CVE-2021-4160: OpenSSL Information Disclosure Flaw

CVE-2021-4160 Overview

CVE-2021-4160 is a carry propagation bug affecting the MIPS32 and MIPS64 squaring procedure in OpenSSL. This cryptographic vulnerability impacts multiple elliptic curve (EC) algorithms, including some of the TLS 1.3 default curves. While the vulnerability requires specific pre-requisites for exploitation—notably the reuse of private keys—it poses potential risks to Diffie-Hellman (DH) key exchanges on MIPS-based systems running affected OpenSSL versions.

Critical Impact

Cryptographic weakness in OpenSSL on MIPS platforms could potentially allow offline attacks against DH private keys, though exploitation requires significant resources and specific conditions including private key reuse.

Affected Products

• OpenSSL versions 1.0.2 through 1.0.2zb
• OpenSSL versions 1.1.1 through 1.1.1l
• OpenSSL version 3.0.0 (including all alpha and beta releases)
• Debian Linux 9.0, 10.0, and 11.0
• Oracle Health Sciences Inform Publisher 6.2.1.1 and 6.3.1.1
• Oracle JD Edwards EnterpriseOne Tools 9.2.6.3
• Oracle JD Edwards World Security A9.4
• Oracle PeopleSoft Enterprise PeopleTools 8.58 and 8.59
• Siemens SINEC INS (versions prior to 1.0 SP2)
• Oracle Enterprise Manager Ops Center 12.4.0.0

Discovery Timeline

• 2021-12-15 - OpenSSL releases security patches in versions 1.1.1m and 3.0.1
• 2022-01-28 - CVE CVE-2021-4160 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-4160

Vulnerability Analysis

The vulnerability exists in the low-level assembly implementation of the squaring operation used in modular exponentiation on MIPS architecture processors. The carry propagation bug can cause incorrect mathematical results during cryptographic operations, specifically affecting the squaring procedure which is fundamental to many asymmetric cryptographic algorithms.

The bug impacts elliptic curve cryptography operations, including curves used as TLS 1.3 defaults. For RSA and DSA algorithms, exploitation is considered extremely difficult due to the mathematical complexity involved. However, Diffie-Hellman key exchanges are theoretically more vulnerable because much of the computational work required to deduce private key information can be performed offline.

A critical mitigating factor is that meaningful attacks against TLS would require the server to share DH private keys among multiple clients—a practice that was deprecated following CVE-2016-0701 and is no longer common. The vulnerability only manifests on MIPS platforms, significantly limiting the scope of affected deployments.

Root Cause

The root cause is an implementation error in the MIPS-specific assembly code that performs big number squaring operations. During the squaring procedure, carry values are not properly propagated between intermediate calculations, leading to potential computational errors. This affects the accuracy of modular arithmetic operations that form the foundation of EC, DH, RSA, and DSA cryptographic algorithms.

Attack Vector

Exploitation of this vulnerability requires a network-based attack where an adversary can observe or interact with cryptographic operations performed by a vulnerable MIPS-based system. The attack is complex and requires:

1. Access to a target system running OpenSSL on MIPS architecture
2. The target must be reusing private keys across multiple operations
3. Significant computational resources to perform offline analysis
4. Multiple observations of cryptographic operations using the same private key

For DH attacks, the adversary would need to collect sufficient handshake data to perform statistical analysis aimed at recovering private key material. The offline nature of the analysis phase means attacks could be attempted without triggering obvious network anomalies, though the resource requirements remain substantial.

Detection Methods for CVE-2021-4160

Indicators of Compromise

• Unusual cryptographic operation failures or errors in OpenSSL logs on MIPS systems
• Unexpected mathematical errors in TLS handshakes or key exchange operations
• Systems running vulnerable OpenSSL versions on MIPS32 or MIPS64 architecture

Detection Strategies

• Inventory all systems running on MIPS architecture and audit their OpenSSL versions
• Monitor for OpenSSL versions 1.0.2 through 1.0.2zb, 1.1.1 through 1.1.1l, or 3.0.0 on MIPS platforms
• Implement cryptographic operation logging to detect abnormal calculation patterns
• Review TLS configurations to identify any systems sharing DH private keys across clients

Monitoring Recommendations

• Deploy software composition analysis (SCA) tools to track OpenSSL versions across MIPS infrastructure
• Monitor security advisories from OpenSSL, Debian, Oracle, and Siemens for related updates
• Establish baseline metrics for cryptographic operation performance to detect anomalies
• Review TLS handshake logs for unusual error rates or patterns on MIPS-based servers

How to Mitigate CVE-2021-4160

Immediate Actions Required

• Identify all MIPS-based systems in your environment running affected OpenSSL versions
• Prioritize patching for systems that handle sensitive cryptographic operations or TLS termination
• Review and disable DH key reuse across multiple client connections if not already configured
• Consider temporarily migrating critical workloads to non-MIPS platforms while patching

Patch Information

OpenSSL has released patches addressing this vulnerability:

• OpenSSL 3.0.1 - Fixes the issue for version 3.0.0 (released December 15, 2021)
• OpenSSL 1.1.1m - Fixes the issue for versions 1.1.1 through 1.1.1l (released December 15, 2021)
• OpenSSL 1.0.2zc - Fixes for the 1.0.2 branch (premium support only; commit 6fc1aaaf3 available in git)

For detailed patch information, refer to the OpenSSL Security Advisory 20220128. Oracle products should be updated according to the Oracle Critical Patch Update April 2022. Debian users should apply updates per Debian Security Announcement DSA-5103. Siemens SINEC INS users should consult the Siemens Security Advisory SSA-637483.

Workarounds

• Ensure DH private keys are not shared across multiple clients (default behavior since CVE-2016-0701 mitigations)
• Avoid reusing private keys in EC, DH, RSA, or DSA operations where possible
• Consider using ECDHE (Ephemeral Elliptic Curve Diffie-Hellman) which generates unique keys per session
• If patching is delayed, evaluate temporarily disabling affected cipher suites on MIPS systems

# Check OpenSSL version and platform
openssl version -a

# Verify MIPS architecture (if applicable)
uname -m

# Update OpenSSL on Debian-based systems
sudo apt-get update && sudo apt-get upgrade openssl

# Verify updated version
openssl version