CVE-2017-3737 in Secure Global Desktop
Summary
by MITRE
OpenSSL 1.0.2 (starting from version 1.0.2b) introduced an "error state" mechanism. The intent was that if a fatal error occurred during a handshake then OpenSSL would move into the error state and would immediately fail if you attempted to continue the handshake. This works as designed for the explicit handshake functions (SSL_do_handshake(), SSL_accept() and SSL_connect()), however due to a bug it does not work correctly if SSL_read() or SSL_write() is called directly. In that scenario, if the handshake fails then a fatal error will be returned in the initial function call. If SSL_read()/SSL_write() is subsequently called by the application for the same SSL object then it will succeed and the data is passed without being decrypted/encrypted directly from the SSL/TLS record layer. In order to exploit this issue an application bug would have to be present that resulted in a call to SSL_read()/SSL_write() being issued after having already received a fatal error. OpenSSL version 1.0.2b-1.0.2m are affected. Fixed in OpenSSL 1.0.2n. OpenSSL 1.1.0 is not affected.
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Analysis
by VulDB Data Team • 03/03/2023
The vulnerability described in CVE-2017-3737 represents a critical flaw in OpenSSL's error state handling mechanism that specifically affects version 1.0.2b through 1.0.2m. This issue stems from a design inconsistency in how OpenSSL manages error states during TLS handshakes, where the error handling behaves correctly for explicit handshake functions but fails when direct SSL_read() or SSL_write() operations are performed. The vulnerability manifests when a fatal error occurs during handshake processing, causing OpenSSL to incorrectly transition to an error state that should prevent further operations but fails to do so when these direct I/O functions are called. The error state mechanism was intentionally implemented to prevent continued operation after handshake failures, ensuring that applications could not inadvertently continue processing data through a broken TLS connection. However, the bug creates a scenario where the error state is not properly enforced during direct I/O operations, allowing data to be processed without proper encryption or decryption.
The technical flaw in CVE-2017-3737 operates at the TLS protocol layer where OpenSSL's internal state management becomes inconsistent. When SSL_read() or SSL_write() is called directly after a handshake failure, the system returns a fatal error on the initial call but then permits subsequent calls to succeed without proper cryptographic processing. This creates a dangerous condition where data flows through the SSL/TLS record layer without the expected encryption or decryption, effectively bypassing the security mechanisms that TLS is designed to provide. The vulnerability is classified under CWE-248 as an "Uncaught Exception" and aligns with ATT&CK technique T1071.101 for application layer protocol: web protocols, as it affects the secure communication layer that applications rely upon. The flaw specifically targets the SSL/TLS implementation's state machine, where the error state transition logic fails to properly gate subsequent operations when direct I/O functions are used rather than explicit handshake functions.
The operational impact of CVE-2017-3737 is severe and potentially exploitable in scenarios where applications continue processing data after encountering handshake errors. This vulnerability enables an attacker to potentially manipulate data flows through a TLS connection that should have been terminated due to handshake failures. The risk is particularly elevated in applications that do not properly handle error conditions or that continue to process data after receiving error codes from OpenSSL. The vulnerability creates a potential data exposure scenario where sensitive information could be transmitted in plaintext or with weakened encryption, depending on the specific application behavior and the timing of the error conditions. This flaw essentially undermines the fundamental security guarantees that TLS provides, allowing for potential man-in-the-middle attacks or data interception that would otherwise be prevented by proper error handling. The impact extends beyond simple data confidentiality to potentially affect data integrity and authentication properties that TLS is designed to ensure.
Mitigation strategies for CVE-2017-3737 require immediate application-level updates and careful code review to ensure proper error handling. Applications should be updated to use OpenSSL version 1.0.2n or later where the vulnerability has been patched, and developers must implement comprehensive error checking that properly terminates TLS sessions upon encountering handshake failures. The recommended approach involves ensuring that SSL_read() and SSL_write() calls are never made after a fatal error has been returned, and applications should implement proper session cleanup and state management to prevent the scenario that leads to exploitation. Organizations should also consider implementing network monitoring to detect unusual patterns of SSL/TLS communication that might indicate exploitation attempts. The fix in OpenSSL 1.0.2n addresses the core issue by ensuring that the error state mechanism works consistently regardless of whether explicit handshake functions or direct I/O operations are used. Additionally, security teams should conduct thorough code reviews focusing on error handling patterns and ensure that all TLS operations include proper state validation before proceeding with data transmission or reception. This vulnerability highlights the critical importance of maintaining proper state management in cryptographic libraries and the potential consequences of inconsistent error handling in security-critical systems.