CVE-2026-25942: FreeRDP Buffer Overflow Vulnerability

CVE-2026-25942 Overview

CVE-2026-25942 is an Out-of-Bounds Read vulnerability in FreeRDP, a free implementation of the Remote Desktop Protocol. Prior to version 3.23.0, the xf_rail_server_execute_result function indexes the global error_code_names[] array (7 elements, indices 0–6) with an unchecked execResult->execResult value received from the server. This allows an out-of-bounds read when the server sends an execResult value of 7 or greater, potentially leading to information disclosure or denial of service.

Critical Impact

A malicious RDP server can trigger an out-of-bounds memory read on connected FreeRDP clients by sending a crafted RAIL execute result with an invalid error code index.

Affected Products

• FreeRDP versions prior to 3.23.0
• FreeRDP X11 client implementations using RAIL (Remote Application Integrated Locally)
• Systems with FreeRDP client libraries linked to vulnerable versions

Discovery Timeline

• February 25, 2026 - CVE-2026-25942 published to NVD
• February 25, 2026 - Last updated in NVD database

Technical Details for CVE-2026-25942

Vulnerability Analysis

This vulnerability exists in the FreeRDP Remote Application Integrated Locally (RAIL) channel implementation. The RAIL channel allows remote applications to run on the local desktop, integrating with the client's window manager. When processing server execute results, the xf_rail_server_execute_result function in client/X11/xf_rail.c uses the server-provided execResult value to directly index into a static array of error code names without performing bounds validation.

The error_code_names[] array contains exactly 7 elements (indices 0 through 6), corresponding to valid RAIL execution error codes. However, because the execResult value comes directly from the remote server without validation, a malicious server can supply any value, causing the client to read memory outside the bounds of this array.

Root Cause

The root cause is improper input validation of server-controlled data before using it as an array index. The execResult->execResult field received from the RDP server is used directly to index the error_code_names[] array without checking whether the value falls within valid bounds (0-6). This is classified as CWE-125: Out-of-bounds Read.

Attack Vector

The attack requires a malicious or compromised RDP server to exploit this vulnerability. When a FreeRDP client connects to such a server and the RAIL channel is active, the server can send a crafted Server Execute Result PDU containing an execResult value of 7 or greater. Upon receiving this PDU, the client attempts to access memory beyond the error_code_names[] array boundaries, potentially exposing sensitive memory contents or causing the client application to crash.

// Vulnerable code pattern (before fix)
// The error_code_names array has only 7 elements (indices 0-6)
static const char* error_code_names[] = { "RAIL_EXEC_S_OK",
                                          "RAIL_EXEC_E_HOOK_NOT_LOADED",
                                          "RAIL_EXEC_E_DECODE_FAILED",
                                          "RAIL_EXEC_E_NOT_IN_ALLOWLIST",
                                          "RAIL_EXEC_E_FILE_NOT_FOUND",
                                          "RAIL_EXEC_E_FAIL",
                                          "RAIL_EXEC_E_SESSION_LOCKED" };

// Server-controlled execResult used without bounds checking
// If execResult >= 7, this causes out-of-bounds read
error_name = error_code_names[execResult->execResult];

Source: FreeRDP Commit Log

The fix replaces the array-based lookup with a safe switch-case function that handles all valid error codes and returns a safe default for unknown values:

// Fixed code - safe string conversion function
static const char* error_code2str(UINT32 code)
{
#define EVCASE(x) \
	case x:       \
		return #x
	switch (code)
	{
		EVCASE(RAIL_EXEC_S_OK);
		EVCASE(RAIL_EXEC_E_HOOK_NOT_LOADED);
		EVCASE(RAIL_EXEC_E_DECODE_FAILED);
		EVCASE(RAIL_EXEC_E_NOT_IN_ALLOWLIST);
		// Additional cases handled safely...
	}
	// Returns safe default for unknown codes
}

Source: FreeRDP Commit Log

Detection Methods for CVE-2026-25942

Indicators of Compromise

• Unexpected FreeRDP client crashes during RAIL session initialization or execution
• Memory access violations or segmentation faults in the xf_rail_server_execute_result function
• Unusual RDP traffic containing RAIL Server Execute Result PDUs with error codes outside the valid range (0-6)

Detection Strategies

• Monitor FreeRDP client processes for unexpected termination or crash events during active RDP sessions
• Implement network-level inspection for RDP RAIL channel traffic with anomalous execute result values
• Review system logs for memory access violations originating from FreeRDP client binaries
• Deploy application-level monitoring to detect buffer over-read attempts in RDP client applications

Monitoring Recommendations

• Enable verbose logging in FreeRDP clients to capture RAIL channel communication details
• Implement endpoint detection rules for FreeRDP process anomalies and memory violations
• Monitor for connections to untrusted or unknown RDP servers that may be attacker-controlled
• Track FreeRDP version deployments across the environment to identify vulnerable installations

How to Mitigate CVE-2026-25942

Immediate Actions Required

• Upgrade FreeRDP to version 3.23.0 or later immediately
• Audit all systems using FreeRDP client functionality and create an inventory of installed versions
• Restrict FreeRDP connections to trusted RDP servers only until patching is complete
• Consider disabling RAIL functionality if remote application integration is not required

Patch Information

The vulnerability is fixed in FreeRDP version 3.23.0. The fix replaces the unsafe array indexing mechanism with a safer switch-case based string conversion function (error_code2str) that properly handles out-of-range values without accessing invalid memory. The security patch is available via the FreeRDP Commit Log. For additional details, refer to the GitHub Security Advisory.

Workarounds

• Limit FreeRDP usage to connections with trusted and verified RDP servers
• Disable RAIL channel support if remote application integration is not required for your use case
• Implement network segmentation to restrict RDP client access to known-good server infrastructure
• Use application sandboxing or containerization for FreeRDP clients to limit the impact of potential exploitation

# Configuration example - Disable RAIL channel if not needed
# Add to FreeRDP connection command line
xfreerdp /v:server.example.com /u:username -rail

# Or configure in connection file to disable RAIL
# RemoteApplicationMode=0