CVE-2016-6302 in OpenSSL
Summary
by MITRE
The tls_decrypt_ticket function in ssl/t1_lib.c in OpenSSL before 1.1.0 does not consider the HMAC size during validation of the ticket length, which allows remote attackers to cause a denial of service via a ticket that is too short.
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Analysis
by VulDB Data Team • 08/29/2024
The vulnerability identified as CVE-2016-6302 resides within the OpenSSL library's implementation of TLS session resumption functionality, specifically within the tls_decrypt_ticket function located in ssl/t1_lib.c. This flaw represents a classic buffer over-read condition that occurs during the processing of session tickets, which are used to maintain session state between client and server connections. The vulnerability affects OpenSSL versions prior to 1.1.0, making it a significant concern for systems running older cryptographic libraries. The issue stems from insufficient validation of ticket length parameters during the decryption process, creating a scenario where maliciously crafted session tickets can trigger unexpected behavior in the cryptographic library.
The technical root cause of this vulnerability can be categorized under CWE-129, which addresses insufficient validation of length parameters, and more specifically aligns with CWE-125, describing out-of-bounds read conditions. During the TLS session resumption process, the server receives a session ticket from the client and attempts to decrypt it to restore the session state. The tls_decrypt_ticket function fails to properly validate that the ticket contains sufficient data to accommodate the expected HMAC signature, which is used to ensure the integrity of the ticket contents. When a short ticket is presented, the function proceeds to read beyond the allocated buffer boundaries in an attempt to extract the HMAC value, leading to memory access violations that can result in application crashes or unpredictable behavior.
From an operational perspective, this vulnerability presents a straightforward denial of service attack vector that requires minimal sophistication from adversaries. Remote attackers can craft specially formatted session tickets that are deliberately shorter than the minimum required size, causing the OpenSSL implementation to attempt memory reads beyond valid ticket boundaries. The attack does not require authentication or specialized privileges, making it particularly dangerous for publicly accessible services. Systems utilizing OpenSSL versions before 1.1.0 are vulnerable regardless of whether they support session resumption, as the flaw exists in the ticket validation logic itself. The impact extends beyond simple service disruption, as successful exploitation can lead to application instability, potential information disclosure through memory corruption, or even system compromise if the memory corruption affects critical library components.
The vulnerability's exploitation aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and represents a specific implementation weakness in cryptographic libraries that can be leveraged by threat actors to disrupt services. Organizations should prioritize immediate patching of affected systems, as the vulnerability exists in the core cryptographic processing functions that are fundamental to secure communications. Mitigation strategies beyond patching include implementing network-level protections such as rate limiting for session resumption requests, monitoring for unusual ticket sizes, and deploying intrusion detection systems that can identify malformed session tickets. Additionally, administrators should consider disabling session resumption functionality entirely if the risk cannot be immediately mitigated through patching, though this approach reduces the performance benefits of TLS session reuse. The vulnerability serves as a reminder of the critical importance of proper input validation in cryptographic implementations and demonstrates how seemingly minor oversights in buffer management can lead to significant security implications in widely deployed software libraries.