CVE-2013-6449 in OpenSSL
Summary
by MITRE
The ssl_get_algorithm2 function in ssl/s3_lib.c in OpenSSL before 1.0.2 obtains a certain version number from an incorrect data structure, which allows remote attackers to cause a denial of service (daemon crash) via crafted traffic from a TLS 1.2 client.
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Analysis
by VulDB Data Team • 06/04/2021
The vulnerability identified as CVE-2013-6449 represents a critical denial of service flaw within the OpenSSL cryptographic library that affected versions prior to 1.0.2. This issue resides in the ssl_get_algorithm2 function located within the ssl/s3_lib.c source file, where the implementation incorrectly retrieves a version number from a data structure that does not contain the expected information. The flaw specifically manifests when processing TLS 1.2 client connections, creating a scenario where malicious actors can exploit the improper data handling to crash the targeted OpenSSL daemon.
The technical nature of this vulnerability stems from a fundamental error in how the function processes version information during the SSL/TLS handshake process. When a TLS 1.2 client establishes a connection, the ssl_get_algorithm2 function attempts to extract version information from a data structure that contains different data than anticipated. This misinterpretation causes the function to read from an incorrect memory location, leading to unpredictable behavior and ultimately resulting in a daemon crash. The improper handling of version numbers occurs during the protocol negotiation phase, where the library should correctly identify and validate the TLS version being negotiated between client and server.
From an operational perspective, this vulnerability presents a significant risk to systems relying on OpenSSL for secure communications, particularly those handling TLS 1.2 connections. Remote attackers can exploit this flaw by crafting specific TLS 1.2 client traffic that triggers the problematic code path, causing the target service to terminate unexpectedly. The impact extends beyond simple service disruption as it can affect availability of critical infrastructure, web servers, email services, and other applications that depend on OpenSSL for secure communication protocols. This vulnerability aligns with CWE-125: Out-of-bounds Read, which describes situations where programs read data from memory locations outside of intended bounds, and represents a classic example of how improper data validation can lead to system instability.
The exploitability of CVE-2013-6449 is relatively straightforward, requiring only that an attacker be able to establish a TLS 1.2 connection to the vulnerable service. This makes it particularly dangerous as it can be leveraged by attackers without requiring authentication or specialized access to the target system. The vulnerability demonstrates a weakness in the input validation mechanisms within OpenSSL's TLS implementation, where the library fails to properly validate the structure of incoming TLS handshake messages before processing them. This flaw underscores the importance of proper bounds checking and data structure validation in cryptographic libraries, as demonstrated by ATT&CK technique T1499.1: Endpoint Denial of Service, which covers methods that cause systems to become unavailable through resource exhaustion or application crashes.
Mitigation strategies for this vulnerability primarily involve upgrading to OpenSSL version 1.0.2 or later, where the problematic code has been corrected to properly handle version information from the correct data structures. System administrators should also implement network monitoring to detect unusual traffic patterns that might indicate exploitation attempts, and consider deploying intrusion detection systems that can identify malformed TLS handshake sequences. Additionally, organizations should maintain regular patching schedules for cryptographic libraries and conduct vulnerability assessments to identify other potential weaknesses in their secure communication infrastructure. The fix implemented in OpenSSL 1.0.2 addresses the root cause by ensuring that version numbers are extracted from the appropriate data structures, preventing the out-of-bounds memory access that previously led to daemon crashes.