CVE-2026-91954 in FreeRDP
Summary
by MITRE • 09/15/2026
FreeRDP before 3.31.0 contains a null pointer dereference vulnerability in gdi_surface_bits when processing Surface Bits commands with NSCodec codec ID. A malicious RDP server can crash a FreeRDP client by sending a crafted Surface Bits command claiming to use NSCodec, even when the codec is disabled.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in versions of FreeRDP prior to 3.31.0 represents a critical memory safety flaw located within the gdi_surface_bits function, specifically during the processing of RDP Surface Bits commands that utilize the NSCodec identifier. This issue stems from an insufficient validation mechanism where the application fails to verify whether the NSCodec library is actually enabled or initialized before attempting to access pointers associated with it. In a properly secured implementation, any codec-specific operation should be gated by a check confirming the availability of the corresponding decoder infrastructure. However, in this flawed state, the software proceeds directly to dereference memory addresses linked to the NSCodec context without ensuring that these structures have been allocated or initialized during the client startup sequence.
From an operational perspective, this defect allows for a remote code execution vector that manifests as a denial of service condition rather than arbitrary code execution due to the nature of the fault being a null pointer dereference. An attacker controlling a malicious RDP server can exploit this by sending a specially crafted Surface Bits command packet that explicitly claims to use the NSCodec compression method. Even if the FreeRDP client has been configured with NSCodec disabled, or if it was not compiled with support for it, the parser ignores these configuration states and attempts to access the null pointer associated with the codec context. This action triggers a segmentation fault on most operating systems, causing the RDP client application to crash immediately upon receiving the malicious packet.
The security implications of this vulnerability are significant because they undermine the reliability and availability of remote desktop sessions for users relying on FreeRDP as their primary client software. Since the attack requires only the reception of a single crafted network packet from an untrusted server, it can be executed remotely without any prior authentication or user interaction beyond establishing a connection to the malicious host. This makes it particularly dangerous in environments where clients connect to public or semi-trusted RDP servers, such as cloud desktop providers or shared terminal services. The crash disrupts productivity and may lead to data loss if unsaved work is present in the client application at the time of exploitation.
This vulnerability aligns with CWE-476, which describes a NULL Pointer Dereference error where software dereferences a pointer that it expects to be valid but is actually null. Furthermore, from an offensive security taxonomy perspective, this exploit technique falls under MITRE ATT&CK tactic T1499, Endpoint Denial of Service, specifically within the sub-technique for application crashes or resource exhaustion via network-based attacks. The lack of proper bounds checking and state validation before memory access is a classic example of input validation failure that leads to unstable program execution.
To mitigate this risk, organizations must ensure that all instances of FreeRDP are updated to version 3.31.0 or later, where the developers have implemented strict checks to verify codec availability prior to dereferencing associated pointers. Until an upgrade is feasible, administrators should restrict client access to only trusted and verified RDP servers to minimize exposure to malicious actors capable of crafting such packets. Additionally, deploying network intrusion detection systems that can identify anomalous RDP traffic patterns may provide a layer of defense against automated exploitation attempts targeting this specific flaw in unpatched environments.