CVE-2026-121407 in FreeRDP
Summary
by MITRE • 11/03/2026
FreeRDP before 3.32.2 contains a denial of service vulnerability in the shadow server's remdesk_server_thread that allows authenticated clients to crash the process via an inverted PDU length check. Attackers can send an 8192-byte remdesk payload declaring an eight-byte PDU length to trigger a fatal Stream_Seek assertion, aborting the server and dropping sessions.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability identified in FreeRDP versions prior to 3.32.2 represents a critical denial of service flaw within the shadow server component, specifically affecting the remdesk_server_thread function. This issue arises from an insufficient validation mechanism when processing Remote Desktop Protocol (RDP) data units known as PDUs. The core technical failure lies in an inverted length check logic that fails to correctly verify the integrity and bounds of incoming payload data before it is processed by internal stream handling routines. Because this flaw exists within a thread responsible for managing shadowing sessions, which typically requires authentication, the attack vector necessitates that an adversary first establish valid credentials or otherwise bypass initial access controls to interact with the server interface.
From a technical perspective, the exploitation mechanism involves sending a specifically crafted remdesk payload of 8192 bytes while simultaneously declaring a significantly smaller PDU length field, such as eight bytes. This discrepancy creates a state where internal buffer pointers and stream iterators become desynchronized from the actual data present in memory. When the server attempts to process this malformed input, it triggers a fatal assertion failure within the Stream_Seek function. Assertions are typically used during development to catch logical errors that should never occur in production code; their presence indicates that the software relies on these checks for basic operational stability rather than robust error handling. The triggering of such an assertion results in an immediate and unhandled termination of the server process, effectively causing a complete crash of the FreeRDP service instance managing those sessions.
The operational impact of this vulnerability is severe due to its direct effect on service availability. Since the shadow server thread handles active remote desktop connections, crashing it causes all associated user sessions to be abruptly dropped without graceful disconnection or data preservation. For organizations relying on FreeRDP for remote administration, virtualization access, or technical support via screen sharing, this results in immediate disruption of business operations. An authenticated attacker can exploit this flaw repeatedly to maintain a persistent denial of service condition, preventing legitimate users from accessing critical systems. This aligns with the Common Weakness Enumeration category CWE-20 Improper Input Validation, as the root cause is the failure to properly validate and constrain user-supplied input data before processing it within sensitive system functions.
Furthermore, this vulnerability maps directly to MITRE ATT&CK technique T1499 Endpoint Denial of Service, specifically under subtechniques involving application exhaustion or resource consumption through malformed requests. The attack does not require privilege escalation beyond the initial authentication level but leverages a logic error in protocol parsing to destabilize the host system's security posture by removing access controls entirely through service unavailability. Mitigation strategies must prioritize immediate patching of FreeRDP to version 3.32.2 or later, where this inverted length check has been corrected and proper bounds checking is implemented for PDU processing. In environments where upgrading is not immediately feasible, network-level segmentation should be enforced to restrict access to the shadow server ports only from trusted IP ranges, thereby reducing the attack surface available to potential adversaries who might attempt to exploit this logic flaw against authenticated services.