CVE-2015-3193 in Xcode
Summary
by MITRE
The Montgomery squaring implementation in crypto/bn/asm/x86_64-mont5.pl in OpenSSL 1.0.2 before 1.0.2e on the x86_64 platform, as used by the BN_mod_exp function, mishandles carry propagation and produces incorrect output, which makes it easier for remote attackers to obtain sensitive private-key information via an attack against use of a (1) Diffie-Hellman (DH) or (2) Diffie-Hellman Ephemeral (DHE) ciphersuite.
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Analysis
by VulDB Data Team • 09/28/2022
The vulnerability identified as CVE-2015-3193 represents a critical flaw in OpenSSL's cryptographic implementation that specifically affects the Montgomery squaring algorithm used in the BN_mod_exp function. This issue exists in OpenSSL versions 1.0.2 and earlier, particularly on x86_64 platforms where the assembly-optimized implementation fails to properly handle carry propagation during modular exponentiation operations. The flaw occurs within the crypto/bn/asm/x86_64-mont5.pl file, which is designed to accelerate cryptographic operations through low-level optimizations but introduces a critical error in its carry handling mechanism.
The technical nature of this vulnerability stems from improper carry propagation in the Montgomery multiplication algorithm, which is fundamental to efficient modular exponentiation operations in public key cryptography. When the algorithm encounters certain input values, it fails to correctly propagate carries between computational words, resulting in incorrect intermediate calculations that ultimately produce erroneous cryptographic outputs. This mathematical error creates a side-channel vulnerability that allows attackers to extract information about the private key through statistical analysis of the incorrect results. The vulnerability is particularly dangerous because it affects Diffie-Hellman and Diffie-Hellman Ephemeral ciphersuites, which are widely used for key exchange in secure communications.
The operational impact of CVE-2015-3193 is severe as it enables remote attackers to perform successful attacks against systems using vulnerable OpenSSL implementations. Attackers can exploit this vulnerability by targeting connections that utilize DH or DHE ciphersuites, potentially recovering private key information through repeated attacks that analyze the incorrect outputs generated by the flawed Montgomery squaring implementation. This weakness directly violates the security principles of public key cryptography where the private key must remain completely confidential. The vulnerability is classified under CWE-194 as "Unexpected Sign Extension" and aligns with ATT&CK technique T1552.004 for "Credentials from Password Stores" and T1071.004 for "Application Layer Protocol: DNS" when attackers leverage compromised keys for further access. The flaw essentially undermines the fundamental security guarantees of the cryptographic protocols it supports.
Mitigation strategies for CVE-2015-3193 primarily involve upgrading to OpenSSL 1.0.2e or later versions where the Montgomery squaring implementation has been corrected to properly handle carry propagation. Organizations should also consider disabling DH and DHE ciphersuites in their configurations when immediate upgrades are not feasible, though this may impact compatibility with older systems. Security teams should monitor for any instances of the vulnerable OpenSSL versions in their infrastructure and conduct thorough vulnerability assessments. The fix implemented in OpenSSL 1.0.2e specifically addresses the carry propagation issue by ensuring that all computational words properly handle carry bits during the Montgomery multiplication process, restoring the mathematical correctness required for secure cryptographic operations. Additionally, system administrators should implement comprehensive monitoring for any anomalous cryptographic behavior that might indicate exploitation attempts.