CVE-2026-22701

Vulnerability Summary

Title: Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock

Affected Component: filelock package - SoftFileLock class File: src/filelock/_soft.py lines 17-27 CWE: CWE-362, CWE-367, CWE-59

Description

A TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly.

The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service.

Attack Scenario

1. Lock attempts to acquire on /tmp/app.lock
2. Permission validation passes
3. [RACE WINDOW] - Attacker creates: ln -s /tmp/important.txt /tmp/app.lock
4. os.open() tries to create lock file
5. Lock operates on attacker-controlled target file or fails

Impact

What kind of vulnerability is it? Who is impacted?

This is a Time-of-Check-Time-of-Use (TOCTOU) race condition vulnerability affecting any application using SoftFileLock for inter-process synchronization.

Affected Users:

• Applications using filelock.SoftFileLock directly
• Applications using the fallback FileLock on systems without fcntl support (e.g., GraalPy)

Consequences:

• Silent lock acquisition failure - applications may not detect that exclusive resource access is not guaranteed
• Denial of Service - attacker can prevent lock file creation by maintaining symlink
• Resource serialization failures - multiple processes may acquire "locks" simultaneously
• Unintended file operations - lock could operate on attacker-controlled files

CVSS v4.0 Score: 5.6 (Medium) Vector: CVSS:4.0/AV:L/AT:L/PR:L/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N

Attack Requirements:

• Local filesystem access to the directory containing lock files
• Permission to create symlinks (standard for regular unprivileged users on Unix/Linux)
• Ability to time the symlink creation during the narrow race window

Patches

Has the problem been patched? What versions should users upgrade to?

Yes, the vulnerability has been patched by adding the O_NOFOLLOW flag to prevent symlink following during lock file creation.

Patched Version: Next release (commit: 255ed068bc85d1ef406e50a135e1459170dd1bf0)

Mitigation Details:

• The O_NOFOLLOW flag is added conditionally and gracefully degrades on platforms without support
• On platforms with O_NOFOLLOW support (most modern systems): symlink attacks are completely prevented
• On platforms without O_NOFOLLOW (e.g., GraalPy): TOCTOU window remains but is documented

Users should:

• Upgrade to the patched version when available
• For critical deployments, consider using UnixFileLock or WindowsFileLock instead of the fallback SoftFileLock

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

For users unable to update immediately:

1. Avoid SoftFileLock in security-sensitive contexts - use UnixFileLock or WindowsFileLock when available (these were already patched for CVE-2025-68146)

2. Restrict filesystem permissions - prevent untrusted users from creating symlinks in lock file directories:

   chmod 700 /path/to/lock/directory
   

3. Use process isolation - isolate untrusted code from lock file paths to prevent symlink creation

4. Monitor lock operations - implement application-level checks to verify lock acquisitions are successful before proceeding with critical operations

References

Are there any links users can visit to find out more?

-…

This package contains a single module, which implements a platform independent file locking mechanism for Python.

Security Fix(es):

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, a TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. This issue has been patched in version 3.20.3.(CVE-2026-22701)

This package contains a single module, which implements a platform independent file locking mechanism for Python.

Security Fix(es):

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, a TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. This issue has been patched in version 3.20.3.(CVE-2026-22701)

This package contains a single module, which implements a platform independent file locking mechanism for Python.

Security Fix(es):

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, a TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. This issue has been patched in version 3.20.3.(CVE-2026-22701)

This package contains a single module, which implements a platform independent file locking mechanism for Python.

Security Fix(es):

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, a TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. This issue has been patched in version 3.20.3.(CVE-2026-22701)

Vulnerability Summary

Title: Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock

Affected Component: filelock package - SoftFileLock class File: src/filelock/_soft.py lines 17-27 CWE: CWE-362, CWE-367, CWE-59

Description

A TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly.

The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service.

Attack Scenario

1. Lock attempts to acquire on /tmp/app.lock
2. Permission validation passes
3. [RACE WINDOW] - Attacker creates: ln -s /tmp/important.txt /tmp/app.lock
4. os.open() tries to create lock file
5. Lock operates on attacker-controlled target file or fails

Impact

What kind of vulnerability is it? Who is impacted?

