CVE-2026-121951 in FreeRDP
Summary
by MITRE • 11/04/2026
FreeRDP 3.0.0 before 3.32.2 server-side RDP peer input parsing skips the six-byte body of a slow-path input event with an unknown message type without checking it lies within the packet's declared length, causing a WinPR stream assertion failure. A remote RDP client with an active session can send a truncated unknown input event to abort assertion-enabled server processes, causing denial of service.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in FreeRDP versions prior to 3.32.2 represents a critical flaw within the server-side Remote Desktop Protocol (RDP) implementation, specifically affecting the parsing logic for peer input events. This issue stems from an insufficient boundary check during the processing of slow-path input messages that possess unknown or unrecognized message types. In standard RDP operations, input events are categorized into fast-path and slow-path mechanisms depending on their complexity and frequency. Slow-path inputs require more extensive parsing because they can carry variable-length data structures. The core technical deficiency lies in how the server handles these slow-path events when the specific message type is not immediately recognized by the application logic. Instead of validating that the entire payload, including a six-byte body associated with the unknown event, falls within the declared length of the incoming packet, the parser proceeds to process the data without verifying its integrity against the packet boundaries. This oversight allows for the transmission of truncated or malformed input events that do not conform to expected structural constraints.
From an operational perspective, this parsing flaw leads directly to a denial of service condition through assertion failures within the WinPR library, which serves as the Windows Protocol implementation layer for FreeRDP. When the server attempts to access memory regions associated with the six-byte body of the truncated event, it triggers an internal assertion check designed to catch programming errors and invalid states. Because the input was not properly bounded by length checks prior to this access, the assertion fails, causing the server process to abort or crash immediately. This mechanism effectively allows a remote attacker who has established an active RDP session with the vulnerable FreeRDP instance to disrupt service availability simply by sending a specifically crafted packet containing a truncated unknown slow-path input event. The impact is severe for environments relying on continuous remote desktop access, as it enables non-privileged users or attackers with valid credentials to crash the server application without requiring further exploitation steps such as code execution.
This vulnerability aligns closely with CWE-20 Improper Input Validation and CWE-119 Buffer Over-read if the assertion failure were replaced by actual memory corruption in different contexts, though here it manifests primarily as a stability issue due to defensive programming practices like assertions. In terms of attack classification under the MITRE ATT&CK framework, this behavior is consistent with T1499 Endpoint Denial of Service, specifically leveraging resource exhaustion or application crashes through malformed input rather than overwhelming system resources with volume. The lack of strict length validation for variable-length fields in protocol parsers is a common root cause for such issues, highlighting the importance of rigorous boundary checks before any data consumption occurs.
Mitigation strategies primarily involve upgrading to FreeRDP version 3.32.2 or later, where this parsing logic has been corrected to ensure that all components of an input event are validated against the packet's declared length prior to processing. For organizations unable to patch immediately due to dependency constraints, implementing network-level access controls can provide a temporary layer of defense by restricting RDP traffic to trusted IP addresses only. Additionally, deploying intrusion detection systems capable of identifying anomalous RDP packet structures may help in detecting attempts to exploit this specific parsing flaw before they result in service disruption. It is also advisable to review application logs for assertion failures or unexpected process terminations following remote input events as an indicator of potential exploitation activity.