CVE-2021-23839 : Exploit Details and Defense Strategies

OpenSSL 1.0.2 has a vulnerability where servers from version 1.0.2s to 1.0.2x incorrectly handle SSLv2 rollback protection, potentially allowing a version rollback attack during RSA signature unpadding.

Understanding CVE-2021-23839

This CVE affects OpenSSL 1.0.2 servers that support SSLv2 and have specific configurations, leading to an incorrect SSLv2 rollback protection implementation.

What is CVE-2021-23839?

OpenSSL 1.0.2 servers from version 1.0.2s to 1.0.2x may accept connections with incorrect padding, potentially exposing them to version rollback attacks.

The Impact of CVE-2021-23839

This vulnerability can be exploited by malicious clients to trick servers into accepting connections that should be rejected, compromising the integrity of SSL communications.

Technical Details of CVE-2021-23839

This vulnerability arises from an error in the RSA_padding_check_SSLv23() function, affecting the RSA_SSLV23_PADDING padding mode.

Vulnerability Description

Servers may erroneously accept connections during version rollback attacks due to the inverted logic in the padding check mechanism.

Affected Systems and Versions

OpenSSL 1.0.2 servers from version 1.0.2s to 1.0.2x are vulnerable to this issue.

Exploitation Mechanism

Attackers can exploit this vulnerability by utilizing incorrect padding during SSLv2 connections, circumventing the expected security checks.

Mitigation and Prevention

To address CVE-2021-23839, OpenSSL 1.0.2 users are advised to upgrade to version 1.0.2y, while users of OpenSSL 1.1.1 are not affected by this issue.

Immediate Steps to Take

Upgrade affected OpenSSL 1.0.2 servers to version 1.0.2y to mitigate the risk of version rollback attacks and enhance security.

Long-Term Security Practices

Ensure SSLv2 support is disabled at compile and runtime, and use up-to-date OpenSSL versions to prevent similar vulnerabilities.

Patching and Updates

OpenSSL 1.0.2 users should update to version 1.0.2y, while OpenSSL 1.1.1 users are not impacted and can continue using the current version.