CVE-2026-33984: FreeRDP Buffer Overflow Vulnerability

CVE-2026-33984 Overview

CVE-2026-33984 is a heap buffer overflow vulnerability in FreeRDP, a free implementation of the Remote Desktop Protocol. The vulnerability exists in the resize_vbar_entry() function within libfreerdp/codec/clear.c. Prior to version 3.24.2, the code incorrectly updates vBarEntry->size to vBarEntry->count before the winpr_aligned_recalloc() call. If the reallocation fails, the size variable is inflated while the pixels pointer still references the old, smaller buffer. On subsequent calls where count is less than or equal to the inflated size value, reallocation is skipped, and the caller writes attacker-controlled pixel data into the undersized buffer, resulting in a heap buffer overflow.

Critical Impact

A remote attacker can exploit this heap buffer overflow vulnerability to potentially execute arbitrary code or cause a denial of service on systems running vulnerable versions of FreeRDP.

Affected Products

• FreeRDP versions prior to 3.24.2
• Systems using FreeRDP as an RDP client
• Applications integrating libfreerdp codec libraries

Discovery Timeline

• 2026-03-30 - CVE CVE-2026-33984 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-33984

Vulnerability Analysis

This vulnerability is classified as CWE-122 (Heap-based Buffer Overflow). The flaw stems from improper memory management in the CLEAR codec implementation used for RDP bitmap decompression.

The vulnerable code path involves the resize_vbar_entry() function which handles dynamic resizing of vertical bar entries used in bitmap processing. The fundamental issue is a premature size variable update that creates a state inconsistency when memory allocation fails. The attack requires network access and user interaction (connecting to a malicious RDP server), but no prior authentication is required.

Root Cause

The root cause is a logic error in memory management within resize_vbar_entry(). The code updates vBarEntry->size to vBarEntry->count before calling winpr_aligned_recalloc(). When the reallocation fails, the function does not roll back the size update, leaving the structure in an inconsistent state where size indicates more allocated memory than actually exists. This creates a classic heap overflow condition exploitable on subsequent operations.

Attack Vector

The attack vector is network-based, requiring a victim to connect to a malicious RDP server. The attacker-controlled server can send specially crafted CLEAR codec data that:

1. Triggers an initial resize operation that will fail due to resource constraints
2. Sends a subsequent request with a count value within the inflated (but incorrect) size limit
3. Delivers attacker-controlled pixel data that overwrites heap memory beyond the actual buffer allocation

The following patch demonstrates the fix applied in version 3.24.2:

 		const UINT32 oldPos = vBarEntry->size * bpp;
 		const UINT32 diffSize = (vBarEntry->count - vBarEntry->size) * bpp;
 
-		vBarEntry->size = vBarEntry->count;
 		BYTE* tmp =
 		    (BYTE*)winpr_aligned_recalloc(vBarEntry->pixels, vBarEntry->count, 1ull * bpp, 32);

Source: GitHub Commit Update

The patch moves the size update to occur only after successful memory allocation, preventing the inconsistent state that enables exploitation.

Detection Methods for CVE-2026-33984

Indicators of Compromise

• Unexpected crashes or segmentation faults in FreeRDP client processes
• Abnormal memory allocation patterns in processes using libfreerdp
• RDP connections to unfamiliar or suspicious server addresses
• Process memory anomalies indicating heap corruption

Detection Strategies

• Monitor FreeRDP client processes for crash events and abnormal termination
• Implement network monitoring for RDP connections (port 3389) to untrusted destinations
• Deploy endpoint detection rules for heap corruption indicators in RDP-related processes
• Enable application crash reporting and analyze dumps for exploitation patterns

Monitoring Recommendations

• Track FreeRDP version deployments across the environment and flag versions prior to 3.24.2
• Monitor for unusual RDP connection patterns, particularly to external or unknown servers
• Implement logging of RDP session establishment events for forensic analysis
• Configure memory protection features (ASLR, DEP) and monitor for bypass attempts

How to Mitigate CVE-2026-33984

Immediate Actions Required

• Update FreeRDP to version 3.24.2 or later immediately
• Audit systems for FreeRDP installations and applications using libfreerdp
• Restrict RDP client connections to trusted servers only via network policies
• Enable memory protection features on systems running FreeRDP clients

Patch Information

FreeRDP has released version 3.24.2 which addresses this vulnerability. The fix ensures that vBarEntry->size is only updated after successful memory allocation in the resize_vbar_entry() function. The patch is available via the GitHub Commit Update. Additional details are available in the GitHub Security Advisory GHSA-8469-2xcx-frf6.

Workarounds

• Restrict FreeRDP client usage to trusted, internal RDP servers only
• Implement network segmentation to limit exposure of RDP client systems
• Use VPN or other secure tunneling when connecting to remote RDP servers
• Disable RDP client functionality on systems where it is not required

# Example: Restrict FreeRDP connections to trusted servers via firewall
# Block outbound RDP to untrusted networks
iptables -A OUTPUT -p tcp --dport 3389 -d 10.0.0.0/8 -j ACCEPT
iptables -A OUTPUT -p tcp --dport 3389 -d 192.168.0.0/16 -j ACCEPT
iptables -A OUTPUT -p tcp --dport 3389 -j DROP