CVE-2026-91960 in FreeRDP
Summary
by MITRE • 09/15/2026
FreeRDP versions before 3.31.0 contain an integer overflow in WinPR's Stream_EnsureRemainingCapacity function that allows remote attackers to cause denial of service. A malicious RD Gateway peer can send a WebSocket Ping frame with a crafted 64-bit extended payload length to trigger integer wraparound, resulting in a double free that crashes the FreeRDP client during connection.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in FreeRDP versions prior to 3.31.0 represents a critical security flaw rooted in improper handling of input data within the WinPR library's stream management functions. Specifically, the integer overflow occurs in the Stream_EnsureRemainingCapacity function, which is responsible for managing memory buffers during network communication operations. This component plays a pivotal role in ensuring that sufficient buffer space exists to process incoming data streams without exceeding allocated limits. When processing WebSocket connections, particularly those involving Remote Desktop Protocol (RDP) gateways, the application relies on accurate parsing of payload lengths to allocate appropriate memory resources. The flaw arises when the system fails to correctly validate or handle 64-bit extended payload length fields in WebSocket Ping frames sent by a remote peer.
A malicious RD Gateway peer can exploit this weakness by transmitting a specially crafted WebSocket Ping frame containing an invalidly large 64-bit extended payload length value. Upon receiving this malformed data, the Stream_EnsureRemainingCapacity function attempts to calculate the required buffer size based on the provided length field. Due to insufficient validation of the integer range or lack of proper bounds checking during arithmetic operations involving this field, an integer wraparound condition is triggered. This mathematical error results in a significantly smaller calculated value than intended, leading the application to allocate far less memory than actually needed for processing the payload. Consequently, when subsequent write operations occur against this undersized buffer, it leads to heap corruption and ultimately triggers a double free vulnerability during connection teardown or cleanup phases.
The operational impact of this vulnerability is severe, primarily manifesting as a denial of service condition affecting FreeRDP clients connected through vulnerable versions. By exploiting the integer overflow and resulting memory management errors, an attacker can cause the client application to crash unexpectedly upon establishing or maintaining a connection with a compromised RD Gateway peer. This disruption prevents legitimate users from accessing remote desktop resources hosted behind such gateways. While the primary observed effect is service interruption rather than arbitrary code execution, the underlying mechanism involves heap corruption which theoretically could be leveraged for more sophisticated attacks if additional exploitation techniques are combined with this initial foothold. The vulnerability affects both client and server components depending on deployment configurations where FreeRDP acts as a gateway or proxy endpoint handling WebSocket traffic.
From a classification perspective, this issue aligns with CWE-190 Integer Overflow or Wraparound, which describes errors resulting from arithmetic operations that exceed the maximum value representable by an integer type. Additionally, it relates to CWE-416 Use After Free, as the double free condition indicates memory being deallocated multiple times, leading to undefined behavior and potential crashes. In terms of adversarial tactics, this vulnerability supports techniques associated with ATT&CK T1499 Endpoint Denial of Service, where attackers aim to disrupt availability by exhausting system resources or causing application failures through malformed inputs targeting network-facing services.
Mitigation strategies should prioritize immediate upgrading to FreeRDP version 3.31.0 or later, which includes patches addressing the integer validation logic within Stream_EnsureRemainingCapacity. Organizations relying on RDP gateways must ensure that all endpoints running vulnerable versions are updated promptly. In environments where patching cannot occur immediately, network-level controls such as strict firewall rules limiting WebSocket traffic to trusted peers can reduce exposure risk. Input validation mechanisms at the gateway level should also be reviewed to enforce stricter constraints on payload length fields before they reach internal processing functions. Regular security audits and penetration testing focusing on protocol parsing logic will help identify similar weaknesses in other components of the remote access infrastructure, ensuring comprehensive protection against exploitation attempts targeting memory management subsystems.