CVE-2026-109679 in FreeRDP
Summary
by MITRE • 10/13/2026
FreeRDP 3.0.0-beta4 before 3.32.0 contains a NULL pointer dereference in arm_handle_request_ok when an ARM endpoint returns an empty HTTP 200 body. Attackers controlling the configured ARM gateway can return a zero-length body to pass NULL to strnlen and crash the connecting client.
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Analysis
by VulDB Data Team • 10/13/2026
The vulnerability identified in FreeRDP versions prior to 3.32.0 represents a critical input validation failure within the Remote Desktop Protocol implementation, specifically affecting the handling of responses from an Azure Relay Manager endpoint. This flaw is categorized under CWE-476, which denotes NULL Pointer Dereference, and aligns with ATT&CK technique T1598.002, reflecting phishing via initially untrusted sources or potentially compromised infrastructure components used in remote access scenarios. The issue arises during the authentication handshake phase when a client connects to an ARM gateway configured for Azure Relay Manager integration. Under normal operational conditions, this endpoint returns HTTP responses containing valid data structures that allow the FreeRDP client to proceed with session establishment. However, due to insufficient boundary checking and error handling logic within the arm_handle_request_ok function, the software fails to account for scenarios where the server responds with an empty body despite returning a standard 200 OK status code.
From a technical perspective, the root cause lies in how the application processes string lengths derived from network inputs without verifying pointer validity first. When an attacker controls or compromises the configured ARM gateway, they can manipulate the response to include a zero-length payload while maintaining the HTTP 200 success indicator. The FreeRDP client interprets this as a successful initial handshake and attempts to process the body content using standard string manipulation functions such as strnlen. Because no data was transmitted in the body, the pointer passed to these functions is null rather than pointing to allocated memory containing valid characters. This mismatch between expected input structure and actual received data triggers an immediate segmentation fault or access violation within the client application thread responsible for handling the connection state machine.
The operational impact of this vulnerability is primarily denial of service against individual clients attempting to establish remote desktop sessions through affected versions of FreeRDP. Since the crash occurs synchronously during the initial request processing, it prevents any further authentication steps from completing and terminates the user session abruptly. While this does not directly allow for arbitrary code execution or privilege escalation on the client machine in its current manifestation, it effectively disrupts availability for users relying on Azure Relay Manager assisted connections. In enterprise environments where centralized remote access is critical, an attacker could exploit this condition to cause widespread connectivity failures by targeting multiple clients simultaneously if they control the intermediary gateway infrastructure.
Mitigation strategies must focus on both immediate patching and defensive coding practices. Organizations should upgrade FreeRDP to version 3.32.0 or later as soon as possible, where developers have implemented proper null checks before invoking string length functions. For environments unable to update immediately, network-level controls such as strict firewall rules limiting access to the ARM gateway can reduce exposure. Additionally, implementing input validation at the application layer to verify that response bodies meet minimum size requirements before processing them would prevent similar flaws in future development cycles. Security teams should also monitor for unusual patterns of connection attempts resulting in immediate client crashes, which may indicate active exploitation of this NULL pointer dereference condition by malicious actors seeking to disrupt remote work capabilities.