CVE-2026-25959: FreeRDP Use After Free Vulnerability

CVE-2026-25959 Overview

CVE-2026-25959 is a Use-After-Free vulnerability affecting FreeRDP, a free implementation of the Remote Desktop Protocol. The flaw exists in the X11 clipboard redirection functionality where the xf_cliprdr_provide_data_ function passes freed pDstData to XChangeProperty. This occurs due to a race condition between the cliprdr channel thread and the X11 event thread, where clipboard data is used without proper locking while being concurrently freed.

Critical Impact

This heap use-after-free vulnerability can lead to denial of service conditions when exploited through concurrent clipboard operations in FreeRDP client sessions.

Affected Products

• FreeRDP versions prior to 3.23.0
• FreeRDP X11 client with clipboard redirection enabled
• Systems using FreeRDP for Remote Desktop Protocol connections

Discovery Timeline

• February 25, 2026 - CVE-2026-25959 published to NVD
• February 26, 2026 - Last updated in NVD database

Technical Details for CVE-2026-25959

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free). The core issue lies in the lack of synchronization between two concurrent threads in the FreeRDP X11 client's clipboard handling code. When clipboard data is being processed by the cliprdr channel thread via xf_cliprdr_server_format_data_response, the X11 event thread can simultaneously invoke xf_cliprdr_clear_cached_data which calls HashTable_Clear. This clearing operation frees the cached data through xf_cached_data_free, while the cliprdr thread still holds a reference to and attempts to use the now-freed memory.

The vulnerability is network-accessible as it can be triggered through Remote Desktop Protocol clipboard synchronization operations between client and server.

Root Cause

The root cause is a classic race condition stemming from missing lock acquisition when accessing shared clipboard cache data. The xf_cliprdr_provide_data_ function retrieves cached data from a hash table and passes it to XChangeProperty without holding a lock on the cache. Concurrently, another thread can clear the cache, freeing the memory that is still being referenced. This TOCTOU (Time-of-Check Time-of-Use) condition results in heap use-after-free when the freed pointer is dereferenced.

Attack Vector

The vulnerability can be triggered through normal clipboard operations during an RDP session. An attacker controlling a malicious RDP server could potentially manipulate clipboard synchronization timing to trigger the race condition, causing the client to crash. The attack requires network access but no user interaction beyond establishing an RDP connection.

The fix implements proper locking around cache access to prevent concurrent modification:

 			DEBUG_CLIPRDR("formatId: 0x%08" PRIx32 ", dstFormatId: 0x%08" PRIx32 "", formatId,
 			              dstFormatId);
 
+			wHashTable* table = clipboard->cachedData;
+			if (rawTransfer)
+				table = clipboard->cachedRawData;
+
+			HashTable_Lock(table);
+
 			if (!rawTransfer)
-				cached_data = HashTable_GetItemValue(clipboard->cachedData,
-				                                     format_to_cache_slot(dstFormatId));
+				cached_data = HashTable_GetItemValue(table, format_to_cache_slot(dstFormatId));
 			else
-				cached_data = HashTable_GetItemValue(clipboard->cachedRawData,
-				                                     format_to_cache_slot(formatId));
+				cached_data = HashTable_GetItemValue(table, format_to_cache_slot(formatId));
 
 			DEBUG_CLIPRDR("hasCachedData: %u, rawTransfer: %d", cached_data ? 1u : 0u, rawTransfer);

Source: FreeRDP Commit Details

Detection Methods for CVE-2026-25959

Indicators of Compromise

• Unexpected FreeRDP client crashes during clipboard operations
• Segmentation faults or heap corruption errors in FreeRDP process logs
• Application crash dumps showing xf_cliprdr_provide_data_ or XChangeProperty in the stack trace

Detection Strategies

• Monitor for abnormal termination of FreeRDP processes with memory corruption signatures
• Implement application crash monitoring to detect heap use-after-free conditions in xfreerdp clients
• Review system logs for recurring FreeRDP crashes during active clipboard synchronization

Monitoring Recommendations

• Enable core dump collection for FreeRDP processes to capture crash details for forensic analysis
• Deploy endpoint detection rules to identify memory corruption patterns in RDP client applications
• Monitor RDP session stability metrics to detect potential exploitation attempts causing repeated crashes

How to Mitigate CVE-2026-25959

Immediate Actions Required

• Upgrade FreeRDP to version 3.23.0 or later which contains the security fix
• If immediate upgrade is not possible, consider disabling clipboard redirection in FreeRDP sessions
• Review and apply the security patch from the FreeRDP Security Advisory

Patch Information

The vulnerability is addressed in FreeRDP version 3.23.0. The fix introduces proper locking mechanisms using HashTable_Lock() to synchronize access to clipboard cache data between the cliprdr channel thread and X11 event thread. The patch commit (d3e8b3b9365be96a4f11dda149d71b3287227d0a) ensures that cache lookups are protected against concurrent modifications.

Organizations should obtain the patched version through their package manager or by building from the official FreeRDP repository. See the FreeRDP Commit Details for the complete patch implementation.

Workarounds

• Disable clipboard redirection by using the /clipboard:- command-line option when launching xfreerdp
• Configure FreeRDP to operate without clipboard synchronization in environments where this feature is not required
• Isolate FreeRDP client systems and limit connections to trusted RDP servers only

# Disable clipboard redirection in FreeRDP
xfreerdp /v:server.example.com /u:username /clipboard:-

# Alternative: Configure via FreeRDP settings file
# Set ClipboardUseSelection=false in ~/.config/freerdp/