CVE-2016-6307 in OpenSSL
Summary
by MITRE
The state-machine implementation in OpenSSL 1.1.0 before 1.1.0a allocates memory before checking for an excessive length, which might allow remote attackers to cause a denial of service (memory consumption) via crafted TLS messages, related to statem/statem.c and statem/statem_lib.c.
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Analysis
by VulDB Data Team • 09/21/2022
The vulnerability identified as CVE-2016-6307 represents a critical memory management flaw within the OpenSSL 1.1.0 cryptographic library implementation. This issue specifically affects the state-machine architecture responsible for processing TLS protocol communications, where the library exhibits improper resource allocation behavior that can be exploited by remote attackers. The flaw resides in the statem/statem.c and statem/statem_lib.c source files, which handle the complex state transitions required for secure TLS handshakes and data transmission. The vulnerability manifests when the OpenSSL library processes crafted TLS messages that contain excessive length parameters, allowing attackers to manipulate the memory allocation process before proper validation occurs.
The technical execution of this vulnerability exploits a fundamental flaw in the library's defensive programming practices, where memory allocation occurs prior to length validation checks. This sequence creates a window of opportunity for malicious actors to submit specially crafted TLS protocol messages containing oversized data structures that trigger excessive memory consumption. The improper ordering of operations means that the system allocates memory resources based on potentially malicious input values before verifying whether these values fall within acceptable parameters. This memory allocation before validation pattern is classified as a weakness under CWE-457, which specifically addresses the use of uninitialized memory and improper resource management in cryptographic implementations.
The operational impact of CVE-2016-6307 extends beyond simple resource exhaustion, as it enables attackers to consume system resources at an accelerated rate, potentially leading to system instability or complete service unavailability. When exploited, this vulnerability allows remote attackers to cause denial of service conditions by forcing the OpenSSL library to allocate excessive amounts of memory, effectively consuming system resources and potentially leading to system crashes or performance degradation. The attack vector requires minimal privileges and can be executed against any system running vulnerable versions of OpenSSL, making it particularly dangerous in production environments where service availability is critical. This vulnerability aligns with ATT&CK technique T1499.004, which describes resource exhaustion attacks targeting network services through malformed protocol messages.
Mitigation strategies for CVE-2016-6307 require immediate deployment of OpenSSL version 1.1.0a or later, which includes patches addressing the improper memory allocation sequence in the state-machine implementation. System administrators should also implement network-level protections such as rate limiting and connection throttling to reduce the impact of potential exploitation attempts. Additionally, monitoring systems should be configured to detect unusual memory consumption patterns that may indicate exploitation attempts. The fix implemented in OpenSSL 1.1.0a corrects the sequence of operations by ensuring proper length validation occurs before memory allocation, thereby preventing attackers from manipulating the state-machine to consume excessive resources. Organizations should also consider implementing intrusion detection systems that can identify and block malformed TLS messages attempting to exploit this specific vulnerability pattern.