CVE-2026-68579: FreeRDP Buffer Overflow Vulnerability

CVE-2026-68579 Overview

CVE-2026-68579 is a heap-based buffer overflow [CWE-787] in FreeRDP versions prior to 3.30.0 (affecting versions up to and including 3.29.0). The flaw resides in the Windows clipboard client's CliprdrStream_Read function, implemented in client/Windows/wf_cliprdr.c. When an OLE paste consumer such as explorer.exe calls IStream::Read with a fixed-size buffer, the function trusts the server-supplied length (req_fsize) rather than the caller's buffer size (cb) when copying file contents. A malicious Remote Desktop Protocol (RDP) server can return an oversized CB_FILECONTENTS_RESPONSE and write attacker-controlled data past the paste consumer's heap buffer.

Critical Impact
A malicious RDP server can corrupt heap memory in the connected client's paste consumer process, enabling denial of service and potential code execution in the context of the user pasting server-offered clipboard data.

Affected Products

FreeRDP 3.x releases up to and including 3.29.0
Windows RDP clients built against the vulnerable FreeRDP wf_cliprdr component
Downstream applications embedding FreeRDP client libraries with clipboard redirection enabled

Discovery Timeline

2026-08-02 - CVE-2026-68579 published to the National Vulnerability Database
2026-08-03 - Last updated in NVD database

Technical Details for CVE-2026-68579

Vulnerability Analysis

The vulnerability exists in FreeRDP's Windows clipboard redirection path, which implements the OLE IStream interface so that Windows shell components can retrieve file contents offered by a remote RDP server. When a user pastes a server-hosted file, the paste consumer (for example, explorer.exe) allocates a heap buffer of cb bytes and calls IStream::Read. FreeRDP's CliprdrStream_Read then issues a CB_FILECONTENTS_REQUEST to the RDP server and copies the returned data into the caller's buffer using the server-supplied req_fsize length. Because req_fsize is never bounded by cb, an out-of-bounds write occurs whenever the server returns more bytes than requested.

Root Cause

The root cause is missing length validation between attacker-controlled and caller-supplied buffer sizes. CliprdrStream_Read in client/Windows/wf_cliprdr.c uses req_fsize from the network-parsed CB_FILECONTENTS_RESPONSE as the copy length instead of clamping it to cb. This violates the OLE contract that a stream read must never write more bytes than the consumer requested and makes the heap layout of the paste consumer directly writable by a malicious server.

Attack Vector

Exploitation requires a victim to connect a vulnerable FreeRDP-based Windows client to an attacker-controlled or compromised RDP server, enable clipboard redirection, and paste server-offered clipboard file contents into a local application. The server responds to the resulting CB_FILECONTENTS_REQUEST with an inflated cbRequested value and oversized payload, corrupting the heap of the paste consumer with attacker-chosen bytes. No authentication on the client side is required beyond the user pasting the offered content.

Code-level details of the fix are available in the upstream commit 5e8e987b469b60a3bafadf8f5afc40f91c09f458. See the GitHub Security Advisory GHSA-m37j-jcr2-8gcc and the VulnCheck advisory for the corrected length-clamping logic and additional context.

Detection Methods for CVE-2026-68579

Indicators of Compromise

Unexpected crashes or heap corruption in explorer.exe or other OLE paste consumers immediately after paste operations from an RDP session.
RDP sessions with clipboard redirection enabled that transmit CB_FILECONTENTS_RESPONSE messages whose payload length exceeds the corresponding CB_FILECONTENTS_REQUESTcbRequested value.
Windows Error Reporting entries citing heap corruption in modules loaded by FreeRDP-based clients such as wfreerdp.exe.

Detection Strategies

Inventory endpoints running FreeRDP 3.29.0 or earlier by hashing or version-checking client binaries and libraries.
Instrument clipboard virtual channel traffic to compare cbRequested and returned payload sizes, alerting on mismatches.
Enable PageHeap or Application Verifier on FreeRDP clients during testing to surface out-of-bounds writes deterministically.

Monitoring Recommendations

Monitor process termination and exception telemetry for FreeRDP client processes and paste consumers correlated with active RDP sessions.
Log outbound RDP connections to untrusted or newly observed servers, especially where clipboard file redirection is negotiated.
Track FreeRDP client versions across the fleet and alert when hosts remain on vulnerable builds after the patch window.

How to Mitigate CVE-2026-68579

Immediate Actions Required

Upgrade all FreeRDP client installations to version 3.30.0 or later, which introduces the length clamp in CliprdrStream_Read.
Restrict outbound RDP connections from managed endpoints to trusted, inventoried servers only.
Disable clipboard file redirection where it is not required for business workflows.

Patch Information

The fix is committed upstream in the FreeRDP repository. Review the FreeRDP commit 5e8e987b and the GitHub Security Advisory GHSA-m37j-jcr2-8gcc for the corrected read path. Distributions and downstream projects embedding FreeRDP should rebuild against 3.30.0 or backport the length-validation change.

Workarounds

Disable clipboard redirection in FreeRDP client invocations by omitting +clipboard or explicitly passing -clipboard.
Avoid pasting files offered by untrusted RDP servers until clients are upgraded.
Route RDP traffic through gateways that enforce server allow-listing and inspect clipboard virtual channels.

bash

# Launch wfreerdp with clipboard redirection disabled as a temporary workaround
wfreerdp /v:rdp.example.internal /u:analyst -clipboard