Detection Guidance

This vulnerability is triggered by a malicious RDP server sending crafted ClearCodec surface commands with out-of-bounds destination rectangles to a FreeRDP client using the GDI surface pipeline (e.g., xfreerdp). Detection involves monitoring RDP traffic for suspicious ClearCodec commands with destination rectangle coordinates that exceed expected surface dimensions.

Specifically, detection can focus on identifying ClearCodec commands where the rectangle offsets (cmd->left, cmd->top) are outside the valid surface bounds, which the vulnerable FreeRDP client fails to validate.

While no explicit commands are provided in the resources, network administrators can use packet capture tools (e.g., Wireshark or tcpdump) to capture RDP traffic and filter for ClearCodec surface commands. Analyzing these packets for out-of-bounds rectangle parameters could indicate exploitation attempts.

On the client system, monitoring for crashes or abnormal behavior in FreeRDP clients (such as xfreerdp) when connecting to RDP servers may also indicate exploitation attempts, as the vulnerability leads to heap corruption and potential crashes.

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Executive Summary

CVE-2026-26955 is a critical heap buffer overflow vulnerability in FreeRDP versions prior to 3.23.0, specifically in the handling of the ClearCodec surface command within the GDI surface pipeline (such as the xfreerdp client).

The vulnerability arises because the function responsible for processing ClearCodec commands, `gdi_SurfaceCommand_ClearCodec()`, does not validate the destination rectangle coordinates against the surface dimensions. This missing bounds check allows a malicious RDP server to send crafted commands with out-of-bounds rectangle values.

As a result, the client performs out-of-bounds writes into the heap-allocated surface pixel buffer, corrupting adjacent memory structures, specifically a pointer to codec data. This corruption can be exploited to hijack control flow and achieve remote code execution on the client.

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Impact Analysis

This vulnerability allows a malicious RDP server to execute arbitrary code on a vulnerable FreeRDP client by exploiting a heap buffer overflow triggered during processing of crafted ClearCodec commands.

The impact includes potential full compromise of the client system, as the attacker can gain control over the instruction pointer and execute code with the privileges of the FreeRDP client process.

Additionally, this can lead to crashes or denial of service if the exploit is not successful, disrupting remote desktop sessions.

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Compliance Impact

I don't know

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Mitigation Strategies

The primary mitigation step is to upgrade FreeRDP clients to version 3.23.0 or later, where the vulnerability has been patched by adding proper bounds validation in the ClearCodec surface command handler.

If upgrading immediately is not possible, consider restricting or monitoring connections to untrusted or potentially malicious RDP servers, as the vulnerability is exploited by malicious servers sending crafted ClearCodec commands.

Additionally, applying any available patches or updates that fix boundary checks in codec decompression routines (such as those described in the commit fixing clear_decompress_subcodecs_data) can help reduce risk.

Implementing network-level protections such as firewall rules or intrusion detection systems to detect and block suspicious RDP traffic patterns may also help mitigate exploitation attempts.

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