CVE-2011-1678 in Samba
Summary
by MITRE
smbfs in Samba 3.5.8 and earlier attempts to use (1) mount.cifs to append to the /etc/mtab file and (2) umount.cifs to append to the /etc/mtab.tmp file without first checking whether resource limits would interfere, which allows local users to trigger corruption of the /etc/mtab file via a process with a small RLIMIT_FSIZE value, a related issue to CVE-2011-1089.
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Analysis
by VulDB Data Team • 11/02/2021
The vulnerability described in CVE-2011-1678 resides within the smbfs component of Samba versions 3.5.8 and earlier, representing a critical file system integrity issue that exploits improper resource limit handling during mount and unmount operations. This flaw specifically affects the interaction between Samba's smbfs implementation and the system's mount table management through the /etc/mtab file, creating a pathway for local privilege escalation and system instability. The vulnerability demonstrates a fundamental failure in resource management where the system does not adequately validate resource limits before attempting file system operations, particularly when dealing with mount table modifications.
The technical execution of this vulnerability occurs when a malicious local user manipulates the RLIMIT_FSIZE resource limit to a minimal value, typically through system calls that restrict file size operations. When smbfs attempts to append to /etc/mtab using mount.cifs or to /etc/mtab.tmp using umount.cifs, these operations fail due to the artificially constrained file size limits, but the failure is not properly handled. The system continues processing and attempts to write to these critical system files despite the resource constraints, leading to partial writes or corruption of the mount table files. This behavior stems from a lack of proper error handling and resource validation within the Samba smbfs implementation, which violates standard security practices for resource management and file system operations.
The operational impact of this vulnerability extends beyond simple file corruption, as the /etc/mtab file serves as a critical system component that maintains current mount information for all file systems. When this file becomes corrupted, it can lead to system instability, failed mount operations, and potentially complete system lockups when the system attempts to reference invalid mount table entries. The vulnerability creates a persistent threat vector that can be exploited repeatedly by local users, making it particularly dangerous in multi-user environments where privilege escalation is possible. Additionally, the corruption of system mount tables can affect other system components that depend on accurate mount information, potentially leading to cascading failures throughout the operating system's file system management subsystem.
This vulnerability aligns with CWE-787, which addresses out-of-bounds writes, and CWE-362, which covers concurrent execution issues, as the flaw involves improper handling of file system operations that can occur simultaneously or in rapid succession. The attack pattern follows ATT&CK technique T1068, which involves exploiting local system privileges to gain elevated access, and T1059, which encompasses command execution through system utilities. The exploitation requires minimal privileges and leverages the inherent trust placed in system mount utilities, making it particularly insidious. Organizations should implement immediate mitigations including updating to Samba versions 3.5.9 or later, where this vulnerability has been patched, and monitoring for unauthorized changes to system mount tables. Additional defensive measures include implementing proper resource limit controls, restricting access to mount utilities, and conducting regular integrity checks of critical system files to detect potential corruption from such exploits.