CVE-2026-121953 in FreeRDP
Summary
by MITRE • 11/04/2026
FreeRDP before 3.32.2 has a use-after-free in the client's rdpgfx dynamic virtual channel: a duplicate-name channel with a new ChannelId overwrites the listener's single callback slot, which rdpgfx_on_close frees without clearing. A malicious RDP server can open and close such a channel, then send a PDU on the original channel to crash the client or potentially execute code.
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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 use-after-free condition within the rdpgfx dynamic virtual channel implementation. This flaw stems from an improper handling of channel identifiers and callback registration mechanisms during the lifecycle management of remote desktop graphics channels. Specifically, when a duplicate-name channel is established with a new ChannelId, it overwrites the listener's single available callback slot without properly invalidating or clearing references to previously allocated resources associated with that slot. This architectural oversight creates a race condition where memory pointers become dangling if not managed correctly during state transitions between open and close operations.
The technical root cause lies in how the rdpgfx module manages its internal data structures for handling incoming protocol data units from RDP servers. When a malicious server initiates a sequence of opening and closing channels with duplicate names but distinct identifiers, it triggers an overwrite of the callback registration structure. Crucially, when the system subsequently processes a close event for one of these channels, the rdpgfx_on_close function is invoked to clean up resources. However, due to the overwritten state, this cleanup routine frees memory associated with the original channel's context without clearing the pointer in the active callback slot. This leaves a dangling pointer that remains accessible within the application's execution flow.
The operational impact of this vulnerability allows for severe consequences ranging from denial of service to arbitrary code execution. A malicious RDP server can exploit this state by sending a carefully crafted Protocol Data Unit on the original channel after the close event has occurred but before the memory is fully reclaimed or overwritten by other processes. Because the callback slot still points to freed memory, executing logic through this pointer leads to undefined behavior. In most scenarios, this results in an immediate crash of the FreeRDP client application, causing a denial of service for users relying on remote desktop connectivity. However, with sophisticated exploitation techniques involving heap grooming and specific payload construction, an attacker could potentially achieve arbitrary code execution by controlling the contents of the freed memory region or manipulating control flow through function pointers stored within that structure.
This vulnerability aligns closely with CWE-416, which describes Use After Free errors where a pointer is used after it has been freed, leading to unpredictable behavior and potential security breaches. From an offensive perspective, this flaw can be mapped to MITRE ATT&CK technique T1203, specifically the exploitation of software vulnerabilities for initial access or execution purposes. The attack vector requires network connectivity to a vulnerable RDP server, classifying it as a remote exploit that does not require user interaction beyond establishing a connection session.
Mitigation strategies primarily involve upgrading FreeRDP to version 3.32.2 or later, where the developers have addressed this memory management flaw by ensuring proper pointer nullification and reference counting during channel lifecycle events. Organizations should also implement network segmentation policies to restrict access to RDP services from untrusted networks. Additionally, deploying intrusion detection systems capable of identifying anomalous patterns in RDP traffic, such as rapid channel open-close sequences with duplicate names, can provide an additional layer of defense against exploitation attempts while patching efforts are underway. Regular security audits and code reviews focusing on memory safety in C-based applications like FreeRDP are essential to prevent similar vulnerabilities from persisting in future releases.