CVE-2016-6309 in OpenSSL
Summary
by MITRE
statem/statem.c in OpenSSL 1.1.0a does not consider memory-block movement after a realloc call, which allows remote attackers to cause a denial of service (use-after-free) or possibly execute arbitrary code via a crafted TLS session.
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Analysis
by VulDB Data Team • 09/21/2022
The vulnerability identified as CVE-2016-6309 represents a critical memory management flaw within OpenSSL 1.1.0a's TLS session handling mechanism. This issue specifically resides in the statem/statem.c file where the software fails to properly account for memory block reallocation during TLS session operations. The flaw manifests when the OpenSSL library reallocates memory blocks to accommodate changing session data requirements, yet does not appropriately update internal references to the relocated memory addresses. This oversight creates a scenario where subsequent memory accesses may reference invalid or already freed memory locations, leading to unpredictable behavior and potential exploitation.
The technical implementation of this vulnerability stems from improper pointer management following dynamic memory allocation operations. When OpenSSL processes TLS sessions, it dynamically allocates and reallocates memory blocks to store session state information including cryptographic parameters, handshake data, and session identifiers. During the realloc operation, the memory block may be moved to a different location in memory while the original pointers remain unchanged, creating a disconnect between the expected memory location and the actual location of the data. This memory management error falls under the CWE-416 category of use-after-free conditions, where memory that has been freed or reallocated is accessed by subsequent operations. The vulnerability is particularly dangerous in TLS contexts because it can be triggered through legitimate network communication channels, making it an attractive target for remote attackers seeking to disrupt services or gain unauthorized system access.
The operational impact of CVE-2016-6309 extends beyond simple denial of service to potentially enable arbitrary code execution, making it a severe security concern for any system relying on OpenSSL for secure communications. Remote attackers can craft specifically formatted TLS session data that triggers the memory reallocation sequence, causing the application to either crash due to memory access violations or potentially execute malicious code if the freed memory contains attacker-controlled data. The vulnerability affects systems using OpenSSL 1.1.0a and potentially earlier versions, impacting web servers, email servers, database servers, and any application that utilizes OpenSSL for TLS/SSL encryption. This flaw aligns with ATT&CK technique T1190 for exploitation of remote services and T1059 for command execution through compromised systems, making it a multi-faceted threat that can be leveraged in various attack scenarios.
Mitigation strategies for this vulnerability require immediate patching of affected OpenSSL installations to versions that properly handle memory reallocation in TLS session handling. System administrators should prioritize updating OpenSSL components across all servers and applications that utilize TLS encryption, particularly those handling sensitive data or providing public services. Additionally, implementing network segmentation and monitoring for unusual TLS session behavior can help detect exploitation attempts. The fix implemented in later OpenSSL versions addresses the root cause by ensuring proper pointer updates after memory reallocation operations and includes additional validation checks for session data integrity. Organizations should also conduct thorough security assessments to identify any custom applications or middleware that may be indirectly affected by this vulnerability through their OpenSSL dependencies, as the issue could potentially be exploited through various attack vectors including man-in-the-middle scenarios and session hijacking attempts.