CVE-2016-8635 in NSS
Summary
by MITRE
It was found that Diffie Hellman Client key exchange handling in NSS 3.21.x was vulnerable to small subgroup confinement attack. An attacker could use this flaw to recover private keys by confining the client DH key to small subgroup of the desired group.
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Analysis
by VulDB Data Team • 04/28/2023
The vulnerability identified as CVE-2016-8635 represents a critical weakness in the Network Security Services (NSS) cryptographic library version 3.21.x, specifically affecting the Diffie-Hellman key exchange implementation. This flaw resides in the client-side handling of Diffie-Hellman key exchanges, where the system fails to properly validate the cryptographic parameters provided by the remote party. The vulnerability stems from insufficient validation of the Diffie-Hellman parameters, particularly the public key components, which allows an attacker to manipulate the key exchange process through a small subgroup confinement attack. Such an attack exploits the mathematical properties of Diffie-Hellman cryptography where certain subgroup elements can be exploited to reduce the effective key space, making the private key recovery computationally feasible.
The technical execution of this vulnerability involves an attacker who can influence the Diffie-Hellman key exchange process by forcing the client to use a small subgroup within the larger cryptographic group. This occurs when the client does not properly validate the public key values received from the server, allowing the attacker to constrain the key exchange to a subgroup with a smaller order. The mathematical foundation of this attack is rooted in the discrete logarithm problem, where the security of Diffie-Hellman relies on the difficulty of computing discrete logarithms in large prime groups. However, when the system operates within a small subgroup, the computational complexity reduces significantly, enabling the attacker to perform a brute force attack or other mathematical techniques to recover the private key components. This vulnerability directly maps to CWE-326, which addresses the weakness in the security of cryptographic algorithms, and specifically relates to improper key validation mechanisms.
The operational impact of CVE-2016-8635 extends beyond simple cryptographic compromise, as it can lead to complete session hijacking and unauthorized access to protected communications. When an attacker successfully exploits this vulnerability, they can intercept and decrypt sensitive data transmitted over SSL/TLS connections that utilize the affected NSS library. This includes web traffic, email communications, and any other encrypted data that relies on the compromised Diffie-Hellman key exchange mechanism. The attack can be particularly devastating in environments where the affected NSS library is used by web servers, email servers, or other network services that handle confidential information. The vulnerability affects not only the confidentiality of communications but also the integrity and authenticity of the connections, potentially enabling man-in-the-middle attacks where the attacker can not only read communications but also modify them. From an ATT&CK framework perspective, this vulnerability aligns with T1566, which covers the exploitation of remote services, and T1571, which addresses the use of application layer protocols for exfiltration.
Mitigation strategies for CVE-2016-8635 primarily involve immediate patching of the affected NSS library to version 3.22 or later, which implements proper subgroup validation and parameter checking. Organizations should also consider implementing additional security measures such as certificate pinning, monitoring for unusual cryptographic behavior, and ensuring that all systems using NSS are regularly updated. The fix addresses the core issue by implementing proper validation of Diffie-Hellman parameters, specifically ensuring that the public key values are within the expected subgroup and that the parameters meet minimum security requirements. Additionally, security teams should conduct comprehensive vulnerability assessments to identify all systems using the affected NSS versions and ensure proper remediation. The vulnerability serves as a reminder of the critical importance of proper cryptographic parameter validation and the potential consequences of insufficient validation in key exchange protocols, particularly in widely deployed security libraries that form the foundation of internet security infrastructure.