CVE-2026-27015: FreeRDP Denial of Service Vulnerability

CVE-2026-27015 Overview

CVE-2026-27015 is a Denial of Service vulnerability affecting FreeRDP, a free implementation of the Remote Desktop Protocol. The vulnerability exists due to a missing bounds check in smartcard_unpack_read_size_align() within libfreerdp/utils/smartcard_pack.c at line 1703. This flaw allows a malicious RDP server to crash the FreeRDP client by triggering a reachable WINPR_ASSERT that leads to an abort() call.

The crash specifically occurs in upstream builds where WITH_VERBOSE_WINPR_ASSERT=ON is enabled, which is the default configuration in FreeRDP 3.22.0 and current WinPR CMake defaults. Exploitation requires that smartcard redirection be explicitly enabled by the user through command-line options such as /smartcard or /smartcard-logon.

Critical Impact

A malicious RDP server can remotely crash FreeRDP clients with smartcard redirection enabled, causing denial of service through an uncontrolled assertion failure.

Affected Products

• FreeRDP versions prior to 3.23.0
• FreeRDP 3.22.0 with default WITH_VERBOSE_WINPR_ASSERT=ON configuration
• Systems using FreeRDP clients with smartcard redirection enabled (/smartcard or /smartcard-logon)

Discovery Timeline

• 2026-02-25 - CVE-2026-27015 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-27015

Vulnerability Analysis

This vulnerability is classified under CWE-617 (Reachable Assertion), representing a class of flaws where assertions intended for development debugging can be triggered through malicious input in production environments. The vulnerability resides in the smartcard packet handling code, specifically in the function responsible for unpacking and aligning read sizes during smartcard communication.

When processing smartcard-related RDP traffic, the smartcard_unpack_read_size_align() function fails to properly validate stream length before performing padding operations. This missing bounds check allows an attacker controlling a malicious RDP server to send crafted packets that trigger the WINPR_ASSERT macro, which in verbose assertion builds calls abort(), immediately terminating the client process.

The attack requires user interaction in that the victim must connect to the malicious RDP server with smartcard redirection enabled. This is not a default configuration, but is commonly used in enterprise environments for smartcard-based authentication workflows.

Root Cause

The root cause is a missing validation of stream length before performing padding alignment operations in the smartcard packet unpacking routine. The function smartcard_unpack_read_size_align() did not check whether sufficient data remained in the stream before attempting to read padding bytes, leading to an assertion failure when the stream was shorter than expected.

Attack Vector

The attack vector is network-based, requiring a victim to connect to a malicious RDP server with smartcard redirection enabled. The attacker must:

1. Operate a malicious RDP server that the victim connects to
2. Wait for the victim to initiate a connection with /smartcard or /smartcard-logon options enabled
3. Send malformed smartcard-related RDP packets designed to trigger the bounds check failure
4. The client's WINPR_ASSERT macro fires, calling abort() and crashing the FreeRDP client

The following patch demonstrates how the vulnerability was fixed by adding proper return value checking:

 	    (ioControlCode != SCARD_IOCTL_RELEASETARTEDEVENT))
 	{
 		offset = (RDPDR_DEVICE_IO_REQUEST_LENGTH + RDPDR_DEVICE_IO_CONTROL_REQ_HDR_LENGTH);
-		smartcard_unpack_read_size_align(s, Stream_GetPosition(s) - offset, 8);
+		if (smartcard_unpack_read_size_align(s, Stream_GetPosition(s) - offset, 8) < 0)
+			return STATUS_INVALID_PARAMETER;
 	}

 	if (Stream_GetPosition(s) < Stream_Length(s))

Source: GitHub Commit Changes

Additional fix in the memory allocation path:

 		return SCARD_E_NO_MEMORY;
 	Stream_Read(s, r, len);
 	const LONG pad = smartcard_unpack_read_size_align(s, len, 4);
+	if (pad < 0)
+	{
+		free(r);
+		return STATUS_INVALID_PARAMETER;
+	}
 	len += (size_t)pad;
 	*data = r;
 	if (plen)

Source: GitHub Commit Changes

Detection Methods for CVE-2026-27015

Indicators of Compromise

• Unexpected FreeRDP client crashes with assertion failure messages referencing smartcard_pack.c
• Core dumps or crash reports showing abort() called from WINPR_ASSERT in smartcard-related functions
• Log entries indicating connection failures during smartcard redirection to untrusted or suspicious RDP servers

Detection Strategies

• Monitor for abnormal FreeRDP client terminations, particularly those with SIGABRT signals
• Implement network monitoring to detect connections to unknown or potentially malicious RDP servers
• Enable verbose logging in FreeRDP clients to capture smartcard-related packet processing errors
• Deploy endpoint detection rules to identify crash patterns consistent with assertion failures

Monitoring Recommendations

• Review FreeRDP client logs for repeated connection failures or crashes when smartcard features are used
• Monitor system crash reporters for FreeRDP-related assertion failures
• Audit RDP server connections in enterprise environments to identify connections to unauthorized servers

How to Mitigate CVE-2026-27015

Immediate Actions Required

• Upgrade FreeRDP to version 3.23.0 or later which contains the security fix
• Disable smartcard redirection when connecting to untrusted RDP servers
• Review and restrict RDP server connections to only trusted, verified servers within your organization
• Build FreeRDP with WITH_VERBOSE_WINPR_ASSERT=OFF as a temporary workaround if upgrade is not immediately possible

Patch Information

The vulnerability is fixed in FreeRDP version 3.23.0. The fix adds proper bounds checking on the return value of smartcard_unpack_read_size_align(), returning STATUS_INVALID_PARAMETER when stream length validation fails instead of triggering an assertion. The patch is available through the GitHub commit 65d59d3b3c2f630f2ea862687ecf5f95f8115244 and the GitHub Security Advisory GHSA-7g72-39pq-4725.

Workarounds

• Avoid using /smartcard or /smartcard-logon options when connecting to untrusted RDP servers
• Build FreeRDP from source with WITH_VERBOSE_WINPR_ASSERT=OFF to prevent assertion-triggered crashes
• Implement network segmentation to prevent connections to unauthorized RDP servers
• Use VPN or other secure tunneling when connecting to remote RDP servers

# Build FreeRDP with verbose assertions disabled as a temporary workaround
cmake -DWITH_VERBOSE_WINPR_ASSERT=OFF ..
make