This is a Time-of-Check-Time-of-Use (TOCTOU) race condition vulnerability affecting any application using SoftFileLock for inter-process synchronization.

Affected Users:

• Applications using filelock.SoftFileLock directly
• Applications using the fallback FileLock on systems without fcntl support (e.g., GraalPy)

Consequences:

• Silent lock acquisition failure - applications may not detect that exclusive resource access is not guaranteed
• Denial of Service - attacker can prevent lock file creation by maintaining symlink
• Resource serialization failures - multiple processes may acquire "locks" simultaneously
• Unintended file operations - lock could operate on attacker-controlled files

CVSS v4.0 Score: 5.6 (Medium) Vector: CVSS:4.0/AV:L/AT:L/PR:L/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N

Attack Requirements:

• Local filesystem access to the directory containing lock files
• Permission to create symlinks (standard for regular unprivileged users on Unix/Linux)
• Ability to time the symlink creation during the narrow race window

Patches

Has the problem been patched? What versions should users upgrade to?

Yes, the vulnerability has been patched by adding the O_NOFOLLOW flag to prevent symlink following during lock file creation.

Patched Version: Next release (commit: 255ed068bc85d1ef406e50a135e1459170dd1bf0)

Mitigation Details:

• The O_NOFOLLOW flag is added conditionally and gracefully degrades on platforms without support
• On platforms with O_NOFOLLOW support (most modern systems): symlink attacks are completely prevented
• On platforms without O_NOFOLLOW (e.g., GraalPy): TOCTOU window remains but is documented

Users should:

• Upgrade to the patched version when available
• For critical deployments, consider using UnixFileLock or WindowsFileLock instead of the fallback SoftFileLock

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

For users unable to update immediately:

1. Avoid SoftFileLock in security-sensitive contexts - use UnixFileLock or WindowsFileLock when available (these were already patched for CVE-2025-68146)

2. Restrict filesystem permissions - prevent untrusted users from creating symlinks in lock file directories:

   chmod 700 /path/to/lock/directory
   

3. Use process isolation - isolate untrusted code from lock file paths to prevent symlink creation

4. Monitor lock operations - implement application-level checks to verify lock acquisitions are successful before proceeding with critical operations

References

Are there any links users can visit to find out more?

-…

Root has patched CVE-2026-22701 in the rootio-filelock package for Root:PyPI. Multiple fixed versions available.

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, a TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. This issue has been patched in version 3.20.3.

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, a TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. This issue has been patched in version 3.20.3.

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, a TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. This issue has been patched in version 3.20.3.

These are all security issues fixed in the python311-filelock-3.20.3-1.1 package on the GA media of openSUSE Tumbleweed.

This update for python-filelock fixes the following issues:

• CVE-2025-68146: TOCTOU race condition may allow local attackers to corrupt or truncate arbitrary user files (bsc#1255244).
• CVE-2026-22701: TOCTOU race condition in the SoftFileLock implementation (bsc#1256457).

This update for python-filelock fixes the following issues:

• CVE-2025-68146: TOCTOU race condition may allow local attackers to corrupt or truncate arbitrary user files (bsc#1255244).
• CVE-2026-22701: TOCTOU race condition in the SoftFileLock implementation (bsc#1256457).

This update for python-filelock fixes the following issues:

• CVE-2026-22701: Fixed TOCTOU race condition in SoftFileLock implementation of he filelock package (bsc#1256457)

This update for python-filelock fixes the following issues:

• CVE-2025-68146: TOCTOU race condition may allow local attackers to corrupt or truncate arbitrary user files (bsc#1255244).
• CVE-2026-22701: TOCTOU race condition in the SoftFileLock implementation (bsc#1256457).

It was discovered that Filelock incorrectly handled symlinks in temp files. A local attacker could possibly use this issue to cause lock operations to fail or behave unexpectedly. (CVE-2026-22701)

It was discovered that the file locking implementation in the Filelock package contained a race condition. A local attacker could possibly use this to cause a denial of service or corrupt arbitrary user files. (CVE-2025-68146)