immediate configuration changes to GnuPG client settings to avoid direct interaction with SKS keyserver network hosts. Administrators should modify their GnuPG keyserver configuration to exclude SKS network hosts and instead use more secure keyserver sources such as those operated by trusted organizations with proper certificate validation mechanisms. The recommended approach includes implementing explicit keyserver exclusions in GnuPG configuration files and updating to GnuPG versions that have addressed the certificate processing flaws. Additionally, organizations should consider implementing network-level controls to prevent automatic keyserver synchronization with the SKS network, as this represents the most effective immediate protection against exploitation of this vulnerability.
The vulnerability highlights fundamental security issues in distributed key management systems and demonstrates how seemingly benign network protocols can be exploited to create persistent denial of service conditions. This attack vector underscores the importance of certificate validation mechanisms in distributed systems and the need for proper resource limiting and validation controls in keyserver implementations. Organizations must maintain awareness of their keyserver configurations and regularly audit their GnuPG settings to prevent accidental exposure to vulnerable network components. The security implications extend to any system that relies on GnuPG for cryptographic operations and key management, making this vulnerability particularly concerning for enterprise security environments where key management is critical to overall security posture.