CVE-2026-63633 in FreeRDP
Summary
by MITRE • 08/19/2026
FreeRDP is a free implementation of the Remote Desktop Protocol. Prior to 3.28.0, freerdp_dsp_decode_opus in libfreerdp/codec/dsp.c calls Stream_EnsureRemainingCapacity on context->common.buffer even though opus_decode writes decoded PCM into the caller-supplied out stream. A malicious RDP server that negotiates WAVE_FORMAT_OPUS with a client built with WITH_OPUS enabled and WITH_DSP_FFMPEG disabled can make libopus write a large decoded frame beyond the 4096-byte StreamPool_Take destination used by channels/rdpsnd/client/rdpsnd_main.c. This can corrupt the client heap, crash the client, and may permit code execution. This issue is fixed in version 3.28.0.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified within FreeRDP prior to version 3.28.0 represents a critical memory safety flaw rooted in improper buffer management during audio decoding operations. FreeRDP serves as an open-source implementation of the Remote Desktop Protocol, facilitating remote access and interaction with Windows-based systems over network connections. The specific defect resides in the libfreerdp/codec/dsp.c module, particularly within the freerdp_dsp_decode_opus function which handles the decoding of Opus audio streams. This component is responsible for converting compressed audio data into playable PCM samples. However, a logical error occurs regarding how memory buffers are allocated and validated before writing decoded data. Specifically, the code invokes Stream_EnsureRemainingCapacity on context->common.buffer to verify or adjust buffer space. While this function checks capacity against expected requirements based on header information or previous state, it fails to account for the actual size of the audio frame being decoded by the underlying libopus library when WITH_DSP_FFMPEG is disabled and WITH_OPUS is enabled.
The technical core of the vulnerability lies in a mismatch between the allocated destination buffer and the output stream provided directly to the opus_decode function. The rdpsnd_main.c module utilizes a StreamPool_Take mechanism that allocates fixed-size buffers, typically capped at 4096 bytes, to handle incoming audio chunks from the server. When an RDP server negotiates WAVE_FORMAT_OPUS as the audio format, it signals its intent to send Opus-encoded data. If the client is configured with OPUS support but without FFmpeg DSP fallbacks, the freerdp_dsp_decode_opus function processes this stream. The opus_decode API writes decoded PCM samples directly into a caller-supplied output buffer. In this flawed implementation, the destination buffer passed to libopus does not align correctly with the safety checks performed by Stream_EnsureRemainingCapacity on the context's common buffer. Consequently, if the server sends an Opus frame that decodes into more than 4096 bytes of PCM data, or simply exploits the boundary condition where the check passes but the write exceeds limits due to internal libopus behavior or misaligned pointers, a heap-based buffer overflow occurs.
This memory corruption allows for severe operational impacts on the client system. The immediate consequence is often a crash of the FreeRDP application due to invalid memory access, leading to a denial of service against remote desktop sessions involving audio transmission. However, given that this is a heap overflow triggered by network-received data from an RDP server, the implications extend far beyond simple instability. An attacker controlling or manipulating the malicious RDP server can craft specific Opus frames designed to overwrite adjacent memory structures on the client's heap. This corruption can alter control flow pointers, such as function return addresses or virtual table pointers, thereby enabling arbitrary code execution with the privileges of the user running the FreeRDP client. This transforms a routine remote desktop session into a potential entry point for full system compromise, allowing attackers to install malware, exfiltrate data, or pivot further into internal networks depending on the security posture of the endpoint.
From a classification perspective, this vulnerability aligns with CWE-120 Buffer Copy without Checking Size of Input in C/C++, as the code performs a copy operation (writing decoded audio) without adequately verifying that the input size fits within the allocated buffer boundaries. It also relates to CWE-787 Out-of-bounds Write, where data is written beyond the intended memory region. In terms of adversary tactics and techniques, this flaw facilitates exploitation via ATT&CK technique T1059 Command and Scripting Interpreter or more directly T1203 Exploitation for Client Execution, as it targets client-side applications to achieve initial access. The attack vector involves a malicious server sending specially crafted network packets that trigger the decoding routine, making it an exploitable remote code execution vulnerability over standard RDP ports.
Mitigation strategies primarily involve upgrading FreeRDP to version 3.28.0 or later, where this logic error has been corrected by ensuring proper bounds checking and buffer allocation alignment before passing data to the decoder. For environments unable to upgrade immediately, administrators should consider disabling OPUS support if not strictly required by configuring build options to exclude WITH_OPUS or enabling WITH_DSP_FFMPEG which may utilize different decoding paths with more robust safety checks. Additionally, deploying network segmentation policies that restrict RDP connections from untrusted networks can reduce the attack surface. Monitoring for anomalous audio traffic patterns in deep packet inspection systems might also help detect exploitation attempts, although encryption typically obscures payload content until decryption occurs at the application layer. Regular patching of all remote access clients remains the most effective defense against such memory corruption vulnerabilities that exploit protocol implementation flaws.