CVE-2026-26271: FreeRDP Buffer Overflow Vulnerability

CVE-2026-26271 Overview

CVE-2026-26271 is a buffer overread vulnerability in FreeRDP, a free implementation of the Remote Desktop Protocol. The vulnerability exists in the freerdp_image_copy_from_icon_data() function located in libfreerdp/codec/color.c. This security flaw can be triggered by crafted RDP Window Icon (TS_ICON_INFO) data sent from a malicious RDP server or through a man-in-the-middle attack scenario.

Critical Impact

Network-reachable buffer overread allowing attackers to potentially leak sensitive memory contents or cause application crashes when processing malicious icon data from RDP connections.

Affected Products

• FreeRDP versions prior to 3.23.0
• Systems using FreeRDP client to connect to untrusted RDP servers
• Network environments susceptible to man-in-the-middle attacks on RDP connections

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-26271 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-26271

Vulnerability Analysis

This vulnerability is classified as CWE-126 (Buffer Over-read), where the freerdp_image_copy_from_icon_data() function fails to properly validate input length parameters before processing icon data. The flaw occurs when handling TS_ICON_INFO structures received from an RDP server, where insufficient bounds checking allows reading beyond the allocated buffer boundaries.

The vulnerability is network-reachable, meaning an attacker controlling an RDP server or positioned as a man-in-the-middle can craft malicious icon data that triggers the overread condition when processed by a vulnerable FreeRDP client.

Root Cause

The root cause is missing input validation in the freerdp_image_copy_from_icon_data() function. Specifically, the function did not properly check for zero-dimension icon data (width or height equal to 0) before performing memory operations. This oversight allowed malformed icon data to bypass expected boundary conditions, resulting in the buffer overread.

Attack Vector

An attacker can exploit this vulnerability through network-based attacks by:

1. Setting up a malicious RDP server that sends crafted TS_ICON_INFO data with invalid dimensions
2. Intercepting legitimate RDP traffic via man-in-the-middle and injecting malicious icon data
3. Triggering the buffer overread when the victim's FreeRDP client processes the icon data

The attack requires the victim to connect to a malicious or compromised RDP server.

// Security patch - adding dimension validation checks
// Source: https://github.com/FreeRDP/FreeRDP/commit/f5e20403d6e325e11b68129803f967fb5aeec1cb

 	if (!pDstData || !bitsColor)
 		return FALSE;
 
+	if ((nWidth == 0) || (nHeight == 0))
+		return TRUE;
+
 	/*
 	 * Color formats used by icons are DIB bitmap formats (2-bit format
 	 * is not used by MS-RDPERP). Note that 16-bit is RGB555, not RGB565,

The patch adds explicit validation to reject icon data with zero width or height dimensions, preventing the buffer overread condition.

Detection Methods for CVE-2026-26271

Indicators of Compromise

• Unexpected FreeRDP client crashes during RDP sessions
• Memory access violations or segmentation faults in FreeRDP processes
• Anomalous RDP traffic containing malformed TS_ICON_INFO structures
• Process memory dumps indicating out-of-bounds read operations in color.c

Detection Strategies

• Monitor for FreeRDP client crashes or abnormal terminations during active RDP sessions
• Implement network-level inspection for malformed RDP packets, particularly those containing TS_ICON_INFO with zero or abnormal dimension values
• Deploy endpoint detection rules to identify memory access violations in FreeRDP processes
• Review application logs for codec-related errors in libfreerdp/codec/color.c

Monitoring Recommendations

• Enable verbose logging in FreeRDP clients to capture detailed connection and processing information
• Monitor network traffic for connections to untrusted or unknown RDP servers
• Implement alerting for repeated FreeRDP process restarts or crashes
• Track version inventory to identify systems running vulnerable FreeRDP versions (< 3.23.0)

How to Mitigate CVE-2026-26271

Immediate Actions Required

• Upgrade FreeRDP to version 3.23.0 or later immediately
• Restrict RDP connections to trusted, verified servers only
• Implement network segmentation to limit exposure to potential man-in-the-middle attacks
• Consider using VPN or other encrypted tunnels for RDP connections over untrusted networks

Patch Information

The vulnerability has been fixed in FreeRDP version 3.23.0. The security patch adds proper input validation to check for zero-dimension icon data before processing. The fix is available in commit f5e20403d6e325e11b68129803f967fb5aeec1cb. For detailed information, refer to the GitHub Security Advisory GHSA-hr4m-ph4g-48j6.

Workarounds

• Only connect to trusted and verified RDP servers until patching is complete
• Use network-level protections (firewalls, IDS/IPS) to filter potentially malicious RDP traffic
• Consider disabling icon processing features if supported by your FreeRDP configuration
• Implement certificate pinning for RDP connections to mitigate man-in-the-middle risks

# Verify FreeRDP version to confirm vulnerability status
xfreerdp --version

# Update FreeRDP to patched version on Debian/Ubuntu
sudo apt update && sudo apt install freerdp2-x11

# For systems building from source, update to version 3.23.0 or later
git clone https://github.com/FreeRDP/FreeRDP.git
cd FreeRDP
git checkout 3.23.0
cmake -DCMAKE_BUILD_TYPE=Release .
make && sudo make install