CVE-2026-63652 in FreeRDP
Summary
by MITRE • 08/19/2026
FreeRDP is a free implementation of the Remote Desktop Protocol. Prior to 3.28.0, rdpsnd_server_recv_formats in channels/rdpsnd/server/rdpsnd_main.c frees context->client_formats on a malformed Client Audio Formats PDU without clearing the owning pointer or num_client_formats. An authenticated RDP client can trigger an error such as a cbSize larger than the remaining record, leave the dangling pointer in the server context, and cause rdpsnd_server_context_free to free the same allocation again at session teardown. This reliably terminates the server and can create allocator-dependent heap corruption. This issue is fixed in version 3.28.0.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability identified in FreeRDP prior to version 3.28.0 represents a critical memory management flaw within the audio subsystem, specifically located in the rdpsnd_server_recv_formats function found in channels/rdpsnd/server/rdpsnd_main.c. This component is responsible for processing incoming Client Audio Formats Protocol Data Units (PDUs) from authenticated Remote Desktop Protocol clients. The core technical deficiency lies in how the server handles malformed input during this negotiation phase. When a client sends a PDU with an invalid structure, such as a cbSize field indicating a size larger than the remaining record data available for parsing, the function attempts to free the memory associated with context->client_formats. However, it fails to perform essential housekeeping operations: specifically, it does not clear the owning pointer or reset the num_client_formats counter in the server context structure. This oversight leaves behind a dangling pointer that references freed heap memory rather than nullifying the reference as required by safe coding practices.
The operational impact of this flaw is severe and directly affects the stability and integrity of the FreeRDP server instance. Because the owning pointer remains valid but points to deallocated memory, subsequent operations within the same session can inadvertently interact with this invalid address. The most critical consequence occurs during session teardown when the rdpsnd_server_context_free function is invoked. This cleanup routine attempts to free the client_formats allocation again based on the stale state of the context. Since the pointer was not cleared after the initial erroneous deallocation triggered by the malformed PDU, this results in a double-free condition. In modern memory allocators, such as those used in Linux and Windows systems, freeing the same block twice reliably corrupts internal allocator metadata structures like free lists or chunk headers. This corruption is not merely theoretical; it can lead to immediate server termination via segmentation faults or more insidious heap corruption that may be exploitable for arbitrary code execution depending on the specific memory layout and timing of subsequent allocations.
From a threat modeling perspective, this vulnerability aligns with CWE-415 Double Free, which describes the error condition where an application frees memory twice without ensuring it is reallocated in between or properly nullified. The attack vector requires authentication as an RDP client, placing this within the scope of authenticated attacks rather than remote unauthenticated exploitation. However, given that many enterprise environments allow a wide range of users to establish RDP sessions, the accessibility of the vulnerability remains high. In terms of adversarial tactics, this behavior is consistent with ATT&CK technique T1496 Resource Hijacking or potentially T1055 Process Injection if an attacker could leverage the heap corruption to gain control over execution flow, although the primary observed impact in standard scenarios is denial of service through server crash. The reliance on allocator-dependent behavior means that exploitation success may vary across different operating systems and FreeRDP build configurations, but the reliability of causing a crash makes it a potent tool for disrupting remote desktop services.
Mitigation strategies must prioritize immediate software updates as the primary defense vector. Organizations running FreeRDP servers should upgrade to version 3.28.0 or later where this logic error has been corrected by properly nullifying pointers and resetting counters after deallocation operations. For environments unable to patch immediately, network-level access controls can be employed to restrict RDP connectivity to only trusted IP ranges, thereby reducing the attack surface available to potential adversaries. Additionally, implementing strict input validation at the protocol parsing layer is essential for long-term resilience against malformed PDU attacks. Security teams should monitor server logs for unexpected process terminations or core dumps associated with the rdpsnd module as an indicator of attempted exploitation. Regular vulnerability scanning and adherence to secure coding standards that mandate nullifying pointers after free operations are critical steps in preventing similar memory safety issues in future development cycles.