CVE-2026-44420: FreeRDP Buffer Overflow Vulnerability

CVE-2026-44420 Overview

CVE-2026-44420 is a heap buffer overflow write vulnerability in FreeRDP, an open-source implementation of the Remote Desktop Protocol (RDP). The flaw resides in the server-side clipboard (cliprdr) channel and affects all versions prior to 3.26.0. A malicious RDP client can send a crafted CB_CLIP_CAPS Protocol Data Unit (PDU) containing a capabilitySetLength value smaller than expected. This corrupts heap memory in the FreeRDP server process and can crash the service, producing a remote denial-of-service condition. Because the overflow corrupts the heap, exploitation for arbitrary code execution may be possible. The vulnerability is classified under [CWE-122] Heap-Based Buffer Overflow and is fixed in FreeRDP 3.26.0.

Critical Impact

An authenticated RDP client can corrupt server heap memory through a malformed clipboard capability PDU, resulting in remote denial of service and potential remote code execution.

Affected Products

• FreeRDP versions prior to 3.26.0
• Any application or server embedding the FreeRDP cliprdr server-side channel
• Remote desktop gateways and session brokers built on vulnerable FreeRDP releases

Discovery Timeline

• 2026-05-29 - CVE-2026-44420 published to the National Vulnerability Database (NVD)
• 2026-06-02 - Last updated in NVD database

Technical Details for CVE-2026-44420

Vulnerability Analysis

The vulnerability exists in FreeRDP's server-side clipboard redirection channel (cliprdr), which negotiates clipboard capabilities with connecting clients. During capability negotiation, the client sends a CB_CLIP_CAPS PDU describing supported clipboard features. The server parses this PDU and processes capability sets based on the capabilitySetLength field supplied by the client.

When a malicious client supplies a capabilitySetLength that is smaller than the actual size required to hold the capability set data, the server allocates an undersized heap buffer. Subsequent write operations overflow the allocation, corrupting adjacent heap metadata and memory. The result is a heap buffer overflow write that crashes the FreeRDP server process and may allow heap-based exploitation primitives.

Root Cause

The root cause is missing or insufficient validation of the attacker-controlled capabilitySetLength field before it is used to size a heap allocation and bound a copy operation. The server trusts the client-supplied length value, producing a classic [CWE-122] heap-based buffer overflow.

Attack Vector

An attacker connects to a FreeRDP server with clipboard redirection enabled and initiates clipboard capability negotiation. The attacker sends a crafted CB_CLIP_CAPS PDU containing an undersized capabilitySetLength. The malformed PDU triggers the out-of-bounds heap write in the server process. The attack requires network access to the RDP service and a valid session capable of exchanging clipboard PDUs.

No public proof-of-concept exploit code has been released for CVE-2026-44420 at the time of publication. Refer to the FreeRDP GHSA-mvpx-xj7r-3p3r Advisory for additional technical detail.

Detection Methods for CVE-2026-44420

Indicators of Compromise

• FreeRDP server process crashes or unexpected restarts coincident with active RDP sessions
• Heap corruption signatures or SIGSEGV faults logged by the FreeRDP server process
• Anomalous CB_CLIP_CAPS PDUs with capabilitySetLength values inconsistent with the trailing payload size
• Repeated short-lived RDP connections from a single source that terminate immediately after capability negotiation

Detection Strategies

• Inspect RDP virtual channel traffic for malformed cliprdr PDUs, particularly during capability exchange
• Alert on FreeRDP server crash signatures in system logs, journalctl, or core dump collection systems
• Correlate RDP session initiation events with subsequent service restarts of FreeRDP-based servers
• Monitor for the installed FreeRDP package version and flag any host running a version below 3.26.0

Monitoring Recommendations

• Enable verbose logging on FreeRDP servers to capture channel negotiation events and abnormal terminations
• Forward host telemetry, including process crash events and RDP session metadata, to a centralized analytics platform for correlation
• Track inbound connections to TCP 3389 and any custom RDP listener ports, alerting on bursts of failed sessions

How to Mitigate CVE-2026-44420

Immediate Actions Required

• Upgrade FreeRDP to version 3.26.0 or later on all affected servers and embedded deployments
• Inventory hosts and applications that link against FreeRDP libraries to identify indirect exposure
• Restrict RDP service exposure to trusted networks using firewall rules or VPN-gated access
• Disable clipboard redirection on FreeRDP servers where the feature is not required by business workflows

Patch Information

The FreeRDP maintainers released the fix in version 3.26.0. The patched code validates the capabilitySetLength value from CB_CLIP_CAPS PDUs before performing heap allocations and writes. Patch details and the official advisory are available in the FreeRDP GHSA-mvpx-xj7r-3p3r Advisory.

Workarounds

• Disable the cliprdr clipboard channel in FreeRDP server configuration until patching is complete
• Place FreeRDP servers behind an authenticating RDP gateway that terminates and validates virtual channel traffic
• Limit RDP exposure to authenticated VPN clients to reduce the attacker population able to reach the vulnerable channel

# Configuration example: disable clipboard redirection at FreeRDP server startup
freerdp-server --disable-channel=cliprdr

# Verify installed FreeRDP version meets the patched release
xfreerdp --version | grep -E '3\.(2[6-9]|[3-9][0-9])'