CVE-2014-9749 in Squid
Summary
by MITRE
Squid 3.4.4 through 3.4.11 and 3.5.0.1 through 3.5.1, when Digest authentication is used, allow remote authenticated users to retain access by leveraging a stale nonce, aka "Nonce replay vulnerability."
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Analysis
by VulDB Data Team • 11/23/2018
The vulnerability identified as CVE-2014-9749 represents a critical authentication flaw in the Squid proxy server software that affects versions ranging from 3.4.4 through 3.4.11 and 3.5.0.1 through 3.5.1. This issue specifically impacts systems utilizing Digest authentication mechanisms, creating a persistent security weakness that allows authenticated attackers to maintain unauthorized access to protected resources. The vulnerability stems from the improper handling of nonce values within the authentication process, which are cryptographic tokens used to prevent replay attacks. When a nonce becomes stale or expires, the system should invalidate it and require a fresh authentication challenge, but this validation mechanism fails in the affected versions, enabling attackers to reuse previously valid nonces.
The technical implementation of this vulnerability resides in the Digest authentication protocol handling within Squid's authentication subsystem. According to CWE-310, this represents a cryptographic vulnerability related to improper use of cryptographic primitives, specifically in the nonce management and validation process. The flaw occurs because the proxy server does not adequately enforce nonce freshness requirements, allowing attackers who have obtained valid authentication credentials to leverage these stale tokens to maintain access to protected resources. This issue directly violates the fundamental security principle that authentication tokens should be single-use or have strict time-based expiration mechanisms. The operational impact is particularly severe because authenticated users who have legitimate access to the proxy can potentially extend their access beyond the intended session duration, while unauthorized attackers who can observe legitimate authentication exchanges can exploit this weakness to gain persistent access to restricted network resources.
The practical exploitation of this vulnerability requires an attacker to first obtain valid credentials through legitimate means or by observing network traffic, then capture a valid nonce value during an authentication exchange. Once obtained, the attacker can reuse this stale nonce to bypass the authentication mechanism and maintain access to protected resources without requiring additional authentication. This creates a persistent threat vector that can remain undetected for extended periods, as the access appears legitimate to network monitoring systems. The vulnerability aligns with ATT&CK technique T1550.001, which describes the use of valid credentials to access systems, but with the added complexity that the attacker leverages a legitimate authentication mechanism while exploiting a flaw in the token validation process. The impact extends beyond simple unauthorized access to include potential data exfiltration, privilege escalation, and the ability to use the proxy as a pivot point for further network exploration.
Organizations affected by this vulnerability should prioritize immediate patching of their Squid proxy servers to versions that properly handle nonce validation and expiration. The recommended mitigations include upgrading to Squid versions 3.4.12 or 3.5.2, which contain the necessary fixes for the nonce replay vulnerability. Additionally, network administrators should implement enhanced monitoring of authentication events and nonce usage patterns to detect potential exploitation attempts. Security teams should also consider implementing additional authentication layers or multi-factor authentication mechanisms to reduce the impact of credential compromise. The vulnerability demonstrates the critical importance of proper cryptographic protocol implementation and the necessity of thorough testing of authentication mechanisms, particularly those involving time-sensitive tokens and nonce management systems. Organizations should conduct comprehensive security assessments of their proxy infrastructure and review their authentication policies to ensure that similar vulnerabilities are not present in other components of their network security infrastructure.