CVE-2025-46805: GNU Screen Race Condition Vulnerability

CVE-2025-46805 Overview

CVE-2025-46805 is a Time-of-Check Time-of-Use (TOCTOU) race condition vulnerability affecting GNU Screen, a popular terminal multiplexer. The vulnerability exists in Screen version 5.0.0 and older version 4 releases when the application is installed with setuid-root privileges. This race condition potentially allows attackers to send SIGHUP and SIGCONT signals to privileged processes, which could result in denial of service conditions affecting critical system processes.

Critical Impact

Local attackers can exploit this TOCTOU race condition to send unauthorized signals to privileged processes when Screen is installed setuid-root, potentially disrupting system operations.

Affected Products

• GNU Screen version 5.0.0
• GNU Screen version 4.x releases (older versions)
• Systems with Screen installed with setuid-root permissions

Discovery Timeline

• 2025-05-26 - CVE-2025-46805 published to NVD
• 2025-05-28 - Last updated in NVD database

Technical Details for CVE-2025-46805

Vulnerability Analysis

This vulnerability is classified as CWE-367 (Time-of-Check Time-of-Use Race Condition). The flaw exists in the signal handling mechanism within GNU Screen when operating with elevated privileges. When Screen is installed with the setuid-root bit set, it runs with root privileges to manage terminal sessions across different users.

The TOCTOU race condition occurs during the validation and use of process identifiers or permissions. In the window between checking a condition (such as process ownership or permissions) and using the result of that check, an attacker can manipulate the state to influence which processes receive signals. This timing window allows an unprivileged local user to potentially target privileged processes with SIGHUP (hangup) or SIGCONT (continue) signals.

The attack requires local access and precise timing to exploit the race window, which limits the practical exploitability but still poses a significant availability risk to systems where Screen is deployed with setuid-root privileges.

Root Cause

The root cause is a classic TOCTOU vulnerability in the signal delivery path. The application performs a security check on target processes at one point in time but uses that information at a later point when conditions may have changed. This temporal gap between checking process permissions and actually sending signals creates an exploitable window where an attacker can substitute or manipulate the target process.

Attack Vector

The attack requires local access to the system with a valid user account. The attacker must exploit a timing window during Screen's signal handling operations to redirect signals to unintended privileged processes. Successful exploitation requires:

1. Screen installed with setuid-root permissions on the target system
2. Local user access to execute Screen
3. Precise timing to win the race condition during signal operations
4. A target privileged process susceptible to SIGHUP or SIGCONT disruption

The vulnerability mechanism involves exploiting the timing gap between Screen's permission validation and signal delivery. When Screen checks whether a user has permission to send a signal to a process and subsequently sends that signal, an attacker can manipulate the target process identifier or related state within this window. For detailed technical analysis, refer to the OpenWall OSS-Security Discussion.

Detection Methods for CVE-2025-46805

Indicators of Compromise

• Unexpected termination or restarts of privileged system processes
• Anomalous SIGHUP or SIGCONT signals received by root-owned processes
• Unusual Screen process activity patterns from non-privileged users
• System log entries indicating signal delivery to processes outside normal operation

Detection Strategies

• Monitor for unusual signal patterns using audit frameworks like auditd to track signal delivery events
• Implement process behavior monitoring to detect privileged processes receiving unexpected signals
• Review system logs for Screen-related activity correlating with service disruptions
• Deploy endpoint detection solutions capable of identifying race condition exploitation attempts

Monitoring Recommendations

• Configure auditd rules to log all kill() system calls involving signals 1 (SIGHUP) and 18 (SIGCONT)
• Monitor Screen process execution patterns, especially from non-privileged accounts
• Implement alerting for unexpected restarts of critical system daemons
• Review setuid-root binary inventory and assess necessity of elevated privileges for Screen

How to Mitigate CVE-2025-46805

Immediate Actions Required

• Audit systems for Screen installations with setuid-root permissions using find / -perm -4000 -name "screen" 2>/dev/null
• Remove setuid-root bit from Screen installations where elevated privileges are not strictly required
• Upgrade to patched versions of GNU Screen when available from your distribution
• Review the SUSE Bugzilla entry for vendor-specific guidance

Patch Information

Organizations should monitor their distribution's security advisories for patches addressing CVE-2025-46805. SUSE has acknowledged this vulnerability in their Bugzilla tracking system. Contact your distribution vendor for specific patch availability and installation instructions.

Workarounds

• Remove the setuid-root bit from the Screen binary: chmod u-s /usr/bin/screen
• Consider alternative terminal multiplexers such as tmux that do not require setuid-root privileges
• Implement mandatory access control policies (SELinux/AppArmor) to restrict Screen's signal capabilities
• Limit Screen access to trusted users through group membership restrictions

# Remove setuid-root permission from Screen
chmod u-s /usr/bin/screen

# Verify the permission change
ls -la /usr/bin/screen

# Alternative: Restrict Screen to specific group
chgrp screen /usr/bin/screen
chmod 750 /usr/bin/screen