CVE-2026-25701: openSUSE sdbootutil Path Traversal Flaw

CVE-2026-25701 Overview

An Insecure Temporary File vulnerability has been identified in openSUSE sdbootutil that allows local users to pre-create a directory to achieve multiple attack effects. This vulnerability enables attackers to gain access to possible private information found in /var/lib/pcrlock.d, manipulate backup data in /tmp/pcrlock.d.bak violating data integrity should it be restored, and overwrite protected system files with data from /var/lib/pcrlock.d by placing symlinks to existing files in the directory tree in /tmp/pcrlock.d.bak.

Critical Impact

Local attackers can exploit insecure temporary file handling to access sensitive PCR lock data, manipulate backup integrity, and potentially overwrite protected system files through symlink attacks.

Affected Products

• openSUSE sdbootutil versions before commit 5880246d3a02642dc68f5c8cb474bf63cdb56bca

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-25701 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-25701

Vulnerability Analysis

This vulnerability is classified as CWE-377 (Insecure Temporary File), a category of file system vulnerabilities where applications create or use temporary files in an insecure manner. The sdbootutil utility, which handles secure boot configuration on openSUSE systems, fails to properly secure its temporary file operations involving the PCR lock backup directory.

The vulnerability requires local access to exploit, meaning an attacker must already have some level of access to the target system. However, this does not diminish its severity, as the vulnerability can lead to privilege escalation through symlink attacks and potential compromise of system integrity.

Root Cause

The root cause of this vulnerability lies in the insecure handling of temporary files and directories by sdbootutil. The utility does not properly validate or secure the creation and use of the /tmp/pcrlock.d.bak directory, allowing local users to pre-create this directory structure before sdbootutil attempts to use it. This race condition (TOCTOU - Time-of-Check Time-of-Use) enables attackers to:

1. Create the backup directory structure in advance
2. Place symbolic links within the directory pointing to sensitive system files
3. Wait for sdbootutil to perform backup or restore operations
4. Exploit the symlinks to read or overwrite protected files

Attack Vector

The attack requires local access to the system and exploits the predictable nature of temporary file paths used by sdbootutil. An attacker with low privileges can leverage this vulnerability through the following attack chain:

Information Disclosure Attack: By pre-creating the directory structure, an attacker can position themselves to capture sensitive PCR lock data from /var/lib/pcrlock.d when backups are created.

Data Integrity Attack: Manipulating contents in /tmp/pcrlock.d.bak allows corruption or modification of backup data, which could lead to system instability or security bypass when restored.

Symlink Attack: The most severe attack vector involves placing symlinks within /tmp/pcrlock.d.bak that point to protected system files. When sdbootutil performs restore operations, it may follow these symlinks and overwrite critical files with attacker-controlled data from /var/lib/pcrlock.d.

Detection Methods for CVE-2026-25701

Indicators of Compromise

• Unexpected creation or modification of /tmp/pcrlock.d.bak directory by non-root users
• Presence of symbolic links within /tmp/pcrlock.d.bak pointing to system files outside the expected directory tree
• Unusual file access patterns to /var/lib/pcrlock.d by unauthorized processes
• Modifications to protected system files that correlate with sdbootutil execution times

Detection Strategies

• Monitor file system activity in /tmp for creation of pcrlock.d.bak directory by processes other than sdbootutil
• Implement file integrity monitoring (FIM) on /var/lib/pcrlock.d and critical system files
• Use auditd rules to track symlink creation in /tmp directories with alerts for suspicious targets
• Configure SentinelOne Singularity Platform to detect TOCTOU race condition patterns and symlink attacks

Monitoring Recommendations

• Deploy real-time file system monitoring on temporary directories used by boot utilities
• Enable audit logging for all sdbootutil executions and associated file operations
• Configure alerts for any symlink creation in /tmp/pcrlock.d.bak or related paths
• Review system logs for failed or unexpected restore operations from sdbootutil

How to Mitigate CVE-2026-25701

Immediate Actions Required

• Update sdbootutil to a version containing commit 5880246d3a02642dc68f5c8cb474bf63cdb56bca or later
• Audit systems for existing unauthorized directories at /tmp/pcrlock.d.bak
• Remove any suspicious symlinks found in temporary directory structures
• Restrict local user access on critical systems until patches can be applied

Patch Information

The vulnerability has been addressed in sdbootutil commit 5880246d3a02642dc68f5c8cb474bf63cdb56bca. Administrators should update to a version of sdbootutil that includes this fix. For detailed information about the vulnerability and patch status, refer to SUSE Bug Report #1258241.

Workarounds

• Restrict write access to /tmp for untrusted local users using mount options or access controls
• Implement mandatory access control (MAC) policies using AppArmor or SELinux to confine sdbootutil file operations
• Create and protect the /tmp/pcrlock.d.bak directory with root-only permissions before sdbootutil execution
• Monitor and alert on any attempts to create symlinks in temporary directories

# Temporary mitigation: Secure the backup directory with restricted permissions
sudo mkdir -p /tmp/pcrlock.d.bak
sudo chown root:root /tmp/pcrlock.d.bak
sudo chmod 700 /tmp/pcrlock.d.bak
# Set immutable attribute to prevent modification
sudo chattr +i /tmp/pcrlock.d.bak