CVE-2026-67300: FreeRDP Use-After-Free Vulnerability

CVE-2026-67300 Overview

CVE-2026-67300 is a client-side heap use-after-free vulnerability in FreeRDP versions prior to 3.29.0. The flaw resides in the async update message proxy for Remote Applications Integrated Locally (RAIL) WINDOW_STATE_ORDER and NOTIFY_ICON_STATE_ORDER when AsyncUpdate is enabled. A malicious or compromised RDP server can trigger memory corruption or a client crash by sending crafted update orders. The message proxy shallow-copies structures containing nested parser-owned pointers, which the parser frees after the callback returns. Queued async messages then dispatch stale pointers. This vulnerability is tracked under CWE-416 (Use After Free).

Critical Impact
Attackers operating a hostile RDP server can corrupt memory on connecting FreeRDP clients, resulting in client crashes or potential exploitation for further impact.

Affected Products

FreeRDP versions prior to 3.29.0
FreeRDP clients with AsyncUpdate enabled
Applications embedding FreeRDP that process RAIL window and notify icon state orders

Discovery Timeline

2026-08-01 - CVE-2026-67300 published to NVD
2026-08-03 - Last updated in NVD database

Technical Details for CVE-2026-67300

Vulnerability Analysis

The vulnerability affects FreeRDP's asynchronous update dispatch path, specifically the message proxy that queues RAIL orders for later processing. When AsyncUpdate is enabled, incoming WINDOW_STATE_ORDER and NOTIFY_ICON_STATE_ORDER structures are copied into queued messages before dispatch. These structures embed nested pointers owned by the parser, including titleInfo.string, windowRects, visibilityRects, and icon buffers. The proxy performs a shallow copy rather than deep-cloning the nested allocations. The parser subsequently frees those nested buffers when the parsing callback returns, leaving the queued message holding dangling pointers.

When the queued async message is later dispatched, it dereferences the freed memory. This produces either memory corruption or a client crash, depending on heap state at the time of dispatch. Successful exploitation requires the victim client to connect to an attacker-controlled RDP server with AsyncUpdate enabled.

Root Cause

The root cause is an ownership mismatch between the RAIL parser and the async message proxy. The parser assumes ownership of nested buffers and releases them after invoking the callback. The proxy assumes the copied structure remains valid until the queued message is processed. Neither component takes responsibility for deep-copying the nested pointers, resulting in a classic use-after-free condition.

Attack Vector

Exploitation requires a client to initiate an RDP session to a malicious server. No user interaction beyond establishing the connection is required. The server transmits crafted RAIL update orders containing WINDOW_STATE_ORDER or NOTIFY_ICON_STATE_ORDER structures. When the client's async update thread processes the queued message, it dereferences freed heap memory. See the FreeRDP Security Advisory GHSA-33gg-h66j-3697 for the full technical description.

text

# Patch reference from cmake/GetProjectVersion.cmake
- set(RAW_VERSION_STRING "3.27.2-dev0")
+ set(RAW_VERSION_STRING "3.28.0")

Source: FreeRDP Commit 5370fb26. The patch bumps the release version as part of the fix series that addresses the async proxy ownership issue.

Detection Methods for CVE-2026-67300

Indicators of Compromise

Unexpected FreeRDP client crashes or segmentation faults during active RDP sessions to untrusted hosts
Heap corruption signatures in crash dumps referencing RAIL order processing functions or async update dispatch frames
Outbound RDP (TCP/3389) connections from workstations to unknown or newly registered external hosts

Detection Strategies

Inventory FreeRDP-based clients including Remmina, GNOME Connections, and third-party embedders, and flag versions below 3.29.0
Correlate client-side application crashes with concurrent RDP session establishment to non-corporate destinations
Monitor endpoint telemetry for FreeRDP process termination followed by core dump generation

Monitoring Recommendations

Log outbound RDP connections at network egress points and alert on connections to destinations outside approved allow lists
Capture and retain FreeRDP client crash telemetry for retrospective analysis against known exploitation patterns
Track software inventory changes to detect deployment of vulnerable FreeRDP builds across managed endpoints

How to Mitigate CVE-2026-67300

Immediate Actions Required

Upgrade all FreeRDP installations to version 3.29.0 or later across managed endpoints
Identify third-party applications that statically link or bundle FreeRDP and apply vendor updates as they become available
Restrict outbound RDP connectivity to trusted destinations at the network perimeter

Patch Information

The fix is included in FreeRDP 3.29.0. Review the upstream commit at FreeRDP Commit 5370fb26 and the VulnCheck Advisory for guidance on downstream package updates. Distribution maintainers should backport the patch to supported FreeRDP branches.

Workarounds

Disable AsyncUpdate in FreeRDP client configurations until patched builds are deployed, which prevents the proxy path from being exercised
Limit RDP client usage to sessions with trusted, internally managed servers only
Apply network segmentation to isolate workstations that require RDP connectivity to external hosts

bash

# Example: launch FreeRDP client with async update disabled
xfreerdp /v:rdp.example.internal /u:user /async-update:off