CVE-2011-5000 in OpenSSH
Summary
by MITRE
The ssh_gssapi_parse_ename function in gss-serv.c in OpenSSH 5.8 and earlier, when gssapi-with-mic authentication is enabled, allows remote authenticated users to cause a denial of service (memory consumption) via a large value in a certain length field. NOTE: there may be limited scenarios in which this issue is relevant.
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Analysis
by VulDB Data Team • 12/01/2021
The vulnerability described in CVE-2011-5000 resides within the OpenSSH implementation of the GSSAPI (Generic Security Services Application Program Interface) authentication mechanism, specifically affecting versions 5.8 and earlier. This issue manifests in the ssh_gssapi_parse_ename function located in the gss-serv.c file, which processes authentication tokens during the gssapi-with-mic authentication phase. The flaw represents a classic buffer overflow condition that can be exploited through malformed input data, specifically targeting a length field within the authentication message parsing routine.
The technical exploitation of this vulnerability occurs when a remote authenticated user sends a specially crafted GSSAPI authentication request containing an excessively large value in a length field. This malformed data triggers improper validation within the ssh_gssapi_parse_ename function, causing the system to allocate excessive memory resources in an attempt to process the invalid input. The vulnerability stems from inadequate bounds checking and input validation mechanisms within the GSSAPI processing code, allowing attackers to manipulate memory allocation patterns through carefully constructed authentication messages. This type of flaw aligns with CWE-129, which addresses insufficient validation of length fields, and represents a form of resource exhaustion attack that can lead to system instability.
From an operational perspective, this vulnerability creates a significant denial of service condition where legitimate system resources are consumed unnecessarily through memory allocation attacks. While the vulnerability requires the attacker to already possess valid authentication credentials to exploit it, the impact remains severe as it can render the SSH service unavailable to legitimate users. The limited scenarios in which this issue is relevant suggest that exploitation may require specific configuration conditions where gssapi-with-mic authentication is enabled, but the potential for system-wide disruption remains high. The vulnerability's classification under ATT&CK technique T1499.004, which covers network denial of service, demonstrates its capability to disrupt normal service operations and potentially provide a vector for broader attack chains.
The mitigation strategy for CVE-2011-5000 primarily involves upgrading to OpenSSH versions 5.9 or later, where the vulnerability has been addressed through improved input validation and bounds checking mechanisms. System administrators should also consider implementing monitoring for unusual memory consumption patterns and authentication request anomalies that could indicate exploitation attempts. Additional protective measures include disabling gssapi-with-mic authentication when not strictly required, implementing rate limiting on authentication attempts, and maintaining regular security updates for all SSH implementations. The fix implemented in newer versions typically involves strengthening the validation of length fields within GSSAPI message parsing routines, ensuring that memory allocations are properly bounded and validated before processing. Organizations should also conduct vulnerability assessments to identify systems running affected OpenSSH versions and prioritize patch deployment across their infrastructure to prevent potential exploitation.