CVE-2026-41051: csync2 Race Condition Vulnerability

CVE-2026-41051 Overview

CVE-2026-41051 affects csync2, a cluster synchronization tool used to replicate file changes across multiple hosts. When the application is compiled with C99 or later, it creates and uses insecure temporary directories. The flaw enables Time-of-Check Time-of-Use (TOCTOU) attacks against those temporary directories [CWE-367]. A local authenticated attacker can exploit the race window to manipulate filesystem state between the check and use operations. The vulnerability is tracked under the SUSE Bug Report CVE-2026-41051.

Critical Impact
A local attacker with low privileges can leverage a race condition against csync2 temporary directories to interfere with file synchronization integrity and availability on the target host.

Affected Products

csync2 builds compiled with C99 or later standards
Linux distributions packaging affected csync2 releases (tracked via SUSE)
Cluster nodes relying on csync2 for file replication

Discovery Timeline

2026-05-13 - CVE-2026-41051 published to NVD
2026-05-13 - Last updated in NVD database

Technical Details for CVE-2026-41051

Vulnerability Analysis

The defect is a TOCTOU race condition rooted in how csync2 provisions and accesses temporary directories on the local filesystem. When compiled against C99 or newer language standards, the resulting binary follows code paths that perform separate check and use operations on temporary directory entries. An attacker with local shell access can race the process by manipulating directory entries between those two operations.

The issue maps to [CWE-367: Time-of-Check Time-of-Use Race Condition]. Exploitation requires local access, low privileges, and user interaction to trigger the vulnerable csync2 invocation. Successful exploitation does not disclose data or alter integrity directly, but it produces a high-impact availability outcome on the affected host. The EPSS probability sits very low at the time of publication, indicating no observed in-the-wild exploitation.

Root Cause

The root cause is reliance on non-atomic filesystem operations against world-writable temporary locations. The C99-or-later compilation path uses temporary directory handling that does not safely bind a directory handle before subsequent file operations. This gap creates an exploitable window between validation and use. See the SUSE Bug Report CVE-2026-41051 for upstream analysis.

Attack Vector

A local user runs preparatory processes that monitor csync2 activity and race the temporary directory operations. By substituting directory contents or replacing entries with symbolic links during the race window, the attacker forces csync2 to operate on attacker-controlled paths. The result is disruption of synchronization tasks and a denial-of-service condition for the cluster service relying on csync2.

No verified public exploit code is available for CVE-2026-41051.
Refer to the SUSE Bugzilla entry for upstream technical details.

Detection Methods for CVE-2026-41051

Indicators of Compromise

Unexpected symbolic links or attacker-owned files appearing inside csync2 temporary directories during synchronization runs.
csync2 process failures, aborted transactions, or corrupted state files on cluster nodes.
Repeated short-lived processes by non-root users that open or stat csync2 temporary paths.

Detection Strategies

Audit filesystem activity in temporary directories used by csync2 with auditd rules targeting open, rename, and symlink syscalls.
Correlate csync2 exit codes and error logs with concurrent local user activity to surface race attempts.
Compare temporary directory ownership and permissions against expected baselines after each csync2 run.

Monitoring Recommendations

Forward csync2 logs and auditd events to a centralized analytics platform for cross-host correlation.
Alert on any non-service-account access to csync2 working directories on cluster members.
Track failure rates of cluster file synchronization jobs to detect availability impact early.

How to Mitigate CVE-2026-41051

Immediate Actions Required

Restrict shell access on cluster nodes running csync2 to trusted administrators only.
Verify that the system temporary directory enforces the sticky bit and is not shared with low-privilege users.
Apply the latest distribution package update for csync2 once vendors publish a fixed build.

Patch Information

No fixed version reference is available in NVD at publication time. Track the SUSE Bug Report CVE-2026-41051 for upstream patch availability and distribution backports. Apply vendor updates through standard package management once released.

Workarounds

Configure csync2 to use a dedicated temporary directory owned by the service account with mode 0700.
Set the TMPDIR environment variable for the csync2 process to a path outside world-writable locations.
Limit interactive logins on cluster nodes to reduce the local attacker population that could win the race.

bash

# Configuration example: dedicated private temp directory for csync2
sudo install -d -o csync2 -g csync2 -m 0700 /var/lib/csync2/tmp
sudo systemctl edit csync2.service
# Add the following override:
# [Service]
# Environment=TMPDIR=/var/lib/csync2/tmp
sudo systemctl daemon-reload
sudo systemctl restart csync2.service