CVE-2026-47289: Remote Desktop Client Buffer Overflow

CVE-2026-47289 Overview

CVE-2026-47289 is a heap-based buffer overflow vulnerability in the Microsoft Remote Desktop Client. An unauthenticated attacker can execute arbitrary code on a target system over a network when a user connects to an attacker-controlled Remote Desktop server. The flaw is classified under [CWE-122] (Heap-based Buffer Overflow) and requires user interaction to trigger. Successful exploitation grants the attacker code execution in the context of the connecting user, with high impact to confidentiality, integrity, and availability.

Critical Impact
Remote attackers can achieve code execution on client systems when users initiate RDP connections to malicious servers, enabling full compromise of the client host.

Affected Products

Microsoft Remote Desktop Client (specific versions listed in the Microsoft Security Response Center advisory)
Windows operating systems shipping the affected Remote Desktop Client component
See the Microsoft Security Update for CVE-2026-47289 for the complete product and version matrix

Discovery Timeline

2026-06-09 - CVE-2026-47289 published to the National Vulnerability Database
2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-47289

Vulnerability Analysis

The vulnerability resides in the Remote Desktop Client's handling of data received from a Remote Desktop Protocol (RDP) server. The client allocates a heap buffer to process server-sourced data but writes beyond the allocated bounds. This out-of-bounds write corrupts adjacent heap metadata and objects in the client process. An attacker who controls the RDP server can craft protocol responses that overwrite function pointers, virtual table entries, or heap chunk headers. The corruption pathway leads to arbitrary code execution within the Remote Desktop Client process when the malformed data is parsed.

Root Cause

The root cause is improper validation of length or size fields in data received from the RDP server prior to copying that data into a fixed or insufficiently sized heap allocation. This is a textbook [CWE-122] heap-based buffer overflow. The client trusts attacker-controlled size values, resulting in a write past the buffer boundary on the heap.

Attack Vector

Exploitation requires a victim to initiate an RDP connection to a server controlled by the attacker. Common delivery techniques include phishing emails containing .rdp files, malicious links that invoke the Remote Desktop Client, or compromised legitimate RDP infrastructure that redirects to attacker servers. Once the client connects, the malicious server returns crafted RDP messages that trigger the heap overflow during parsing. No prior authentication to the target client is needed, but user interaction is mandatory to initiate the connection.

No verified public proof-of-concept code is available at the time of publication. See the Microsoft Security Update for CVE-2026-47289 for vendor technical details.

Detection Methods for CVE-2026-47289

Indicators of Compromise

Outbound RDP connections (TCP/UDP 3389 or alternate configured ports) from user workstations to untrusted or newly observed external IP addresses
Unexpected child processes spawned by mstsc.exe or the Remote Desktop Client process
Crash events or Windows Error Reporting entries naming the Remote Desktop Client with heap corruption exception codes such as 0xC0000374 or 0xC0000005
Execution of .rdp files originating from email attachments, downloads, or removable media

Detection Strategies

Monitor process lineage where mstsc.exe is the parent of shells, scripting engines, or LOLBins such as powershell.exe, cmd.exe, or rundll32.exe
Alert on .rdp file creation in user download, temp, or email attachment directories followed by Remote Desktop Client execution
Correlate Remote Desktop Client crashes with subsequent suspicious network or process activity on the same host

Monitoring Recommendations

Log and review all outbound RDP traffic from endpoint subnets, flagging connections to external or unsanctioned destinations
Enable Windows Defender Exploit Guard and capture Application Error (Event ID 1000) and WER (Event ID 1001) events referencing Remote Desktop Client binaries
Centralize endpoint telemetry in a SIEM or data lake to enable retrospective hunting for anomalous RDP client behavior

How to Mitigate CVE-2026-47289

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update for CVE-2026-47289 advisory across all systems running the Remote Desktop Client
Block outbound RDP (default port 3389) at the perimeter firewall except to explicitly approved destinations
Instruct users to avoid opening .rdp files received via email or from untrusted sources
Inventory endpoints running affected Remote Desktop Client versions and prioritize patching for high-value users

Patch Information

Microsoft has issued security updates addressing CVE-2026-47289. Administrators should consult the Microsoft Security Update for CVE-2026-47289 for the complete list of affected builds and corresponding patch KB articles. Apply updates through Windows Update, Microsoft Update Catalog, or the organization's existing patch management workflow.

Workarounds

Restrict outbound RDP connections at the network boundary to a known allowlist of internal and partner RDP servers
Use Group Policy or AppLocker to block execution of .rdp files from email attachment and download folders
Train users to verify RDP destinations and report unsolicited connection prompts to the security team

bash

# Block outbound RDP (TCP 3389) to non-allowlisted destinations on Windows endpoints
New-NetFirewallRule -DisplayName "Block Outbound RDP" `
  -Direction Outbound `
  -Protocol TCP `
  -RemotePort 3389 `
  -Action Block `
  -Profile Any