CVE-2015-1340 in lxd
Summary
by MITRE
LXD before version 0.19-0ubuntu5 doUidshiftIntoContainer() has an unsafe Chmod() call that races against the stat in the Filepath.Walk() function. A symbolic link created in that window could cause any file on the system to have any mode of the attacker's choice.
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Analysis
by VulDB Data Team • 09/06/2023
The vulnerability described in CVE-2015-1340 affects LXD containers prior to version 0.19-0ubuntu5, specifically within the doUidshiftIntoContainer() function. This represents a critical race condition that exploits improper file system permissions handling during container initialization. The flaw occurs when the system attempts to adjust user and group identifiers for files within containerized environments, creating a window of opportunity for malicious actors to manipulate file permissions through symbolic link manipulation.
The technical implementation of this vulnerability stems from an unsafe chmod() operation that occurs concurrently with file system traversal through filepath.Walk() function. During the container setup process, LXD performs uid/gid shifting operations on files to ensure proper ownership within the containerized environment. However, the timing between the chmod() call and the stat() operation within the file traversal creates a temporal gap where attackers can insert malicious symbolic links. This race condition allows adversaries to create symbolic links that point to arbitrary system files, enabling them to modify permissions of critical system components through the container's file access mechanisms.
The operational impact of this vulnerability extends beyond simple permission manipulation, as it enables attackers to potentially escalate privileges and compromise the entire host system. When an attacker successfully exploits this race condition, they can set any file mode they choose, including making critical system files executable or writable by unauthorized users. This vulnerability directly relates to CWE-367, which addresses Time-of-Check to Time-of-Use (TOCTOU) race conditions, and can be mapped to ATT&CK technique T1068, which involves exploiting vulnerabilities to gain elevated privileges. The attack vector typically involves container creation or initialization processes where the vulnerable code path is executed, allowing adversaries to manipulate file permissions through carefully crafted symbolic link placement.
Mitigation strategies for this vulnerability require immediate patching of LXD to version 0.19-0ubuntu5 or later, which resolves the race condition by ensuring proper synchronization between file system operations. System administrators should also implement strict container isolation policies and monitor for unauthorized symbolic link creation in container environments. Additional protective measures include deploying file system integrity monitoring tools, implementing mandatory access controls, and ensuring that container runtime environments have minimal privileges. The fix addresses the root cause by eliminating the unsafe concurrent file system operations and establishing proper atomic operations for uid/gid shifting, preventing attackers from exploiting the temporal window that previously enabled this type of privilege escalation attack. Organizations should also conduct comprehensive security audits of their containerized environments to identify any potential exploitation attempts and ensure that all container runtime components are kept up to date with security patches.