CVE-2026-23732: FreeRDP Buffer Overflow Vulnerability

CVE-2026-23732 Overview

CVE-2026-23732 is a heap buffer overflow vulnerability in FreeRDP, a free implementation of the Remote Desktop Protocol. Prior to version 3.21.0, the FastGlyph parsing code trusts the cbData/remaining length values without validating them against the minimum size implied by the glyph dimensions (cx/cy). This allows a malicious RDP server to send crafted FastGlyph orders that trigger a client-side global buffer overflow, leading to a crash and denial of service.

Critical Impact

A malicious RDP server can exploit this vulnerability to crash FreeRDP clients, causing denial of service. Organizations relying on FreeRDP for remote access may experience disruption of services.

Affected Products

• FreeRDP versions prior to 3.21.0
• Applications and Linux distributions bundling vulnerable FreeRDP libraries
• Remote desktop clients built on the FreeRDP library

Discovery Timeline

• 2026-01-19 - CVE CVE-2026-23732 published to NVD
• 2026-01-20 - Last updated in NVD database

Technical Details for CVE-2026-23732

Vulnerability Analysis

This vulnerability exists in FreeRDP's handling of FastGlyph orders, which are part of the RDP protocol used to efficiently transmit glyph (character/font) data from server to client. The flaw is classified as CWE-122 (Heap-based Buffer Overflow).

When processing incoming FastGlyph orders, the client parses glyph data based on the cbData field provided by the server. The vulnerable code path fails to validate that this data length is sufficient to accommodate the glyph dimensions specified by the cx (width) and cy (height) fields. A malicious server can provide dimensions that imply a larger data requirement than the actual buffer allocated, causing subsequent operations to write beyond the buffer boundaries.

The vulnerability affects multiple components within the FreeRDP codebase, including the glyph cache module (glyph.c), color handling (color.c), graphics rendering (graphics.c), and order processing (orders.c).

Root Cause

The root cause is insufficient input validation in the FastGlyph parsing routine. The code trusts server-supplied cbData and remaining length values without computing and verifying the minimum buffer size required based on the glyph dimensions (cx × cy). This missing bounds check allows the server to specify arbitrary dimensions while providing an undersized data buffer, leading to heap corruption when the client attempts to process the glyph data.

Attack Vector

The attack vector is network-based, requiring a victim to connect to a malicious RDP server. The attacker operates a rogue RDP server that sends specially crafted FastGlyph orders containing mismatched dimension values and data lengths. When a vulnerable FreeRDP client connects and processes these orders, the buffer overflow occurs.

The exploitation scenario involves a victim client initiating an RDP connection to an attacker-controlled server. The server responds with malformed FastGlyph orders where cx/cy values imply a data size larger than cbData. The client allocates a buffer based on cbData but processes data as if it were sized according to cx/cy, resulting in out-of-bounds memory access. For technical implementation details, refer to the GitHub Security Advisory GHSA-7qxp-j2fj-c3pp and the associated source code review.

Detection Methods for CVE-2026-23732

Indicators of Compromise

• Unexpected FreeRDP client crashes when connecting to RDP servers
• Core dumps or crash logs showing segmentation faults in glyph.c, color.c, graphics.c, or orders.c modules
• Anomalous RDP traffic containing malformed FastGlyph orders with mismatched dimension and data length fields

Detection Strategies

• Monitor FreeRDP client processes for abnormal termination signals (SIGSEGV, SIGABRT)
• Implement network intrusion detection rules to identify RDP sessions with suspicious FastGlyph order patterns
• Deploy endpoint detection to alert on repeated FreeRDP crashes that may indicate active exploitation attempts

Monitoring Recommendations

• Enable verbose logging in FreeRDP clients to capture order processing details
• Monitor system logs for repeated application crashes associated with xfreerdp or custom FreeRDP-based applications
• Track connections to untrusted or unknown RDP servers in network flow data

How to Mitigate CVE-2026-23732

Immediate Actions Required

• Upgrade FreeRDP to version 3.21.0 or later immediately
• Audit all systems and applications using FreeRDP libraries and update accordingly
• Restrict RDP connections to trusted, known servers only
• Implement network segmentation to limit exposure of FreeRDP clients to potentially malicious servers

Patch Information

The FreeRDP project has addressed this vulnerability in version 3.21.0. The patch adds proper validation to ensure that the cbData/remaining length values are checked against the minimum size required by the glyph dimensions (cx/cy) before processing. Users should upgrade to FreeRDP version 3.21.0 or later. For detailed information about the fix, consult the GitHub Security Advisory.

Workarounds

• Avoid connecting to untrusted or unknown RDP servers until patching is complete
• Use network-level controls (firewalls, VPNs) to restrict RDP connectivity to verified internal servers
• Consider using alternative RDP clients if immediate patching is not feasible
• Implement application whitelisting to control which RDP servers clients can connect to

# Verify FreeRDP version to ensure patched version is installed
xfreerdp --version
# Expected output should show version 3.21.0 or higher