CVE-2013-2566 in Communications BRM
Summary
by MITRE
The RC4 algorithm, as used in the TLS protocol and SSL protocol, has many single-byte biases, which makes it easier for remote attackers to conduct plaintext-recovery attacks via statistical analysis of ciphertext in a large number of sessions that use the same plaintext.
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Analysis
by VulDB Data Team • 05/22/2026
The RC4 algorithm vulnerability identified as CVE-2013-2566 represents a critical weakness in cryptographic implementations that has profound implications for secure communications. This vulnerability specifically targets the RC4 stream cipher when employed within the Transport Layer Security and Secure Sockets Layer protocols, where it has been widely deployed for decades despite known cryptographic weaknesses. The flaw stems from inherent biases in the algorithm's keystream generation process, particularly affecting the first few bytes of the cipher output, making it susceptible to sophisticated statistical analysis attacks that can recover plaintext data.
The technical core of this vulnerability lies in the mathematical properties of RC4's pseudo-random number generator, which exhibits significant non-uniformity in its output distribution. These biases manifest as predictable patterns in the keystream bytes, with certain values appearing more frequently than others due to the algorithm's internal state management and permutation processes. When attackers can observe multiple encrypted sessions using the same plaintext, they can leverage these statistical anomalies to perform plaintext recovery attacks, essentially breaking the confidentiality guarantees that TLS and SSL protocols are designed to provide. This weakness is particularly dangerous because it operates at the protocol level rather than at application-specific encryption implementations.
The operational impact of CVE-2013-2566 extends far beyond simple cryptographic failure, affecting millions of systems worldwide that relied on RC4 for secure communications. Organizations using vulnerable implementations face potential exposure of sensitive data including login credentials, personal information, financial transactions, and confidential business communications. The vulnerability has been classified under CWE-327, which specifically addresses the use of weak or broken cryptographic algorithms, and aligns with ATT&CK technique T1552.001 for unsecured credentials and T1046 for network service scanning that can expose vulnerable systems. The attack vectors typically involve passive network monitoring and active session interception, where adversaries collect sufficient ciphertext samples to perform statistical analysis and recover plaintext content.
Mitigation strategies for this vulnerability require immediate implementation of cryptographic protocol updates and configuration changes. Organizations must disable RC4 support in their TLS implementations and transition to stronger cipher suites such as AES-GCM or ChaCha20-Poly1305. System administrators should conduct comprehensive audits of all TLS-enabled services and update cryptographic libraries to versions that either remove RC4 support entirely or implement proper mitigations. The vulnerability demonstrates the critical importance of ongoing cryptographic hygiene and regular security assessments, as the weaknesses in RC4 were well-documented before this specific CVE was published, yet widespread deployment continued for years beyond the known risks. Security frameworks such as NIST SP 800-57 and ISO/IEC 15408 provide guidance on cryptographic algorithm selection and implementation practices that would have prevented or reduced the impact of this vulnerability.