CVE-2011-0091 in Windows
Summary
by MITRE
Kerberos in Microsoft Windows Server 2008 R2 and Windows 7 does not prevent a session from changing from strong encryption to DES encryption, which allows man-in-the-middle attackers to spoof network traffic and obtain sensitive information via a DES downgrade, aka "Kerberos Spoofing Vulnerability."
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Analysis
by VulDB Data Team • 03/18/2021
The vulnerability identified as CVE-2011-0091 represents a critical weakness in the Kerberos authentication protocol implementation within Microsoft Windows Server 2008 R2 and Windows 7 operating systems. This flaw stems from the protocol's failure to enforce strong encryption policies during session establishment, creating an exploitable condition where attackers can manipulate encryption strength negotiations. The vulnerability specifically targets the Kerberos authentication mechanism's ability to maintain encryption integrity throughout a session, allowing for cryptographic downgrade attacks that compromise the security assurances typically provided by the protocol.
The technical nature of this vulnerability lies in the Kerberos implementation's inability to prevent session encryption strength degradation from robust cryptographic algorithms to the weak Data Encryption Standard (DES) encryption method. When a Kerberos session negotiates encryption parameters, the system should maintain the strongest available encryption algorithm throughout the entire authentication process. However, this vulnerability allows attackers to force a downgrade to DES encryption, which is inherently weak and susceptible to cryptographic analysis and brute force attacks. The flaw operates at the protocol negotiation level where encryption strength preferences are exchanged between client and server, creating a window where an attacker can intercept and manipulate these negotiations.
The operational impact of this vulnerability extends beyond simple authentication compromise to enable sophisticated man-in-the-middle attacks that can effectively spoof network traffic. Attackers exploiting this weakness can intercept and modify Kerberos authentication messages, potentially gaining unauthorized access to network resources while maintaining the appearance of legitimate authentication. The downgrade to DES encryption creates a significant security risk because DES has known cryptographic weaknesses and can be broken with sufficient computational resources. This vulnerability particularly affects enterprise environments where Kerberos is commonly used for single sign-on and network authentication, potentially allowing attackers to escalate privileges and access sensitive corporate data.
Organizations affected by CVE-2011-0091 face substantial risk exposure given that Kerberos is fundamental to many enterprise security infrastructures. The vulnerability enables attackers to perform credential theft and session hijacking attacks that can persist undetected within network environments. From a cybersecurity perspective, this weakness aligns with attack patterns described in the MITRE ATT&CK framework under credential access and defense evasion techniques, specifically targeting Kerberos ticket manipulation and protocol downgrade attacks. The vulnerability also relates to CWE-310, which addresses cryptographic weaknesses in encryption implementations, and CWE-312, which covers the exposure of sensitive information through improper encryption.
Mitigation strategies for this vulnerability require immediate implementation of security patches provided by Microsoft through their regular security updates. Organizations should also implement network monitoring to detect unusual Kerberos encryption negotiation patterns and establish stronger encryption policies that enforce the use of robust cryptographic algorithms such as AES. Network segmentation and the deployment of additional authentication layers can help reduce the attack surface while patches are being applied. System administrators should also review and enforce proper Kerberos configuration settings, ensuring that DES encryption is disabled and that strong encryption algorithms are prioritized during authentication negotiations. The vulnerability underscores the importance of maintaining up-to-date security patches and implementing defense-in-depth strategies to protect against cryptographic downgrade attacks that can compromise fundamental authentication mechanisms.