CVE-2026-91951 in FreeRDP
Summary
by MITRE • 09/15/2026
FreeRDP versions before 3.31.0 contain an out-of-bounds write vulnerability in the urbdrc client channel's urb_send_current_frame_number_result() function. A malicious RDP server can send a crafted 28-byte USB redirection message to trigger a 4-byte write past the allocated 16-byte buffer, causing denial of service when verbose asserts are enabled.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in FreeRDP versions prior to 3.31.0 represents a critical memory safety flaw located within the urbdrc client channel implementation. Specifically, the defect resides in the urb_send_current_frame_number_result function, which is responsible for handling USB redirection data streams during remote desktop sessions. This component acts as an intermediary between the local system and the remote server's virtualized hardware devices, making it a high-value target for attackers seeking to compromise the integrity of the connection or the underlying host machine. The root cause of this issue is a failure in boundary checking when processing incoming USB redirection messages from the RDP server side.
From a technical perspective, the vulnerability manifests as an out-of-bounds write operation triggered by a specifically crafted input payload. When a malicious RDP server sends a 28-byte USB redirection message to the client, the urbdrc module fails to validate that the data length fits within the expected buffer constraints. Consequently, the function attempts to perform a four-byte write operation beyond the boundaries of an allocated sixteen-byte buffer. This memory corruption occurs because the code does not adequately verify whether the incoming packet size exceeds the static allocation limits before proceeding with the copy or assignment operations. Such errors are characteristic of improper input validation where the software assumes that received data will always conform to expected dimensions without rigorous runtime checks.
The operational impact of this vulnerability is primarily centered on service availability and system stability. In environments where verbose assertions are enabled, typically during development or debugging phases, triggering this buffer overflow results in an immediate denial of service due to assertion failures halting the application execution. However, even in production builds where such asserts may be disabled, out-of-bounds writes pose a severe risk as they can corrupt adjacent memory structures, lead to undefined behavior, or potentially allow for arbitrary code execution if the attacker can control the data written past the buffer boundary. This aligns with common attack patterns found in network-facing services that process untrusted input from remote peers without sufficient sanitization.
This flaw is categorized under CWE-787: Out-of-bounds Write, which describes writing to a memory location outside of the intended destination buffer. It also relates closely to CWE-20: Improper Input Validation, as the core failure lies in not verifying that the input data conforms to expected size constraints before processing. From an offensive security perspective, this vulnerability can be leveraged within the context of MITRE ATT&CK technique T1498: Network Denial of Service, where the attacker aims to disrupt normal operations by crashing the FreeRDP client process. Additionally, depending on how memory is laid out and whether further exploitation techniques are applied, it could potentially facilitate privilege escalation or remote code execution if combined with other vulnerabilities in the stack.
To mitigate this risk, organizations relying on FreeRDP must ensure that all instances of the software are upgraded to version 3.31.0 or later, where the boundary checks have been corrected and validated by upstream developers. For environments unable to patch immediately due to compatibility constraints, deploying network-level access control lists to restrict connections from untrusted RDP servers can reduce exposure. Furthermore, enabling strict memory protection mechanisms such as Address Space Layout Randomization (ASLR) and Data Execution Prevention (DEP) on client systems can help mitigate the potential impact of any successful exploitation attempts by making it more difficult for attackers to predict memory addresses or execute injected code. Regular security audits focusing on input validation in network protocol handlers are also recommended to prevent similar issues across other components of the remote desktop infrastructure.