Compliance Impact

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Executive Summary

CVE-2026-26271 is a buffer overread vulnerability in FreeRDP versions prior to 3.23.0, specifically in the function freerdp_image_copy_from_icon_data() which processes Remote Desktop Protocol (RDP) Window Icon data.

The vulnerability occurs because while the size of the color data buffer is properly validated, the size of the mask data buffer is not. When the image format includes an alpha channel, the function applies an alpha mask by reading the mask data buffer without verifying it is large enough.

A malicious RDP server or a man-in-the-middle attacker can send crafted icon data with a small mask buffer size but large image dimensions, causing the function to read beyond the allocated mask buffer, leading to a buffer overread.

This issue was fixed in FreeRDP version 3.23.0 by adding proper input length validation to prevent reading beyond the buffer.

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Impact Analysis

This vulnerability can be triggered remotely when a FreeRDP client processes crafted Window Icon data sent by a malicious RDP server or a man-in-the-middle attacker.

The buffer overread may lead to reading memory beyond the intended buffer, which can cause application crashes or potentially expose sensitive information from memory.

Since the vulnerability is reachable over the network without authentication, it increases the risk of exploitation in environments where FreeRDP clients connect to untrusted or compromised servers.

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Detection Guidance

This vulnerability can be detected by monitoring RDP client traffic for crafted Window Icon (TS_ICON_INFO) data that triggers the buffer overread in the function freerdp_image_copy_from_icon_data(). Detection involves identifying RDP Window Order (Rail) PDUs containing suspiciously small mask buffer sizes (cbBitsMask) combined with large image dimensions (nWidth and nHeight).

Since the vulnerability arises when processing icon data from an RDP server or a man-in-the-middle, inspecting RDP client logs for errors related to icon data processing or buffer overreads may help detect exploitation attempts.

Specific commands are not provided in the resources, but general approaches include:

• Using network packet capture tools (e.g., Wireshark) to filter and analyze RDP traffic for Window Icon orders with abnormal icon data sizes.
• Enabling verbose logging in FreeRDP clients to capture errors or warnings related to icon data processing, especially after the patch which logs detailed errors on length check failures.
• Using intrusion detection systems (IDS) with signatures targeting malformed RDP Window Icon data or unusual RDP Window Order PDUs.

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Mitigation Strategies

The primary mitigation step is to upgrade FreeRDP to version 3.23.0 or later, where the vulnerability has been fixed by adding proper input length validation and error handling in the function freerdp_image_copy_from_icon_data().

Until the upgrade can be applied, consider the following steps:

• Avoid connecting to untrusted or potentially malicious RDP servers that could send crafted icon data.
• Use network-level protections such as firewalls or RDP gateways to restrict RDP traffic to trusted sources.
• Monitor client logs for errors related to icon data processing which may indicate attempted exploitation.

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