Executive Summary

CVE-2026-40033 is a heap-buffer-overflow vulnerability in the FreeRDP client library, specifically in the gdi_CacheToSurface function of the RDPGFX component.

The vulnerability occurs because the rectangle validation clamps coordinates to UINT16_MAX, but the actual copy operation uses the original, unclamped cache entry dimensions. This mismatch allows a malicious RDP server to cause large out-of-bounds writes to heap memory.

An attacker can exploit this by sending crafted RDPGFX PDUs to a FreeRDP client with RDPGFX enabled, potentially causing client crashes or enabling remote code execution.

The vulnerability affects FreeRDP versions up to and including 3.25.0 and was fixed in version 3.26.0.

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Impact Analysis

This vulnerability can have serious impacts including remote code execution and client crashes.

A remote attacker controlling a malicious RDP server can exploit this flaw to write out-of-bounds heap memory, potentially executing arbitrary code on the client system without requiring any privileges.

User interaction is limited to connecting to the malicious server, making it a high-risk vulnerability.

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Mitigation Strategies

To mitigate the CVE-2026-40033 vulnerability, you should upgrade FreeRDP to version 3.26.0 or later, where the heap-buffer-overflow issue in the gdi_CacheToSurface function has been fixed.

Avoid connecting to untrusted or potentially malicious RDP servers, as the vulnerability can be triggered by crafted RDPGFX PDUs sent from such servers.

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Detection Guidance

This vulnerability occurs when a FreeRDP client connects to a malicious RDP server that sends specially crafted RDPGFX PDUs triggering a heap-buffer-overflow in the gdi_CacheToSurface function.

Detection on the network or system would involve monitoring for connections to potentially malicious RDP servers or analyzing FreeRDP client behavior for crashes or abnormal memory writes during RDP sessions.

Since the vulnerability is triggered by crafted RDPGFX packets, one approach is to identify FreeRDP client versions before 3.26.0 running on your systems, as these are vulnerable.

Suggested commands to detect vulnerable FreeRDP versions on your system include:

• Check FreeRDP version installed: `xfreerdp /version` or `freerdp-shadow-cli --version`
• Use package manager queries, e.g., `dpkg -l | grep freerdp` on Debian-based systems or `rpm -qa | grep freerdp` on RPM-based systems.

For network detection, monitoring RDP traffic for unusual or malformed RDPGFX PDUs would require deep packet inspection tools or custom IDS/IPS signatures, but no specific commands or signatures are provided in the available resources.

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Compliance Impact

The provided information does not specify any direct impact of the CVE-2026-40033 vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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