CVE-2026-112909 in filelock
Summary
by MITRE • 10/20/2026
filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. This issue is fixed in version 4.2.1.
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Analysis
by VulDB Data Team • 10/20/2026
The vulnerability identified in the Python library filelock prior to version 4.2.1 represents a significant integrity and confidentiality risk stemming from improper handling of filesystem operations during lock acquisition on Unix-based systems. The core technical flaw resides within the UnixFileLock implementation, which relies on standard POSIX system calls such as rename() and open() with specific flags to establish exclusive file locks. In environments where local users possess permissions to rename files or create hard links into the target directory under a permissive policy, an attacker can exploit race conditions or symlink-like behaviors inherent in how these operations interact with existing filesystem entries. Specifically, if a regular file is renamed onto the lock path while another user holds actual rename permission over that location, the locking mechanism may inadvertently truncate the content of that pre-existing file rather than creating a new empty lock file as intended. This behavior occurs because certain implementations of open() combined with O_TRUNC or similar flags will clear the contents of an existing file if it is opened for writing, even if the intent was merely to create a new inode for locking purposes.
This truncation effect poses severe operational risks in multi-tenant environments or shared hosting scenarios where multiple users have write access to common directories used by applications relying on this library. An attacker with local user privileges can deliberately place a sensitive file at the expected lock path location before an application attempts to acquire the lock. When the vulnerable version of filelock executes its locking logic, it will truncate that sensitive data, effectively causing a denial of service through data destruction or potentially leading to information loss if critical configuration files, logs, or user-generated content are targeted. Furthermore, in affected versions ranging from 3.10.2 up to but not including 4.2.1, the vulnerability extends beyond simple truncation. If an explicit file mode is requested during lock creation, the library may also alter the permissions of the target file. This allows a local attacker who can influence the path resolution or pre-existence of files at the lock location to modify access control lists and permission bits on arbitrary files they have write access to rename into that directory. Such unauthorized modification of file modes constitutes an escalation of privilege vector, potentially allowing further exploitation by altering read/write/execute permissions for other users or services running under different security contexts.
From a classification perspective, this vulnerability aligns with CWE-367, Time-of-check Time-of-use (TOCTOU) race conditions, as the library checks for file existence and then performs operations that assume no intervening changes have occurred by another process or user. Additionally, it relates to CWE-20 Improper Input Validation regarding the handling of filesystem paths and permissions, and CWE-732 Incorrect Permission Assignment for Critical Resource due to the unintended modification of file modes. In terms of adversary tactics, this flaw supports ATT&CK technique T1485 Data Destruction, as an attacker can reliably delete or corrupt data by exploiting the truncation behavior during lock acquisition. It also touches upon T1068 Exploitation for Privilege Escalation if the permission modification aspect is leveraged to weaken security controls on other files within the same directory structure.
Mitigation strategies primarily involve upgrading the filelock library to version 4.2.1 or later, where these issues have been resolved through stricter validation of filesystem states and safer handling of open flags that prevent unintended truncation or permission changes. For organizations unable to immediately upgrade, defensive coding practices should be adopted within applications using this library. Developers can implement pre-checks for file existence and permissions before invoking lock acquisition routines, although care must be taken as these checks themselves are subject to TOCTOU vulnerabilities if not atomic with the subsequent operation. A more robust approach involves utilizing dedicated directories with restricted write access owned by the application user alone, ensuring that no other local users can place files in the path where locks are created. Additionally, configuring filesystems with stricter hard-link policies and disabling unnecessary rename permissions for shared directories can reduce the attack surface available to local attackers seeking to exploit this vulnerability. Regular auditing of file system permissions and monitoring for unexpected truncation events in application logs can also aid in early detection of exploitation attempts.