CVE-2016-7055 in OpenSSL
Summary
by MITRE
There is a carry propagating bug in the Broadwell-specific Montgomery multiplication procedure in OpenSSL 1.0.2 and 1.1.0 before 1.1.0c that handles input lengths divisible by, but longer than 256 bits. Analysis suggests that attacks against RSA, DSA and DH private keys are impossible. This is because the subroutine in question is not used in operations with the private key itself and an input of the attacker's direct choice. Otherwise the bug can manifest itself as transient authentication and key negotiation failures or reproducible erroneous outcome of public-key operations with specially crafted input. Among EC algorithms only Brainpool P-512 curves are affected and one presumably can attack ECDH key negotiation. Impact was not analyzed in detail, because pre-requisites for attack are considered unlikely. Namely multiple clients have to choose the curve in question and the server has to share the private key among them, neither of which is default behaviour. Even then only clients that chose the curve will be affected.
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Analysis
by VulDB Data Team • 10/03/2022
The vulnerability identified as CVE-2016-7055 represents a critical flaw in OpenSSL's cryptographic implementation that specifically affects the Broadwell processor architecture. This issue manifests in the Montgomery multiplication procedure used for elliptic curve cryptography operations, particularly impacting the Brainpool P-512 curve. The flaw stems from a carry propagation bug that occurs when processing input lengths that are divisible by but exceed 256 bits, creating a condition where mathematical operations may produce incorrect results due to improper handling of bit-level carry operations in the optimized assembly code designed for Intel's Broadwell processors.
The technical nature of this vulnerability places it squarely within the domain of CWE-129, which addresses improper handling of buffer overflows and memory access violations. The issue affects OpenSSL versions 1.0.2 and 1.1.0 before 1.1.0c, representing a significant portion of the cryptographic library's user base during that period. The vulnerability is particularly concerning because it operates at the mathematical computation level rather than the protocol level, making it more insidious and harder to detect through traditional network monitoring approaches. The flaw specifically impacts the Montgomery multiplication algorithm which is fundamental to elliptic curve operations, where the carry propagation error can cause incorrect computations that may appear random but are actually deterministic under specific conditions.
From an operational perspective, the impact of this vulnerability is nuanced and somewhat limited in practical exploitation scenarios. While the bug could theoretically affect RSA, DSA, and DH private key operations, analysis indicates that direct attacks against private keys are impossible because the vulnerable subroutine is not used in private key operations. However, the vulnerability can manifest as transient authentication failures or reproducible erroneous outcomes in public-key operations with specially crafted inputs. This creates a scenario where legitimate cryptographic operations may fail intermittently or produce incorrect results, potentially leading to service disruptions or authentication challenges that could be exploited in specific attack contexts.
The most significant concern with CVE-2016-7055 lies in its potential impact on ECDH key negotiation, particularly when using the Brainpool P-512 curve. The vulnerability's requirement for specific preconditions makes it less likely to be exploited broadly, but the potential exists for targeted attacks in environments where multiple clients consistently use the affected curve and servers share private keys among clients. This scenario aligns with ATT&CK technique T1592, which involves reconnaissance to identify specific cryptographic implementations and configurations that may be vulnerable to exploitation. The attack prerequisites are considered unlikely by default because standard configurations do not typically involve the specific curve requirements, making this vulnerability more of a potential threat in specialized environments rather than a widespread concern.
The mitigation strategies for this vulnerability primarily involve upgrading to OpenSSL versions 1.1.0c or later where the bug has been corrected. System administrators should prioritize updating their OpenSSL installations, particularly in environments where the affected elliptic curve operations are actively used. Additionally, organizations should consider implementing monitoring to detect unusual authentication failures or cryptographic operation errors that might indicate exploitation attempts. The fix addresses the underlying carry propagation issue in the Montgomery multiplication routine, ensuring that mathematical operations produce correct results regardless of input length, while maintaining the performance optimizations for the Broadwell architecture. Security teams should also review their cryptographic configuration to ensure that the Brainpool P-512 curve is not being used unnecessarily, as this curve is the primary target of the vulnerability and represents a specific risk vector that can be eliminated through configuration changes.