CVE-2024-2511 in OpenSSL
Summary
by MITRE • 04/08/2024
Issue summary: Some non-default TLS server configurations can cause unbounded memory growth when processing TLSv1.3 sessions
Impact summary: An attacker may exploit certain server configurations to trigger unbounded memory growth that would lead to a Denial of Service
This problem can occur in TLSv1.3 if the non-default SSL_OP_NO_TICKET option is being used (but not if early_data support is also configured and the default anti-replay protection is in use). In this case, under certain conditions, the session cache can get into an incorrect state and it will fail to flush properly as it fills. The session cache will continue to grow in an unbounded manner. A malicious client could deliberately create the scenario for this failure to force a Denial of Service. It may also happen by accident in normal operation.
This issue only affects TLS servers supporting TLSv1.3. It does not affect TLS clients.
The FIPS modules in 3.2, 3.1 and 3.0 are not affected by this issue. OpenSSL 1.0.2 is also not affected by this issue.
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Analysis
by VulDB Data Team • 10/19/2024
This vulnerability represents a critical memory management flaw in OpenSSL's TLSv1.3 implementation that can lead to unbounded memory growth under specific server configurations. The issue manifests when servers employ the non-default SSL_OP_NO_TICKET option while simultaneously supporting TLSv1.3 sessions. This particular combination creates a condition where the session cache mechanism fails to properly flush expired entries, resulting in continuous memory allocation without cleanup. The vulnerability operates at the protocol level within the TLS handshake and session management components, specifically affecting the server-side session cache implementation that handles TLS session tickets and state tracking. The flaw is categorized under CWE-772, which addresses Missing Release of Memory after Effective Lifetime, and represents a classic resource exhaustion attack vector that can be exploited through deliberate manipulation of connection parameters.
The operational impact of this vulnerability extends beyond simple denial of service to potentially compromise entire server availability and system stability. When the session cache becomes unable to flush properly, it continues to accumulate session data indefinitely, consuming available memory resources until the system becomes unresponsive or crashes entirely. This behavior creates a predictable attack surface where malicious actors can deliberately construct TLS connections that trigger the problematic code path, forcing servers into a memory exhaustion state that effectively renders them unavailable to legitimate users. The vulnerability affects all OpenSSL versions supporting TLSv1.3 except for the FIPS modules in versions 3.2, 3.1, and 3.0, as well as the older OpenSSL 1.0.2 release, indicating that the issue was introduced in specific TLSv1.3 implementation paths rather than being a fundamental protocol flaw. The attack vector aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion.
Mitigation strategies for this vulnerability require careful configuration management and system hardening approaches. Organizations should avoid using the SSL_OP_NO_TICKET option in TLSv1.3 configurations unless absolutely necessary, as this option directly triggers the problematic code path. The recommended approach involves either enabling default anti-replay protection when early_data support is configured or implementing proper session cache size limits and monitoring mechanisms. System administrators should also consider implementing connection rate limiting and memory monitoring to detect abnormal growth patterns. The vulnerability can be addressed through OpenSSL version upgrades to patched releases, with the specific fix targeting the session cache management logic in TLSv1.3 implementations. Additionally, implementing proper resource limits through containerization or system-level memory constraints can provide defensive measures against exploitation attempts. Security teams should monitor for unusual memory consumption patterns and implement automated alerting for session cache growth anomalies, as this vulnerability can also occur accidentally during normal operation due to specific traffic patterns or connection sequences that trigger the flawed cache management behavior.