CVE-2026-121961 in FreeRDP
Summary
by MITRE • 11/04/2026
FreeRDP before 3.32.2 contains a use-after-free vulnerability in the freerdp-proxy DRDYNVC tracker when intercepting the drdynvc channel. A malicious target server can repeatedly send CREATE_REQUEST_PDU for an active channel id, racing front DATA handling to free a borrowed context and crash the proxy process.
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Analysis
by VulDB Data Team • 11/04/2026
The identified vulnerability resides within the FreeRDP software suite prior to version 3.32.2, specifically affecting the freerdp-proxy component during its operation as a Dynamic Virtual Channel tracker for the drdynvc channel. This security flaw is classified as a use-after-free error, which represents one of the most dangerous classes of memory corruption vulnerabilities in systems programming. The core technical issue arises from an improper handling of resource lifecycle management when the proxy intercepts traffic on the drdynvc channel. In this context, the software borrows a context object to manage communication states between clients and servers. Under normal operations, these contexts are allocated, utilized, and then freed according to strict state transitions. However, the implementation fails to adequately synchronize access to these borrowed contexts when handling incoming protocol data units from the remote server side of the connection.
The operational mechanism of this exploit involves a malicious target server actively manipulating the timing of Protocol Data Units sent over the network. Specifically, the attacker repeatedly sends CREATE_REQUEST_PDU messages referencing an already active channel identifier. This action triggers a race condition within the proxy process. The vulnerability allows for a scenario where the system attempts to free a borrowed context that is still being referenced or processed by another thread handling incoming DATA PDU traffic. Because the memory deallocation occurs while other parts of the application are still holding pointers to this now-invalidated memory region, subsequent access operations result in undefined behavior. This race condition effectively bypasses standard safety checks because the timing window between the allocation reference and the actual free operation is exploited by high-frequency network requests from a compromised or malicious server endpoint.
The impact of successfully exploiting this vulnerability is primarily denial of service against the FreeRDP proxy process. When the use-after-free condition triggers, it typically leads to immediate memory corruption that causes the application to crash. For users relying on freerdp-proxy for transparent interception and analysis of Remote Desktop Protocol traffic, such a crash disrupts connectivity and halts any security monitoring or logging activities dependent on this middleware. While modern operating systems with features like Address Space Layout Randomization may mitigate some aspects of exploitation, the immediate result is still service disruption. In more complex scenarios involving heap corruption, there could be potential for arbitrary code execution if an attacker can precisely control the data written to the freed memory region before it is reallocated, although the primary documented impact remains process termination due to the instability introduced by accessing invalid pointers.
From a classification perspective, this vulnerability aligns with CWE-416, which defines use-after-free errors where software continues to use a pointer after it has been freed. The attack vector leverages timing differences in concurrent processing threads, characteristic of race conditions often associated with CWE-362. In terms of the MITRE ATT&CK framework, this vulnerability facilitates techniques related to Denial of Service (T1499) and potentially Execution if memory corruption is fully exploited for code injection. The attack requires network access to a server that can send crafted drdynvc packets, placing it within the context of remote exploitation via protocol manipulation rather than local privilege escalation or physical access.
Mitigation strategies focus primarily on software updates and architectural hardening. Organizations running FreeRDP versions older than 3.32.2 must upgrade immediately to the patched version where developers have addressed the synchronization issues in the drdynvc tracker logic. The fix likely involves implementing proper reference counting mechanisms, adding mutex locks around context access during state transitions, or ensuring that borrowed contexts are not freed while active data handling operations are pending. Additionally, network-level defenses such as intrusion detection systems can be configured to monitor for anomalous patterns of CREATE_REQUEST_PDU messages targeting existing channel IDs, although this is a secondary measure since the root cause lies in the application logic itself rather than external traffic anomalies alone. Regular patch management and keeping all RDP proxy components up to date are essential practices to prevent exploitation of such memory safety vulnerabilities.