CVE-2019-1563 in OpenSSL
Summary
by MITRE
In situations where an attacker receives automated notification of the success or failure of a decryption attempt an attacker, after sending a very large number of messages to be decrypted, can recover a CMS/PKCS7 transported encryption key or decrypt any RSA encrypted message that was encrypted with the public RSA key, using a Bleichenbacher padding oracle attack. Applications are not affected if they use a certificate together with the private RSA key to the CMS_decrypt or PKCS7_decrypt functions to select the correct recipient info to decrypt. Fixed in OpenSSL 1.1.1d (Affected 1.1.1-1.1.1c). Fixed in OpenSSL 1.1.0l (Affected 1.1.0-1.1.0k). Fixed in OpenSSL 1.0.2t (Affected 1.0.2-1.0.2s).
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Analysis
by VulDB Data Team • 11/26/2024
The vulnerability described in CVE-2019-1563 represents a critical padding oracle attack against OpenSSL implementations of CMS/PKCS7 decryption operations. This flaw specifically affects applications that utilize the OpenSSL library for decrypting messages encrypted with RSA public keys, creating a scenario where an attacker can exploit timing information or automated notification responses to gradually recover encryption keys or decrypt messages. The vulnerability stems from improper handling of PKCS#1 v1.5 padding validation during decryption processes, which exposes the system to a classic Bleichenbacher attack pattern that has been well-documented in cryptographic literature. The attack exploits the fact that applications provide feedback on whether decryption attempts succeed or fail, allowing an attacker to perform iterative queries that reveal information about the padding structure.
The technical implementation of this vulnerability lies in the OpenSSL library's handling of RSA decryption operations within CMS/PKCS7 contexts. When OpenSSL processes a decryption request, it performs padding validation that should be constant-time to prevent information leakage. However, in affected versions, the library's error handling reveals whether padding was valid or invalid, creating a timing oracle that an attacker can leverage. The attack operates by sending specially crafted encrypted messages and observing the response behavior, which provides sufficient information to reconstruct the original plaintext through mathematical analysis. This approach aligns with the established Bleichenbacher attack methodology that targets PKCS#1 v1.5 padding schemes, where the oracle provides information about the validity of the padding structure.
The operational impact of this vulnerability extends far beyond simple decryption failures, potentially compromising entire cryptographic systems that rely on RSA encryption for message confidentiality. Attackers can use this vulnerability to recover CMS/PKCS7 transported encryption keys, which would allow them to decrypt any messages encrypted with the corresponding public key. Additionally, the ability to decrypt RSA-encrypted messages using the public key represents a fundamental breach of cryptographic security principles, as it essentially allows attackers to perform decryption without possessing the private key. This vulnerability particularly affects applications that handle email encryption, digital signatures, and secure message transport protocols where CMS/PKCS7 is commonly used, making it a significant threat to enterprise security infrastructure. The vulnerability's impact is amplified when applications provide automated feedback mechanisms that indicate decryption success or failure, as these responses serve as the oracle that enables the attack.
Organizations affected by this vulnerability should immediately implement mitigations that address both the immediate security risk and prevent future exploitation attempts. The most effective mitigation involves upgrading to OpenSSL versions 1.1.1d, 1.1.0l, or 1.0.2t, which contain the necessary fixes for the padding oracle vulnerability. However, additional protective measures should include implementing constant-time padding validation in custom applications, disabling automated notification responses during decryption operations, and ensuring that certificate-based decryption methods are properly utilized to prevent the vulnerability from being exploited. Security teams should also consider implementing monitoring for unusual decryption patterns that might indicate an active attack attempt, as the vulnerability requires extensive interaction with the target system to be effective. This vulnerability aligns with CWE-209, which addresses information exposure through error messages, and represents a classic example of how timing-based side-channel attacks can compromise cryptographic systems. The ATT&CK framework categorizes this as a credential access technique through exploitation of cryptographic weaknesses, highlighting the need for comprehensive cryptographic security reviews and proper implementation of constant-time operations to prevent similar vulnerabilities from being exploited in other cryptographic libraries or custom implementations.