CVE-2026-121954 in FreeRDP
Summary
by MITRE • 11/04/2026
FreeRDP before 3.32.2 has a heap out-of-bounds write in the planar codec because freerdp_bitmap_planar_context_reset commits new dimensions before buffer reallocation succeeds, and a failed ResetGraphics is only logged. A malicious RDP server can send an oversized ResetGraphics PDU, then a GFX PLANAR command, to corrupt client heap memory and crash the client.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in FreeRDP versions prior to 3.32.2 represents a critical security flaw within the planar codec implementation, specifically affecting how bitmap context resets are handled during remote desktop sessions. This issue stems from an improper sequence of operations where the application commits new buffer dimensions before verifying that the memory reallocation has successfully completed. In standard software development practices for handling dynamic data structures, it is imperative to ensure that resource allocation succeeds and returns a valid pointer or status code before updating internal state variables that reference those resources. By committing the new dimensions prematurely, FreeRDP creates a race condition-like scenario where subsequent operations assume the existence of a buffer with specific properties that may not actually exist in memory if the reallocation failed. This design flaw is characteristic of CWE-190, which describes integer overflow or wraparound issues leading to improper resource allocation, although in this specific context, it manifests more directly as an out-of-bounds write due to incorrect state management relative to buffer availability.
The operational mechanism of this vulnerability involves a malicious Remote Desktop Protocol server exploiting the client's handling of graphics commands. An attacker can craft and send an oversized ResetGraphics Protocol Data Unit (PDU) designed to trigger a memory reallocation failure or cause instability in the allocation process. Following this, the attacker sends a GFX PLANAR command that instructs the client to write bitmap data into the context using the newly committed but potentially invalid dimensions. Because the application has already updated its internal state to reflect these new dimensions without confirming successful buffer creation, it proceeds with memory writes based on incorrect assumptions about available space. This results in a heap out-of-bounds write, where data is written beyond the allocated boundaries of the heap segment. Such memory corruption can overwrite adjacent metadata or other critical application structures, leading to immediate instability and crashes of the FreeRDP client process.
From an offensive security perspective, this vulnerability aligns with MITRE ATT&CK technique T1203, which covers Exploitation for Client Execution, as it involves leveraging a flaw in a widely used client-side application through network interaction. The ability to corrupt heap memory opens pathways beyond simple denial of service; while the primary observed impact is crashing the client, heap corruption vulnerabilities are frequently chained with other exploits to achieve arbitrary code execution if specific conditions regarding memory layout and allocator behavior are met. This makes the vulnerability particularly dangerous in enterprise environments where FreeRDP is used for remote access, as a successful exploitation could allow an attacker to gain control over the user's system or pivot further into internal networks. The lack of robust error handling means that even partial failures in graphics processing can lead to severe security consequences rather than graceful degradation or safe termination.
Mitigation strategies must focus on immediate patching and defensive coding practices. Organizations should upgrade FreeRDP to version 3.32.2 or later, where the logic has been corrected to ensure that buffer reallocation is verified for success before any state changes are committed. For environments unable to update immediately, network-level controls such as firewalls can be configured to inspect and limit the size of incoming RDP PDUs, particularly ResetGraphics commands, to prevent oversized payloads from reaching the vulnerable client software. Additionally, enabling heap protection mechanisms like Address Sanitizer in development builds or utilizing operating system-level mitigations such as Data Execution Prevention (DEP) and Stack Canaries can help detect or mitigate exploitation attempts. Continuous monitoring of FreeRDP logs for unusual patterns in graphics commands may also aid in identifying attempted exploits before they result in successful memory corruption.