CVE-2011-5094 in Network Security Services
Summary
by MITRE
** DISPUTED ** Mozilla Network Security Services (NSS) 3.x, with certain settings of the SSL_ENABLE_RENEGOTIATION option, does not properly restrict client-initiated renegotiation within the SSL and TLS protocols, which might make it easier for remote attackers to cause a denial of service (CPU consumption) by performing many renegotiations within a single connection, a different vulnerability than CVE-2011-1473. NOTE: it can also be argued that it is the responsibility of server deployments, not a security library, to prevent or limit renegotiation when it is inappropriate within a specific environment.
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Analysis
by VulDB Data Team • 03/04/2025
The vulnerability described in CVE-2011-5094 pertains to Mozilla Network Security Services version 3.x where specific configurations of the SSL_ENABLE_RENEGOTIATION option fail to properly restrict client-initiated renegotiation within SSL and TLS protocols. This represents a significant security concern as it creates an avenue for remote attackers to exploit the protocol's renegotiation mechanism. The issue manifests when certain SSL_ENABLE_RENEGOTIATION settings are enabled, allowing malicious actors to perform excessive renegotiation operations within a single connection, ultimately leading to substantial CPU consumption and potential denial of service conditions. This vulnerability operates at the protocol level and specifically targets the fundamental security mechanisms that govern secure communication channels.
The technical flaw underlying this vulnerability resides in the improper restriction of client-initiated renegotiation capabilities within the SSL/TLS implementation. When SSL_ENABLE_RENEGOTIATION is configured inappropriately, the system fails to adequately limit or control the frequency and conditions under which renegotiation can occur. This creates a scenario where an attacker can continuously initiate renegotiation requests without proper rate limiting or validation mechanisms. The flaw essentially allows unlimited renegotiation attempts within a single connection, consuming significant processing resources as the system repeatedly performs the computationally expensive operations required for renegotiation. This behavior directly violates the established security principles for protocol implementation and resource management.
The operational impact of this vulnerability extends beyond simple resource exhaustion, creating potential for widespread service disruption across affected systems. Remote attackers can exploit this weakness to consume CPU cycles continuously, effectively creating a denial of service condition that can render systems unresponsive or significantly degraded. The vulnerability is particularly dangerous because it operates at the core SSL/TLS functionality, meaning any system utilizing affected NSS versions with improper renegotiation settings becomes vulnerable. The computational overhead from repeated renegotiation operations can quickly overwhelm system resources, especially when multiple attackers target the same service simultaneously, leading to cascading failures in network infrastructure and application availability.
From a cybersecurity framework perspective, this vulnerability aligns with CWE-307, which addresses improper restriction of excessive authentication attempts, and relates to the broader category of resource exhaustion attacks. The issue also connects to ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion. The vulnerability demonstrates the critical importance of proper protocol configuration and the need for robust access control mechanisms within cryptographic libraries. Security practitioners must understand that this issue represents a configuration problem rather than a fundamental flaw in the cryptographic implementation itself, though the library's behavior in handling renegotiation settings creates an exploitable condition.
Mitigation strategies for CVE-2011-5094 should focus on proper configuration management and protocol restriction implementation. Organizations should disable SSL_ENABLE_RENEGOTIATION when it is not required for specific applications, or implement strict rate limiting and monitoring for renegotiation attempts. The recommended approach involves configuring systems to either completely disable renegotiation or to enforce strict controls on when and how often renegotiation can occur. Additionally, network monitoring solutions should be deployed to detect unusual patterns of renegotiation activity that could indicate exploitation attempts. System administrators should also consider implementing connection pooling and resource quotas to limit the impact of potential abuse, while maintaining appropriate logging and alerting mechanisms to detect and respond to anomalous behavior.
The debate surrounding this vulnerability's classification as a library issue versus a deployment responsibility reflects broader discussions in cybersecurity about where security boundaries should be drawn. While the vulnerability exists within the NSS library implementation, the argument that server deployments should handle renegotiation restrictions highlights the importance of proper system hardening and configuration management. This case study emphasizes the need for comprehensive security policies that address both library-level vulnerabilities and deployment-specific configurations. Organizations must recognize that cryptographic libraries provide the foundation for security, but proper operational security practices and configuration management are equally critical for protecting against exploitation of library-level vulnerabilities. The vulnerability serves as a reminder that security is not solely dependent on the strength of individual components but also on how those components are properly configured and managed within specific operational contexts.