CVE-2026-25997: FreeRDP Use-After-Free Vulnerability

CVE-2026-25997 Overview

CVE-2026-25997 is a Use After Free vulnerability in FreeRDP, the free implementation of the Remote Desktop Protocol. The vulnerability exists in the X11 clipboard functionality where the xf_clipboard_format_equal function reads freed lastSentFormats memory. This occurs because xf_cliprdr_formats_free (called from the cliprdr channel thread during auto-reconnect) frees the array while the X11 event thread concurrently iterates it in xf_clipboard_changed, creating a race condition that triggers a heap use after free condition.

Critical Impact
This race condition in clipboard handling could allow an attacker to cause denial of service through application crashes, or potentially achieve further exploitation through memory corruption during FreeRDP auto-reconnect operations.

Affected Products

FreeRDP versions prior to 3.23.0
FreeRDP X11 client with clipboard functionality enabled
Systems utilizing FreeRDP for RDP connections with auto-reconnect enabled

Discovery Timeline

2026-02-25 - CVE-2026-25997 published to NVD
2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-25997

Vulnerability Analysis

This vulnerability represents a classic race condition leading to a use after free condition in multi-threaded software. The FreeRDP X11 client maintains clipboard format information in the lastSentFormats array, which is shared between two threads: the cliprdr channel thread and the X11 event thread.

During an auto-reconnect scenario, the cliprdr channel thread calls xf_cliprdr_formats_free to clean up the clipboard formats, which frees the lastSentFormats array. However, the X11 event thread may be concurrently iterating over this same array in the xf_clipboard_changed function via xf_clipboard_format_equal. Without proper synchronization, the X11 thread can access memory that has already been freed, resulting in a heap use after free condition (CWE-416).

The vulnerability is network-accessible as it can be triggered during RDP connection auto-reconnect sequences, though exploitation requires specific timing conditions to be met.

Root Cause

The root cause is the lack of proper thread synchronization between the cliprdr channel thread and the X11 event thread when accessing shared clipboard format data. The lastSentFormats array is accessed by multiple threads without adequate locking mechanisms, allowing one thread to free memory while another thread is still reading from it.

Attack Vector

The vulnerability can be triggered through the following sequence:

Establish an RDP connection using FreeRDP X11 client with clipboard functionality enabled
Trigger an auto-reconnect scenario (network disruption or server-initiated reconnect)
During reconnect, the cliprdr thread frees clipboard formats while X11 events are being processed
The X11 event thread attempts to access the freed lastSentFormats memory
This results in a use after free condition that could cause a crash or potentially be exploited for further memory corruption

The vulnerability is accessible over the network (attack vector: Network) but requires specific timing conditions during auto-reconnect operations.

// Security patch demonstrating the fix - adding thread synchronization
 {
 	WINPR_ASSERT(clipboard);

+	/* Synchronize RDP/X11 thread with channel thread */
+	xf_lock_x11(clipboard->xfc);
 	xf_cliprdr_free_formats(clipboard->lastSentFormats, clipboard->lastSentNumFormats);
+	xf_unlock_x11(clipboard->xfc);
+
 	clipboard->lastSentFormats = NULL;
 	clipboard->lastSentNumFormats = 0;
 }

Source: FreeRDP Commit Changes

Detection Methods for CVE-2026-25997

Indicators of Compromise

FreeRDP client crashes during auto-reconnect operations with clipboard activity
Segmentation faults or memory access violations in xf_clipboard_format_equal or xf_clipboard_changed functions
Core dumps indicating heap corruption in clipboard-related memory regions
Application crashes specifically when clipboard operations occur during RDP reconnection

Detection Strategies

Monitor FreeRDP client processes for unexpected crashes or segmentation faults during reconnect operations
Implement memory debugging tools (Valgrind, AddressSanitizer) in development/testing environments to detect use after free conditions
Review system logs for FreeRDP crash reports that reference clipboard functions (xf_cliprdr.c)
Deploy endpoint detection to identify abnormal termination patterns in RDP client applications

Monitoring Recommendations

Enable application crash monitoring for FreeRDP processes on endpoints
Configure logging to capture RDP session reconnection events and clipboard operations
Set up alerts for repeated FreeRDP client crashes that may indicate exploitation attempts
Monitor for unusual network reconnection patterns that could be used to trigger the vulnerability

How to Mitigate CVE-2026-25997

Immediate Actions Required

Upgrade FreeRDP to version 3.23.0 or later immediately
If immediate upgrade is not possible, consider temporarily disabling clipboard redirection in FreeRDP connections
Review and update all systems using FreeRDP X11 clients to the patched version
Assess exposure of systems using vulnerable FreeRDP versions in production environments

Patch Information

The vulnerability is fixed in FreeRDP version 3.23.0. The fix adds proper thread synchronization using xf_lock_x11() and xf_unlock_x11() mutex operations around the clipboard format cleanup code to prevent the race condition.

For detailed patch information, see the FreeRDP security fix commit and the GitHub Security Advisory.

Workarounds

Disable clipboard redirection by using the /clipboard:- option when launching FreeRDP connections
Disable auto-reconnect functionality using /auto-reconnect:- if clipboard functionality is required
Restrict FreeRDP usage to controlled network environments where reconnection triggers are less likely
Consider using alternative RDP clients until upgrade to version 3.23.0 is completed

bash

# Disable clipboard redirection as a temporary workaround
xfreerdp /v:server.example.com /u:username /clipboard:-

# Alternative: Disable auto-reconnect functionality
xfreerdp /v:server.example.com /u:username /auto-reconnect:-