CVE-2026-91956 in FreeRDP
Summary
by MITRE • 09/15/2026
FreeRDP before 3.31.0 contains an out-of-bounds read vulnerability in the URBDRC channel's func_get_ep_desc function that indexes interface arrays by position instead of protocol field InterfaceNumber. A malicious RDP server can send a crafted SELECT_CONFIGURATION message with permuted InterfaceNumber values to read past allocated heap memory and crash the client.
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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 security flaw within the Universal Remote Desktop Channel (URBDRC) implementation, specifically affecting the func_get_ep_desc function. This component is responsible for handling USB redirection features that allow clients to access peripheral devices connected to remote servers. The core technical deficiency lies in how the software processes interface descriptors received from the server during the configuration selection phase of a connection session. Instead of validating and utilizing the InterfaceNumber field provided by the protocol, which serves as the authoritative identifier for specific device interfaces, the application incorrectly indexes into internal data structures using an arbitrary position value derived from the message structure itself. This fundamental logic error creates a direct path to memory corruption because the index used does not correspond to the actual bounds of the allocated interface array.
From a technical perspective, this flaw constitutes an out-of-bounds read vulnerability that falls under CWE-125 in common weakness enumeration standards. When a malicious RDP server constructs and transmits a crafted SELECT_CONFIGURATION message containing permuted or excessively large InterfaceNumber values, it exploits the mismatch between the expected index range and the actual array size. The application proceeds to dereference memory addresses beyond the allocated heap region without performing adequate boundary checks. This action results in reading sensitive data from adjacent memory locations that were not intended for access by this function. In many execution contexts, such as when the read operation is followed by further processing of the retrieved data or when it triggers a segmentation fault due to accessing unmapped pages, the immediate operational impact is a denial of service characterized by client application crashes.
The security implications extend beyond simple availability loss. While the primary reported consequence is a crash, out-of-bounds reads can potentially leak confidential information stored in adjacent memory blocks, such as authentication tokens or other session-specific data structures. This aligns with ATT&CK technique T1005, which involves Data from Local System, although the specific exploitation vector here relies on remote code execution capabilities being absent unless combined with additional vulnerabilities to achieve arbitrary read primitives. The attack requires a man-in-the-middle scenario or direct connection to an untrusted RDP server that can inject these malformed packets during the initial handshake and configuration negotiation phases of the URBDRC channel initialization.
Mitigation strategies primarily involve upgrading FreeRDP to version 3.31.0 or later, where this indexing logic has been corrected to strictly validate InterfaceNumber values against actual array bounds before access. For environments unable to upgrade immediately, network-level controls such as firewalls should be configured to restrict RDP connections to trusted servers only, thereby preventing exposure to malicious endpoints capable of sending crafted URBDRC messages. Additionally, enabling strict protocol validation features within the client configuration can help detect and drop malformed packets that deviate from expected standard behaviors defined in Microsoft Remote Desktop Protocol specifications. Regular security patching remains the most effective defense against such implementation flaws in widely used open-source remote desktop clients.