CVE-2011-5095 in OpenSSL
Summary
by MITRE
The Diffie-Hellman key-exchange implementation in OpenSSL 0.9.8, when FIPS mode is enabled, does not properly validate a public parameter, which makes it easier for man-in-the-middle attackers to obtain the shared secret key by modifying network traffic, a related issue to CVE-2011-1923.
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Analysis
by VulDB Data Team • 12/04/2021
The vulnerability described in CVE-2011-5095 represents a critical weakness in the cryptographic implementation of OpenSSL 0.9.8 when operating in FIPS mode. This flaw specifically targets the Diffie-Hellman key-exchange mechanism, which forms the foundation of secure key establishment in many cryptographic protocols including TLS, SSH, and various VPN implementations. The issue arises from insufficient validation of public parameters during the key exchange process, creating a pathway for adversaries to manipulate network traffic without detection. The vulnerability is particularly concerning because it directly impacts the security assurances provided by FIPS 140-2 compliant cryptographic implementations, which are widely deployed in government and enterprise environments where data protection is paramount.
The technical root cause of this vulnerability stems from the failure to properly validate the public parameter g^x mod p during the Diffie-Hellman key exchange process. In a properly implemented system, the public parameter should be validated to ensure it meets specific mathematical criteria that prevent weak key exchanges. When FIPS mode is enabled, OpenSSL is expected to enforce strict cryptographic standards, but this implementation flaw allows attackers to modify the public parameter values in transit. The vulnerability is categorized under CWE-327, which addresses the use of weak cryptographic algorithms or modes, and more specifically relates to improper validation of cryptographic parameters. This weakness enables attackers to perform man-in-the-middle attacks by altering the Diffie-Hellman parameters, potentially allowing them to compute the shared secret key without proper authentication.
The operational impact of CVE-2011-5095 extends beyond simple cryptographic compromise, as it undermines the fundamental security model of TLS and other protocols that rely on Diffie-Hellman key exchange. Network traffic interception and modification becomes significantly easier when attackers can manipulate the public parameters, potentially compromising all communications secured by affected OpenSSL implementations. This vulnerability affects a wide range of systems including web servers, email servers, VPN gateways, and any network service that utilizes OpenSSL for secure communications. The attack vector is particularly insidious because it requires no special privileges or access to the target system, merely the ability to intercept and modify network traffic. This aligns with ATT&CK technique T1566, which covers "Phishing with Malicious Attachments" and similar network-based attack methods that exploit cryptographic weaknesses in transit.
The security implications of this vulnerability are severe and multifaceted, as it effectively nullifies the confidentiality protections provided by Diffie-Hellman key exchange when FIPS mode is active. Organizations using affected OpenSSL versions face potential exposure of sensitive communications, including user credentials, personal data, financial transactions, and proprietary information. The vulnerability's relationship to CVE-2011-1923 demonstrates a pattern of weak parameter validation in cryptographic implementations that can be exploited to compromise key exchange security. Remediation efforts must include immediate patching of OpenSSL installations to versions that properly validate Diffie-Hellman parameters, along with comprehensive network monitoring to detect potential exploitation attempts. System administrators should also consider implementing additional security controls such as certificate pinning, network segmentation, and enhanced traffic inspection to mitigate the risk of successful attacks exploiting this vulnerability. The fix typically involves ensuring that public parameters are validated against known secure criteria and that the implementation properly rejects invalid parameter values that could lead to weak key exchanges.