How to Mitigate Acrobat and Reader NTLM Credential Theft Exploit
Question details
Users need to understand the NTLM credential theft vulnerability associated with PDF files in Adobe Acrobat and Reader, and learn how to implement mitigations since Adobe relies on Microsoft's security enhancements for a fix.
- Product
- Adobe Acrobat and Reader
- Device & OS
- Windows
- Scenario
- Securing corporate networks and personal devices against NTLM credential exposure via malicious PDF files.
- Observed behavior
- PDF files may expose NTLM credentials to external attackers when opened. Adobe has stated it will not release a specific patch, relying instead on Microsoft OS-level mitigations.
Consult with your IT or security department before changing network authentication protocols, as modifying NTLM settings can affect internal single sign-on (SSO) capabilities.
Apply Microsoft's NTLM Authentication Mitigation
Since Adobe does not plan to modify Acrobat, the primary fix is to use Microsoft's optional security enhancement to disable NTLM SSO for public resources.
Microsoft provides OS-level group policies to restrict NTLM traffic to remote servers, effectively blocking malicious PDFs from silently passing credentials outside your network.
Press the Win + R keys simultaneously, type 'gpedit.msc' into the Run dialog, and press Enter to open the Local Group Policy Editor.
In the left pane, navigate to Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options.
Scroll down to locate the policy named 'Network security: Restrict NTLM: Outgoing NTLM traffic to remote servers'. Double-click it and set the value to 'Deny all' or 'Audit all' depending on your organization's testing phase.
Review Firewall and Endpoint Security Policies
Block outbound SMB traffic (TCP port 445) at the perimeter firewall to prevent NTLM hashes from leaving your network entirely.
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- 1. Download the installer: Visit the official WPS Office website and click the free download button for your operating system.
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Frequently Asked Questions
Why isn't Adobe releasing a patch for this NTLM exploit in Acrobat?
Adobe considers this an OS-level authentication issue rather than a specific flaw in Acrobat. They rely on Microsoft's optional security enhancements that allow network administrators to disable or restrict NTLM single sign-on authentication for public resources.
How does the PDF NTLM credential theft exploit work?
Attackers embed malicious remote SMB file paths inside a PDF document. When the PDF is opened, the application automatically tries to authenticate with the remote server, unknowingly sending the user's NTLM hash, which can then be captured and cracked by the attacker.
Does blocking port 445 protect against this PDF vulnerability?
Yes, blocking outbound TCP port 445 at your perimeter firewall is highly effective. It prevents the SMB traffic required to transmit the NTLM hashes from leaving your internal corporate network.
Will restricting NTLM authentication break internal network access?
It can. NTLM is still widely used in legacy internal network environments for file sharing and application authentication. It is highly recommended to use the 'Audit all' policy first to identify legitimate NTLM traffic before enforcing a strict 'Deny all' rule.
