CVE-2026-23531 in FreeRDP
Summary
by MITRE • 01/19/2026
FreeRDP is a free implementation of the Remote Desktop Protocol. Prior to version 3.21.0, in ClearCodec, when `glyphData` is present, `clear_decompress` calls `freerdp_image_copy_no_overlap` without validating the destination rectangle, allowing an out-of-bounds read/write via crafted RDPGFX surface updates. A malicious server can trigger a client‑side heap buffer overflow, causing a crash (DoS) and potential heap corruption with code‑execution risk depending on allocator behavior and surrounding heap layout. Version 3.21.0 contains a patch for the issue.
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Analysis
by VulDB Data Team • 01/30/2026
The vulnerability identified as CVE-2026-23531 affects FreeRDP, an open-source implementation of the Remote Desktop Protocol that enables remote desktop connections across various platforms. This issue specifically resides within the ClearCodec component, which handles graphics compression and decompression for RDPGFX surface updates. The flaw manifests when the system processes glyph data within RDP graphics updates, creating a critical security gap that can be exploited by malicious RDP servers to compromise client systems.
The technical root cause involves improper bounds checking within the `clear_decompress` function in ClearCodec. When glyph data is present in RDPGFX surface updates, the function invokes `freerdp_image_copy_no_overlap` without first validating the destination rectangle coordinates. This lack of input validation creates an out-of-bounds memory access condition that can result in either read or write operations occurring beyond the allocated memory boundaries. The vulnerability stems from CWE-129, which addresses insufficient validation of length of input buffers, and aligns with ATT&CK technique T1203 for exploitation of input validation flaws in remote desktop protocols.
The operational impact of this vulnerability extends beyond simple denial of service to potentially enable remote code execution. A malicious RDP server can craft specially formatted graphics updates that trigger a heap buffer overflow on the client system. This heap corruption can lead to unpredictable behavior including application crashes, system instability, or in worst-case scenarios, arbitrary code execution. The risk level is particularly concerning given that RDP is widely used for remote administration and access, making this vulnerability exploitable in numerous enterprise environments where FreeRDP clients are deployed.
Organizations should immediately update to FreeRDP version 3.21.0 or later, which contains the necessary patch addressing this vulnerability. System administrators should also implement network segmentation and access controls to limit RDP server access to trusted sources only. Additional mitigations include monitoring for unusual RDP traffic patterns and implementing intrusion detection systems that can identify malformed RDPGFX updates. The patch addresses the core issue by adding proper bounds checking to validate destination rectangle parameters before any memory operations are performed, preventing the out-of-bounds access that previously enabled exploitation.