CVE-2026-25952: FreeRDP Use-After-Free Vulnerability

CVE-2026-25952 Overview

CVE-2026-25952 is a Use After Free vulnerability in FreeRDP, a free implementation of the Remote Desktop Protocol. Prior to version 3.23.0, the xf_SetWindowMinMaxInfo function dereferences a freed xfAppWindow pointer due to a race condition between the main thread and the RAIL channel thread. The vulnerability occurs because xf_rail_get_window in xf_rail_server_min_max_info returns an unprotected pointer from the railWindows hash table, and the main thread can concurrently delete the window (via a window delete order) while the RAIL channel thread is still using the pointer.

Critical Impact
This race condition could allow a remote attacker to trigger a denial of service condition by exploiting the use-after-free, potentially causing application crashes during Remote Desktop sessions.

Affected Products

FreeRDP versions prior to 3.23.0
FreeRDP X11 client implementations using RAIL (Remote Application Integrated Locally) functionality
Systems using FreeRDP for remote desktop connections with RemoteApp features

Discovery Timeline

2026-02-25 - CVE-2026-25952 published to NVD
2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-25952

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free). The flaw resides in the FreeRDP X11 client's handling of RAIL windows, which are used for RemoteApp functionality where individual remote applications appear as native windows on the local desktop.

The core issue stems from inadequate thread synchronization when accessing the railWindows hash table. The RAIL channel operates on its own thread to handle asynchronous messages from the RDP server, while the main X11 event loop thread processes window operations. When the server sends a window delete order, the main thread removes the window from the hash table and frees the associated xfAppWindow structure. However, if the RAIL thread is simultaneously processing a MinMaxInfo order for the same window, it may still hold a pointer to the now-freed window structure.

Root Cause

The root cause is a missing synchronization mechanism when accessing shared window pointers from the railWindows hash table. The xf_rail_get_window function returns a raw pointer without any reference counting or locking, creating a Time-of-Check Time-of-Use (TOCTOU) race condition. Between the time the RAIL thread retrieves the window pointer and the time it dereferences it in xf_SetWindowMinMaxInfo, the main thread can delete the window, invalidating the pointer.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. A malicious RDP server (or a man-in-the-middle attacker) could craft a specific sequence of RAIL protocol messages to trigger the race condition:

Send a legitimate window creation order to establish a RAIL window
Send concurrent MinMaxInfo orders while simultaneously sending a window delete order
Time the messages to maximize the probability of the race condition occurring

The following code snippet demonstrates the security patch that addresses this vulnerability by adding proper locking around appWindow access:

 	}
 	if (xfc->remote_app)
 	{
+		Window w = 0;
+		HashTable_Lock(xfc->railWindows);
 		if (!xfc->appWindow)
-		{
 			WLog_WARN(TAG, "xf_Pointer: Invalid appWindow");
-			return 0;
-		}
-		return xfc->appWindow->handle;
+		else
+			w = xfc->appWindow->handle;
+		HashTable_Unlock(xfc->railWindows);
+		return w;
 	}
 	else
 	{

Source: FreeRDP Commit History

The patch also ensures proper cleanup by calling xf_rail_return_window after window operations:

 	activate.windowId = (UINT32)appWindow->windowId;
 	activate.enabled = enabled;
 	xfc->rail->ClientActivate(xfc->rail, &activate);
+	xf_rail_return_window(appWindow);
 }
 
 BOOL xf_rail_send_client_system_command(xfContext* xfc, UINT64 windowId, UINT16 command)

Source: FreeRDP Commit History

Detection Methods for CVE-2026-25952

Indicators of Compromise

Unexpected FreeRDP client crashes during RemoteApp sessions
Segmentation fault signals (SIGSEGV) in FreeRDP process logs referencing xf_rail.c or xf_window.c
Core dumps showing stack traces involving xf_SetWindowMinMaxInfo or xf_rail_server_min_max_info

Detection Strategies

Monitor for abnormal termination of FreeRDP client processes with memory access violations
Implement application crash monitoring to detect patterns of use-after-free exploitation attempts
Review RDP session logs for unusual sequences of RAIL protocol messages, particularly rapid window create/delete operations

Monitoring Recommendations

Deploy endpoint detection to monitor FreeRDP process behavior and flag unexpected crashes
Enable core dump collection for forensic analysis of potential exploitation attempts
Monitor network traffic for suspicious RDP connections from untrusted servers

How to Mitigate CVE-2026-25952

Immediate Actions Required

Upgrade FreeRDP to version 3.23.0 or later immediately
If immediate patching is not possible, avoid connecting to untrusted RDP servers
Disable RAIL/RemoteApp functionality if not required by using the /without-rail build option or avoiding RemoteApp connections

Patch Information

The vulnerability has been fixed in FreeRDP version 3.23.0. The fix implements proper hash table locking using HashTable_Lock() and HashTable_Unlock() around all accesses to the railWindows hash table, ensuring thread-safe window pointer access. The patch also introduces xf_rail_return_window() to properly release window references after use.

For detailed information, refer to the FreeRDP Security Advisory and the FreeRDP Commit History.

Workarounds

Disable RemoteApp/RAIL functionality by avoiding connections that use remote applications integrated locally
Connect only to trusted, known RDP servers until patches can be applied
Use network segmentation to limit exposure of FreeRDP clients to potentially malicious RDP servers

bash

# Build FreeRDP without RAIL support as a temporary workaround
cmake -DWITH_RAIL=OFF ..
make && make install