CVE-2026-109255: FreeRDP Buffer Overflow Vulnerability

CVE-2026-109255 Overview

CVE-2026-109255 is a heap buffer overflow vulnerability in FreeRDP, an open-source implementation of the Remote Desktop Protocol (RDP). The flaw exists in the license_check_stream_length function and stems from an integer truncation issue affecting 32-bit builds. A malicious RDP server can send a crafted SERVER_LICENSE_REQUEST message with an oversized scopeCount value that bypasses length validation, triggering out-of-bounds writes on the heap. Exploitation can lead to denial of service or remote code execution on the connecting client. The vulnerability affects FreeRDP versions 3.0.0 through 3.31.x and is fixed in 3.32.0. The weakness is classified as [CWE-122] Heap-based Buffer Overflow.

Critical Impact

A malicious RDP server can corrupt heap memory on connecting 32-bit FreeRDP clients, enabling denial of service or potential remote code execution.

Affected Products

• FreeRDP 3.0.0 through 3.31.x (32-bit client builds)
• Applications and distributions bundling the vulnerable FreeRDP library
• RDP clients using FreeRDP on 32-bit platforms

Discovery Timeline

• 2026-10-12 - CVE-2026-109255 published to NVD
• 2026-10-15 - Last updated in NVD database

Technical Details for CVE-2026-109255

Vulnerability Analysis

The vulnerability resides in license_check_stream_length, a helper function that validates the remaining bytes in a stream before parsing fields of a server license message. On 32-bit builds, size calculations involving the scopeCount field undergo integer truncation when multiplied by the per-scope element size. The truncated value then satisfies the length check even though the actual data referenced exceeds the buffer. Subsequent parsing writes attacker-controlled data past the allocated heap region. The condition requires a victim to initiate an RDP connection to a server under attacker control, meaning user interaction is required. Successful exploitation targets integrity and availability of the client process, which may run with the privileges of the connecting user.

Root Cause

The root cause is an integer truncation defect in the length validation logic of license_check_stream_length. When a large scopeCount is supplied in a SERVER_LICENSE_REQUEST, arithmetic performed with 32-bit size_t wraps or truncates, producing a small value that passes the bounds check. The parser then processes an unchecked number of scope entries into an undersized heap allocation.

Attack Vector

An attacker must operate or control an RDP server and convince a user to connect using a vulnerable 32-bit FreeRDP client. During the licensing negotiation phase, the server returns a crafted SERVER_LICENSE_REQUEST with an inflated scopeCount. The malformed message triggers the overflow before any authentication occurs, allowing memory corruption during the initial handshake.

No verified proof-of-concept code is published. See the FreeRDP GitHub Security Advisory and the Vulncheck Heap Overflow Advisory for technical references.

Detection Methods for CVE-2026-109255

Indicators of Compromise

• Unexpected crashes or segmentation faults in xfreerdp, wlfreerdp, or applications linking the FreeRDP library during RDP connection setup
• Outbound RDP (TCP/3389) connections from workstations to untrusted or newly observed external hosts
• Heap corruption signatures in client-side crash dumps referencing license_check_stream_length or the licensing module

Detection Strategies

• Inventory endpoints running 32-bit FreeRDP builds and identify versions prior to 3.32.0 using software asset management data
• Alert on RDP sessions initiated from client workstations to Internet-facing or non-corporate RDP endpoints
• Monitor process telemetry for abnormal child processes spawned by FreeRDP-based clients following outbound RDP sessions

Monitoring Recommendations

• Capture and retain crash telemetry from RDP client processes to detect exploitation attempts resulting in process termination
• Baseline normal RDP destinations per user and flag deviations, particularly connections to residential or hosting-provider IP ranges
• Correlate FreeRDP client execution with subsequent suspicious process creation, file writes, or network callbacks

How to Mitigate CVE-2026-109255

Immediate Actions Required

• Upgrade FreeRDP to version 3.32.0 or later on all affected systems, prioritizing 32-bit builds
• Audit third-party applications that bundle FreeRDP and apply vendor updates incorporating the fixed library
• Restrict outbound RDP (TCP/3389) at perimeter firewalls to known, trusted destinations only

Patch Information

The vulnerability is remediated in FreeRDP 3.32.0. Patch details and commit references are available in the FreeRDP GitHub Security Advisory GHSA-hxw3-57rq-3v4m. Linux distribution maintainers typically backport fixes to supported package versions; verify your distribution's security tracker for the corresponding update.

Workarounds

• Use 64-bit FreeRDP builds where feasible, as the integer truncation condition is specific to 32-bit size arithmetic
• Avoid connecting to untrusted or unknown RDP servers until patched builds are deployed
• Enforce RDP connections only to internal, authenticated infrastructure through network segmentation or VPN gateways

# Verify installed FreeRDP version
xfreerdp --version

# Debian/Ubuntu example: update to patched package
sudo apt update && sudo apt install --only-upgrade freerdp3-x11 libfreerdp3

# Fedora/RHEL example
sudo dnf upgrade freerdp