CVE-2026-67305 Overview
CVE-2026-67305 is a heap buffer overflow vulnerability in the FreeRDP Windows client prior to version 3.29.0. The flaw resides in the clipboard virtual channel, specifically in the handling of CLIPRDR_FILE_CONTENTS_RESPONSE Protocol Data Units (PDUs). The client fails to validate the server-provided size against the destination buffer, allowing a malicious Remote Desktop Protocol (RDP) server to corrupt heap memory. A user connecting to an attacker-controlled server can trigger the overflow by performing a paste operation, potentially resulting in remote code execution on the client host. The weakness is classified as [CWE-122] Heap-based Buffer Overflow.
Critical Impact
A malicious RDP server can achieve arbitrary heap memory corruption on a connecting FreeRDP client, enabling remote code execution triggered by a routine clipboard paste action.
Affected Products
- FreeRDP Windows client versions prior to 3.29.0
- Applications and distributions bundling vulnerable FreeRDP builds
- Clipboard virtual channel (cliprdr) component of FreeRDP
Discovery Timeline
- 2026-08-01 - CVE-2026-67305 published to NVD
- 2026-08-05 - Last updated in NVD database
Technical Details for CVE-2026-67305
Vulnerability Analysis
The FreeRDP clipboard virtual channel (cliprdr) implements file-transfer semantics between an RDP server and client. When a client requests file contents, the server responds with a CLIPRDR_FILE_CONTENTS_RESPONSE PDU that carries a size field and a data payload. The vulnerable client code path trusts the server-declared payload length and copies the data into a heap buffer sized according to the original client request. A server that returns significantly more data than requested overruns the destination allocation. The overflow occurs entirely within heap memory managed by the client process, giving an attacker control over adjacent heap structures and function pointers.
Root Cause
The root cause is missing bounds validation between the server-provided payload size and the client-side destination buffer capacity. The parser accepts the length field from the untrusted server without cross-checking it against the buffer allocated for the pending file-contents request. This is a classic input-trust failure in a protocol handler that mixes size negotiation with data transfer in a single PDU.
Attack Vector
Exploitation requires a victim to initiate an RDP connection to a malicious server and perform a paste operation involving clipboard file content. The server sends a crafted CLIPRDR_FILE_CONTENTS_RESPONSE PDU with an oversized payload. Because the clipboard channel is enabled by default in most FreeRDP configurations, no additional protocol negotiation is required. User interaction is limited to the paste action, which security teams should treat as a low bar in interactive desktop workflows. Successful exploitation yields heap corruption suitable for arbitrary code execution in the client process context. Refer to the GitHub Security Advisory and the VulnCheck Security Advisory for protocol-level detail.
Detection Methods for CVE-2026-67305
Indicators of Compromise
- FreeRDP client processes (wfreerdp.exe, freerdp.exe) crashing or spawning child processes shortly after establishing an outbound RDP session.
- Outbound RDP connections (TCP/3389) from workstations to untrusted or newly registered internet-facing hosts.
- Unexpected memory access violations logged by Windows Error Reporting for FreeRDP binaries.
Detection Strategies
- Inventory installed FreeRDP versions across managed endpoints and flag any build earlier than 3.29.0.
- Alert on RDP client processes initiating suspicious child processes such as cmd.exe, powershell.exe, or rundll32.exe.
- Monitor for anomalous outbound RDP traffic patterns, particularly to hosts outside sanctioned administrative ranges.
Monitoring Recommendations
- Capture process creation, module load, and network connection telemetry from endpoints that run FreeRDP.
- Correlate FreeRDP process crashes with prior outbound RDP sessions to identify potential exploitation attempts.
- Track clipboard virtual channel usage where instrumentation is available, focusing on file-transfer sub-protocols.
How to Mitigate CVE-2026-67305
Immediate Actions Required
- Upgrade all FreeRDP Windows client installations to version 3.29.0 or later.
- Restrict outbound RDP connections to an allowlist of trusted server endpoints via network policy.
- Advise users to avoid paste operations when connected to untrusted or third-party RDP hosts.
Patch Information
The FreeRDP project addressed the heap buffer overflow in the cliprdr channel in release 3.29.0. Administrators should deploy the updated binaries across all systems running FreeRDP or applications that embed the library. Review the GitHub Security Advisory for the authoritative fix and version guidance.
Workarounds
- Disable the clipboard virtual channel redirection when connecting to untrusted RDP servers by omitting the +clipboard flag or explicitly passing /clipboard:off.
- Enforce egress firewall rules that block outbound TCP/3389 to non-corporate destinations.
- Segment workstations that require RDP client functionality from sensitive assets to limit blast radius from a successful exploit.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
