CVE-2026-109240 in FreeRDP
Summary
by MITRE • 10/12/2026
FreeRDP 3.11.0 before 3.32.1 contains a reachable assertion vulnerability in urb_isoch_transfer that allows malicious RDP servers to crash clients by sending unvalidated OutputBufferSize values. Attackers can send a TS_URB_ISOCH_TRANSFER IN-direction request with OutputBufferSize above INT32_MAX over URBDRC, triggering an assertion in libusb_udev_isoch_transfer that terminates the client.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability identified within FreeRDP versions prior to 3.32.1 represents a critical security flaw located specifically within the urb_isohc_transfer function. This component is responsible for handling USB redirection traffic, which allows clients to access peripheral devices connected to remote servers over an RDP session. The core technical issue stems from insufficient validation of input parameters received during the processing of TS_URB_ISOCH_TRANSFER requests. Specifically, when a malicious Remote Desktop Protocol server sends an IN-direction request via the URBDRC channel with an OutputBufferSize value that exceeds INT32_MAX, it triggers an assertion failure within the libusb_udev_isoch_transfer routine. In software engineering terms, this is not merely a logic error but a fundamental lack of boundary checking on integer values derived from external network inputs. The assertion acts as a safety check intended to catch programming errors during development; however, in production builds where assertions are often enabled or handled abruptly, the failure results in an immediate termination of the client application process rather than graceful error handling.
From a technical perspective, this vulnerability is classified under CWE-20 Improper Input Validation and more specifically relates to CWE-190 Integer Overflow or Wraparound if the validation logic itself was flawed, though here it manifests as a failure to validate against expected bounds before passing data to lower-level libraries. The attack vector requires an active RDP connection where the client is configured for USB redirection. An attacker controlling the remote server side can craft specific network packets containing malformed TS_URB_ISOCH_TRANSFER structures. By setting the OutputBufferSize field to a value greater than 2,147,483,647 (INT32_MAX), the FreeRDP client attempts to process this invalid size during USB isochronous transfer handling. The subsequent assertion failure causes the application to crash instantly, leading to a Denial of Service condition for that specific session and potentially disrupting any ongoing work or data transmission associated with the remote desktop connection.
The operational impact of this vulnerability is primarily focused on availability rather than confidentiality or integrity in its current manifestation. A successful exploit allows an unauthenticated attacker who has established an RDP connection to crash the FreeRDP client application remotely. This denial of service can be used to disrupt business operations, force users to reconnect and potentially expose them to further attacks during re-authentication phases, or simply serve as a nuisance attack against infrastructure relying on stable remote desktop sessions. While the immediate effect is a crash, such instability in core connectivity software can have cascading effects if it occurs within automated systems or critical operational technology environments where consistent RDP availability is required for monitoring and control tasks.
Mitigation strategies should focus primarily on upgrading to FreeRDP version 3.32.1 or later, as this release contains the necessary patches to validate input sizes properly before processing them through USB redirection channels. Organizations relying on older versions must implement network-level controls such as firewalls or intrusion prevention systems that can inspect RDP traffic for anomalous packet structures, although deep packet inspection of encrypted RDP streams may be limited without specific decryption capabilities. Additionally, administrators should restrict URBDRC usage to trusted networks and enforce strict access control policies to ensure only authorized servers can initiate USB redirection sessions with clients. Regular patch management cycles are essential to address such vulnerabilities promptly, ensuring that the software stack remains resilient against known exploitation techniques targeting input validation failures in network-facing components.