CVE-2026-109256: FreeRDP Buffer Overflow Vulnerability

CVE-2026-109256 Overview

CVE-2026-109256 is a heap buffer overflow [CWE-122] in FreeRDP, an open source implementation of the Remote Desktop Protocol (RDP). The flaw resides in the WinPR png_write_data callback, which writes past the allocated buffer after a realloc failure. A malicious RDP clipboard peer can supply a large CF_DIB (Device-Independent Bitmap) image that triggers an out-of-bounds memcpy during Portable Network Graphics (PNG) conversion. The result is a client-side crash. FreeRDP versions 3.10.0 through 3.32.2 are affected.

Critical Impact
A malicious RDP server or clipboard peer can crash a connected FreeRDP client by sending a crafted CF_DIB clipboard payload, producing an availability impact on the client process.

Affected Products

FreeRDP 3.10.0 through 3.32.2
WinPR library component shipped with affected FreeRDP releases
Downstream applications and distributions embedding vulnerable FreeRDP builds

Discovery Timeline

2026-10-12 - CVE-2026-109256 published to the National Vulnerability Database (NVD)
2026-10-14 - Last updated in NVD database

Technical Details for CVE-2026-109256

Vulnerability Analysis

The vulnerability occurs during clipboard data conversion within WinPR, the portable Windows runtime library used by FreeRDP. When an RDP peer sends a CF_DIB clipboard payload, FreeRDP converts the bitmap into PNG format using libpng. The png_write_data callback appends encoded bytes into a dynamically grown heap buffer backed by realloc.

When realloc fails under memory pressure or when confronted with an oversized image, the callback does not abort the write. Instead, it proceeds to call memcpy against the prior buffer pointer, writing attacker-controlled PNG stream bytes past the allocation boundary. This produces a classic heap buffer overflow that destabilizes the client and causes it to crash.

Because the trigger lives on the clipboard conversion path, the attack surface is exposed on both standalone client sessions and server-mode deployments that accept clipboard data from peers.

Root Cause

The root cause is missing error handling after a failed realloc call inside the png_write_data callback. The code assumes the reallocation succeeds and advances the write offset without checking the return value. Attacker-controlled image dimensions make it trivial to force a large allocation that exceeds available heap space.

Attack Vector

Exploitation requires an attacker to control an RDP peer that participates in clipboard exchange with a vulnerable FreeRDP client. User interaction is required to establish the session. Once connected, the peer sends a crafted CF_DIB clipboard image sized to force the PNG encoder into a realloc failure, triggering the out-of-bounds write and client crash. No verified public exploit code or proof-of-concept has been published at the time of writing. See the GitHub Security Advisory GHSA-wf3x-658c-52m5 for upstream details.

Detection Methods for CVE-2026-109256

Indicators of Compromise

Repeated FreeRDP client process crashes correlated with active RDP sessions to untrusted hosts
Operating system crash dumps referencing png_write_data, winpr, or libpng frames
Unexpected clipboard transfers of unusually large CF_DIB payloads from remote peers

Detection Strategies

Inventory endpoints and servers for FreeRDP binaries in the 3.10.0–3.32.2 range using software asset management or endpoint detection and response (EDR) telemetry
Alert on process termination events for xfreerdp, wfreerdp, or embedded FreeRDP consumers immediately following outbound RDP connections
Review network telemetry for RDP sessions to untrusted or newly observed destinations

Monitoring Recommendations

Centralize crash and reliability telemetry from workstations that use FreeRDP-based clients
Monitor outbound TCP port 3389 traffic to non-corporate destinations
Track clipboard channel usage within RDP sessions where supported by endpoint tooling

How to Mitigate CVE-2026-109256

Immediate Actions Required

Upgrade FreeRDP to a fixed release above 3.32.2 as published in the FreeRDP security advisory
Audit third-party applications that embed WinPR or FreeRDP and apply vendor updates when available
Restrict RDP connections from managed endpoints to known, trusted servers only

Patch Information

Refer to the upstream GitHub Security Advisory GHSA-wf3x-658c-52m5 and the VulnCheck advisory for fixed version information and patch commits. Rebuild and redistribute any downstream packages that statically link WinPR.

Workarounds

Disable the clipboard redirection channel in FreeRDP client invocations where clipboard sharing is not required
Limit RDP client usage to trusted networks pending patch deployment
Apply application allow-listing to prevent FreeRDP clients from connecting to arbitrary destinations

bash

# Disable clipboard redirection when launching xfreerdp as a workaround
xfreerdp /v:rdp.example.internal /u:analyst -clipboard