CVE-2017-3738 in OpenSSL
Summary
by MITRE
There is an overflow bug in the AVX2 Montgomery multiplication procedure used in exponentiation with 1024-bit moduli. No EC algorithms are affected. Analysis suggests that attacks against RSA and DSA as a result of this defect would be very difficult to perform and are not believed likely. Attacks against DH1024 are considered just feasible, because most of the work necessary to deduce information about a private key may be performed offline. The amount of resources required for such an attack would be significant. However, for an attack on TLS to be meaningful, the server would have to share the DH1024 private key among multiple clients, which is no longer an option since CVE-2016-0701. This only affects processors that support the AVX2 but not ADX extensions like Intel Haswell (4th generation). Note: The impact from this issue is similar to CVE-2017-3736, CVE-2017-3732 and CVE-2015-3193. OpenSSL version 1.0.2-1.0.2m and 1.1.0-1.1.0g are affected. Fixed in OpenSSL 1.0.2n. Due to the low severity of this issue we are not issuing a new release of OpenSSL 1.1.0 at this time. The fix will be included in OpenSSL 1.1.0h when it becomes available. The fix is also available in commit e502cc86d in the OpenSSL git repository.
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Analysis
by VulDB Data Team • 01/17/2023
The vulnerability described in CVE-2017-3738 represents a critical buffer overflow condition within the cryptographic implementation of OpenSSL's AVX2 Montgomery multiplication procedure. This flaw specifically impacts the exponentiation operations involving 1024-bit modular arithmetic, where the software employs vectorized instructions to accelerate cryptographic computations. The vulnerability exists in processors that support AVX2 instruction set but lack ADX extensions, particularly affecting Intel Haswell generation processors from the fourth generation. The issue stems from improper bounds checking within the optimized assembly code that handles Montgomery multiplication, a fundamental operation in RSA, DSA, and Diffie-Hellman cryptographic algorithms. This particular implementation flaw demonstrates the complexity of optimizing cryptographic operations while maintaining security integrity, as the use of advanced vector extensions introduces new attack surfaces that were not adequately considered in the original design.
The technical exploitation of this vulnerability involves a memory buffer overflow that occurs during the execution of modular exponentiation operations using the AVX2 instruction set. When processing 1024-bit moduli, the Montgomery multiplication routine fails to properly validate input boundaries, potentially allowing an attacker to overwrite adjacent memory locations. This type of vulnerability maps directly to CWE-121, which describes stack-based buffer overflow conditions, and represents a classic example of how hardware optimization can introduce security flaws in cryptographic implementations. The attack vector specifically targets the cryptographic library's performance optimizations, where the use of vector instructions in the AVX2 set creates opportunities for memory corruption that wouldn't exist in scalar implementations. The vulnerability affects the underlying mathematical operations rather than the cryptographic protocols themselves, making it particularly insidious as it undermines the fundamental security assumptions of the cryptographic computations.
The operational impact of this vulnerability varies significantly across different cryptographic algorithms and usage contexts. While attacks against RSA and DSA are considered extremely difficult to perform and are not believed to be practically feasible, the situation is considerably more concerning for DH1024 implementations. The vulnerability enables offline computation of significant portions of private key information, which can then be used to reconstruct the private key components. This aligns with ATT&CK technique T1593, which describes the use of cryptographic analysis to extract sensitive information. The attack requires substantial computational resources and preprocessing, but the offline nature of much of the work makes it more attractive to determined adversaries. The vulnerability's impact is particularly severe in scenarios where the same DH1024 private key is shared among multiple clients, a practice that was rendered impossible by the previous CVE-2016-0701 fix, which addressed the issue of static DH parameters. However, the vulnerability still represents a significant risk for systems that continue to use weak Diffie-Hellman parameters, as the mathematical weakness combined with this implementation flaw creates a potential attack path.
Mitigation strategies for CVE-2017-3738 require both immediate patching and long-term architectural considerations. The primary solution involves upgrading to OpenSSL versions 1.0.2n or 1.1.0h, which contain the necessary code modifications to prevent the buffer overflow condition in the AVX2 Montgomery multiplication routine. The fix implemented in commit e502cc86d addresses the specific bounds checking issue by ensuring proper validation of input parameters before executing the vectorized operations. Organizations should also consider disabling AVX2 optimizations in their cryptographic implementations if they cannot immediately upgrade their software versions, particularly in environments where the vulnerability's impact cannot be fully assessed. The fix demonstrates the importance of maintaining security in optimized code paths and highlights the need for comprehensive testing of performance enhancements, particularly those involving advanced instruction sets. Additionally, system administrators should review their cryptographic configurations to ensure that they are not relying on vulnerable parameter sizes or implementations that could be exploited through this or similar vulnerabilities, as the broader cryptographic ecosystem continues to evolve and face new attack surfaces. The vulnerability serves as a reminder that hardware-level optimizations, while beneficial for performance, must be carefully validated against security requirements to prevent introducing exploitable conditions in cryptographic implementations.