CVE-2026-29776: FreeRDP Integer Underflow RCE Vulnerability

CVE-2026-29776 Overview

CVE-2026-29776 is an Integer Underflow vulnerability affecting FreeRDP, a free implementation of the Remote Desktop Protocol. The vulnerability exists in the update_read_cache_bitmap_order function within FreeRDP's core library. Prior to version 3.24.0, insufficient input validation allows an integer underflow condition to occur when processing bitmap cache orders, potentially leading to denial of service conditions.

Critical Impact

An integer underflow in FreeRDP's bitmap order processing could allow remote attackers to cause service disruption through malformed RDP data, affecting availability of systems running vulnerable FreeRDP versions.

Affected Products

• FreeRDP versions prior to 3.24.0

Discovery Timeline

• 2026-03-13 - CVE-2026-29776 published to NVD
• 2026-03-17 - Last updated in NVD database

Technical Details for CVE-2026-29776

Vulnerability Analysis

This vulnerability stems from a classic integer underflow condition (CWE-190) in the update_read_cache_bitmap_order function located in FreeRDP's core library (libfreerdp/core/orders.c). The flaw occurs during the processing of bitmap compression headers when handling RDP cache bitmap orders.

The vulnerable code performs an arithmetic subtraction on the bitmapLength field without first verifying that the value is large enough to accommodate the subtraction. When processing bitmap data with compression headers, the code reads an 8-byte bitmapComprHdr structure and then subtracts 8 from the bitmapLength value. If bitmapLength is less than 8, this subtraction causes an integer underflow, wrapping the value to an extremely large number.

This underflow can lead to subsequent memory access violations or unexpected behavior as the corrupted length value is used in downstream processing operations. The network attack vector requires user interaction and has high attack complexity, which limits the practical exploitability of this vulnerability.

Root Cause

The root cause is missing bounds checking before performing arithmetic operations on the bitmapLength variable. The code assumes that bitmapLength will always be at least 8 bytes when compression headers are present, but fails to validate this assumption. This violates the principle of defensive programming where all input values should be validated before use, especially when performing arithmetic that could result in underflow or overflow conditions.

Attack Vector

The vulnerability is exploitable via a network-based attack where a malicious RDP server or man-in-the-middle attacker sends specially crafted RDP data containing a cache bitmap order with a bitmapLength value less than 8. When a vulnerable FreeRDP client processes this malformed data, the integer underflow occurs. The attack requires user interaction (the victim must connect to the malicious server or have their connection intercepted) and has high complexity due to the specific conditions required.

The following patch from the FreeRDP security fix demonstrates the required validation:

 			goto fail;

 		Stream_Read(s, bitmapComprHdr, 8); /* bitmapComprHdr (8 bytes) */
+		if (cache_bitmap->bitmapLength < 8)
+			goto fail;
 		cache_bitmap->bitmapLength -= 8;
 	}
}

Source: GitHub FreeRDP Commit

The patch adds a bounds check to verify that bitmapLength is at least 8 before performing the subtraction, preventing the integer underflow condition.

Detection Methods for CVE-2026-29776

Indicators of Compromise

• Unexpected FreeRDP client crashes or segmentation faults during RDP sessions
• Abnormal memory consumption patterns in FreeRDP processes
• RDP connection failures with error messages related to bitmap processing
• Log entries indicating malformed RDP data or protocol violations

Detection Strategies

• Monitor FreeRDP client processes for unexpected termination or crashes during active RDP sessions
• Implement network-level detection for malformed RDP packets with suspicious bitmap order structures
• Deploy application crash monitoring to detect potential exploitation attempts
• Use endpoint detection to identify FreeRDP versions prior to 3.24.0

Monitoring Recommendations

• Enable verbose logging in FreeRDP clients to capture protocol-level errors and anomalies
• Configure endpoint security solutions to alert on FreeRDP process crashes
• Implement network traffic analysis for RDP connections to detect malformed bitmap cache orders
• Maintain an inventory of FreeRDP installations and their versions across the environment

How to Mitigate CVE-2026-29776

Immediate Actions Required

• Upgrade all FreeRDP installations to version 3.24.0 or later immediately
• Review and audit all systems using FreeRDP clients for vulnerable versions
• Consider temporarily restricting RDP connections to trusted servers until patching is complete
• Implement network segmentation to limit exposure of systems running vulnerable FreeRDP versions

Patch Information

The vulnerability has been fixed in FreeRDP version 3.24.0. The fix adds proper input validation to ensure that bitmapLength is at least 8 bytes before subtracting the compression header size. Organizations should update to this version or later to remediate the vulnerability.

For detailed patch information, refer to:

• GitHub FreeRDP Security Commit
• GitHub Security Advisory GHSA-c747-x4wf-cqrr

Workarounds

• Restrict RDP connections to only known, trusted servers to reduce attack surface
• Implement network-level filtering to block connections from untrusted RDP endpoints
• Consider using alternative RDP clients until FreeRDP can be updated
• Deploy application allowlisting to control which RDP client versions can execute

# Verify FreeRDP version to check if patching is required
xfreerdp --version

# Update FreeRDP on Debian/Ubuntu systems
sudo apt update && sudo apt install freerdp2-x11

# Update FreeRDP on Fedora/RHEL systems
sudo dnf update freerdp