CVE-2016-8871 in Botan
Summary
by MITRE
In Botan 1.11.29 through 1.11.32, RSA decryption with certain padding options had a detectable timing channel which could given sufficient queries be used to recover plaintext, aka an "OAEP side channel" attack.
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Analysis
by VulDB Data Team • 05/20/2019
The vulnerability identified as CVE-2016-8871 represents a critical timing side channel attack affecting the Botan cryptographic library version 1.11.29 through 1.11.32. This flaw specifically targets RSA decryption operations when utilizing certain padding schemes, creating a detectable timing channel that can be exploited by adversaries to recover plaintext data. The vulnerability stems from the implementation's failure to maintain constant-time execution during RSA decryption processes, particularly when employing OAEP (Optimal Asymmetric Encryption Padding) schemes that are designed to provide semantic security. The timing variations occur during the padding validation phase of the decryption process, where different execution times are observable depending on the validity of the padding structure. This timing discrepancy provides attackers with sufficient information to perform statistical analysis and gradually reconstruct the original plaintext through repeated decryption queries. The attack vector relies on the ability to make numerous decryption requests and measure the time differences between successful and failed padding validations, ultimately enabling the recovery of sensitive data without possessing the private key.
The technical flaw manifests in the cryptographic library's handling of padding validation during RSA decryption operations, where the implementation does not employ constant-time algorithms for checking padding integrity. This deviation from best practices creates a timing channel that leaks information about the padding structure, allowing attackers to determine whether specific padding bytes match expected values. The vulnerability specifically affects the OAEP padding scheme, which is designed to be secure against chosen ciphertext attacks but becomes vulnerable due to the timing variations introduced by non-constant time operations. According to CWE-385, this represents a timing attack vulnerability where the attacker can exploit timing differences to infer information about the cryptographic operations. The flaw violates the fundamental principle of constant-time execution that is essential for preventing side channel attacks, as outlined in the NIST SP 800-57 standard for cryptographic key management and the recommendations provided in the OWASP Cryptographic Storage Cheat Sheet.
The operational impact of this vulnerability extends beyond simple information disclosure, as it fundamentally compromises the security guarantees provided by RSA encryption with OAEP padding. Attackers can leverage this vulnerability to perform practical attacks against systems that rely on Botan for secure communications, potentially compromising encrypted messages, digital signatures, and other cryptographic operations that depend on the library's RSA implementation. The attack requires a significant number of queries to be effective, making it a chosen ciphertext attack that can be executed by adversaries with sufficient access to the decryption service. Organizations using affected versions of Botan may experience data breaches where sensitive information such as user credentials, financial data, or confidential communications can be recovered through careful timing analysis. The vulnerability affects the confidentiality and integrity of encrypted data, potentially enabling man-in-the-middle attacks, credential theft, and other malicious activities that exploit the timing channel to bypass the security provided by the cryptographic padding schemes.
Mitigation strategies for CVE-2016-8871 require immediate updates to the Botan library to versions that implement constant-time padding validation during RSA decryption operations. System administrators should prioritize upgrading to Botan 1.11.33 or later, which contains the necessary fixes to eliminate the timing channel. Organizations should also implement monitoring for unusual decryption patterns that might indicate timing channel attacks, particularly in systems where the library is used for sensitive operations. The fix typically involves modifying the padding validation logic to ensure that all branches of the algorithm execute in constant time regardless of input values, preventing any timing variations that could leak information about the padding structure. Security teams should conduct thorough reviews of all cryptographic libraries in use to identify similar timing channel vulnerabilities, as this type of flaw is commonly found in implementations that do not properly account for side channel considerations during cryptographic operations. Additional mitigations include implementing rate limiting on decryption services to prevent brute force timing attacks, using hardware security modules that provide constant-time cryptographic operations, and ensuring that all cryptographic implementations follow the ATT&CK framework's guidance for preventing side channel attacks. The vulnerability serves as a reminder of the critical importance of constant-time implementation practices in cryptographic software, as specified in the NIST Cryptographic Algorithm Validation Program requirements and the ISO/IEC 14888-3 standard for cryptographic protection of data.