CVE-2013-4548 in OpenSSH
Summary
by MITRE
The mm_newkeys_from_blob function in monitor_wrap.c in sshd in OpenSSH 6.2 and 6.3, when an AES-GCM cipher is used, does not properly initialize memory for a MAC context data structure, which allows remote authenticated users to bypass intended ForceCommand and login-shell restrictions via packet data that provides a crafted callback address.
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Analysis
by VulDB Data Team • 06/01/2021
The vulnerability described in CVE-2013-4548 represents a critical memory initialization flaw within the OpenSSH implementation that affects versions 6.2 and 6.3. This issue specifically targets the monitor_wrap.c file within the sshd daemon, where the mm_newkeys_from_blob function fails to properly initialize memory structures associated with MAC context data when AES-GCM cipher mode is employed. The flaw exists in the privileged monitoring process that handles authentication and key management, creating a potential path for authenticated remote attackers to manipulate the system's security controls.
The technical nature of this vulnerability stems from improper memory initialization within the cryptographic context handling mechanism. When AES-GCM cipher mode is used, the function does not adequately clear or initialize the MAC context data structure before processing incoming packet data. This incomplete initialization creates a memory state where residual data from previous operations might persist and influence the current cryptographic processing. The vulnerability becomes exploitable through crafted packet data that manipulates the callback address within the MAC context, allowing an attacker to effectively bypass the intended security restrictions.
The operational impact of this vulnerability is significant as it enables authenticated remote attackers to circumvent critical security controls including ForceCommand directives and login-shell restrictions. These restrictions are fundamental to system security as they limit what commands can be executed and what shells can be accessed by authenticated users. The ability to bypass these controls through memory manipulation represents a privilege escalation vector that could allow attackers to execute arbitrary commands with elevated privileges or gain access to restricted system resources. The vulnerability specifically affects systems where AES-GCM cipher mode is configured for SSH connections, which was a common configuration in the affected versions.
From a cybersecurity perspective, this vulnerability aligns with CWE-1286 which addresses improper initialization of data structures and can be categorized under the MITRE ATT&CK framework as a privilege escalation technique through memory corruption. The attack requires an authenticated session, making it less accessible than some other vulnerabilities but still dangerous given that it can be exploited by users who already have legitimate access to the system. The vulnerability demonstrates the importance of proper memory management in security-critical components and highlights how seemingly minor initialization flaws can have major security implications in cryptographic implementations.
The recommended mitigations for this vulnerability include immediate upgrade to OpenSSH versions that contain the fix, which was implemented in versions 6.4 and later. System administrators should also consider disabling AES-GCM cipher mode if it is not strictly required, as this removes the attack vector entirely. Additionally, monitoring for unusual authentication patterns and implementing proper access controls can help detect potential exploitation attempts. Organizations should conduct thorough security assessments of their SSH configurations and ensure that all systems are running patched versions to prevent exploitation of this memory initialization flaw that could compromise system integrity and user authentication controls.