CVE-2026-27951: FreeRDP Denial of Service Vulnerability

CVE-2026-27951 Overview

CVE-2026-27951 is an Integer Overflow vulnerability affecting FreeRDP, a free implementation of the Remote Desktop Protocol. Prior to version 3.23.0, the Stream_EnsureCapacity function can create an endless blocking loop due to improper integer handling. This vulnerability affects all client and server implementations using FreeRDP, though practical exploitation is limited to 32-bit systems where available physical memory is greater than or equal to SIZE_MAX.

Critical Impact

Remote attackers can cause denial of service conditions by triggering an infinite loop in the stream capacity allocation function, potentially making FreeRDP clients and servers unresponsive.

Affected Products

• FreeRDP versions prior to 3.23.0
• All FreeRDP client implementations
• All FreeRDP server implementations

Discovery Timeline

• 2026-02-25 - CVE-2026-27951 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27951

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The root cause lies in improper buffer size calculations within FreeRDP's stream handling functions. When the Stream_EnsureCapacity function processes requests for buffer allocation or growth, integer overflow conditions can cause the function to enter an endless blocking loop.

The vulnerability is network-exploitable without requiring authentication or user interaction, making it accessible to remote attackers. However, practical exploitation is constrained to 32-bit systems where available physical memory meets or exceeds SIZE_MAX, significantly limiting the attack surface in modern computing environments.

Root Cause

The vulnerability stems from integer overflow issues in buffer size calculations. As shown in the security patches, the code was incorrectly calculating buffer lengths using multiplication by 2 instead of properly using sizeof(WCHAR). This arithmetic error in buffer size computation can lead to integer wraparound conditions that cause the stream capacity function to behave unexpectedly, resulting in an infinite loop scenario.

Attack Vector

The attack vector is network-based, allowing remote exploitation without authentication. An attacker could send specially crafted RDP traffic to a vulnerable FreeRDP client or server implementation. When the malicious data triggers the Stream_EnsureCapacity function with values that cause integer overflow, the affected system enters an endless loop, resulting in denial of service.

// Patch showing corrected buffer size calculation in drive_file.c
 	else if (!FindNextFileW(file->find_handle, &file->find_data))
 		goto out_fail;
 
-	length = _wcslen(file->find_data.cFileName) * 2;
+	length = _wcslen(file->find_data.cFileName) * sizeof(WCHAR);
 
 	switch (FsInformationClass)
 	{

Source: GitHub Commit Update

// Patch showing corrected buffer size calculation in remdesk_common.c
 	}
 
 	const size_t ChannelNameLen =
-	    (strnlen(header->ChannelName, sizeof(header->ChannelName)) + 1) * 2;
+	    (strnlen(header->ChannelName, sizeof(header->ChannelName)) + 1) * sizeof(WCHAR);
 	WINPR_ASSERT(ChannelNameLen <= ARRAYSIZE(header->ChannelName));
 
 	Stream_Write_UINT32(s, (UINT32)ChannelNameLen); /* ChannelNameLen (4 bytes) */

Source: GitHub Commit Update

Detection Methods for CVE-2026-27951

Indicators of Compromise

• Unusual CPU usage spikes in FreeRDP processes indicating potential infinite loop conditions
• FreeRDP client or server processes becoming unresponsive during RDP sessions
• Memory allocation patterns showing abnormal growth in stream buffers
• Network traffic anomalies with malformed RDP packets targeting FreeRDP implementations

Detection Strategies

• Monitor FreeRDP process CPU utilization for sustained high usage patterns that may indicate infinite loops
• Implement network intrusion detection rules to identify malformed RDP traffic targeting known vulnerable functions
• Deploy application-level monitoring to detect FreeRDP service hangs or unresponsiveness
• Use endpoint detection solutions to identify denial of service conditions affecting RDP services

Monitoring Recommendations

• Enable verbose logging for FreeRDP implementations to capture stream allocation events
• Configure process monitoring to alert on FreeRDP processes that exceed normal CPU or memory thresholds
• Implement network flow analysis to detect unusual RDP traffic patterns
• Set up service availability monitoring for FreeRDP-based remote desktop services

How to Mitigate CVE-2026-27951

Immediate Actions Required

• Upgrade FreeRDP to version 3.23.0 or later immediately
• Audit all systems for FreeRDP installations and identify versions prior to 3.23.0
• Prioritize patching 32-bit systems as they are most susceptible to practical exploitation
• Implement network segmentation to limit exposure of vulnerable FreeRDP services

Patch Information

FreeRDP version 3.23.0 contains the security patch that addresses this vulnerability. The fix corrects the buffer size calculations by replacing hardcoded multiplication by 2 with proper sizeof(WCHAR) usage, preventing integer overflow conditions in the Stream_EnsureCapacity function.

For detailed patch information, refer to:

• GitHub Commit Update
• GitHub Security Advisory

Workarounds

• No known workarounds are available for this vulnerability according to the security advisory
• Upgrade to FreeRDP version 3.23.0 is the only effective remediation
• Consider temporarily restricting network access to FreeRDP services until patching is complete
• Implement rate limiting on RDP connections to reduce exploitation impact

# Verify FreeRDP version
xfreerdp --version
# Upgrade FreeRDP on Debian/Ubuntu
sudo apt update && sudo apt install freerdp2-x11
# Verify updated version
xfreerdp --version