Enabling In-Browser Pdf Rendering In Sharepoint 2010: Security Considerations And Best Practices

Understanding the Risks of Enabling PDF Rendering

Enabling in-browser PDF rendering in SharePoint 2010 introduces additional attack surfaces that can be exploited by malicious actors. The PDF rendering software itself as well as the integration points with SharePoint need to be secured to prevent attacks such as cross-site scripting and other injection threats.

The PDF rendering software parses PDF files and converts them into an interactive document that can be displayed in a web browser. This complex processing of arbitrary user-uploaded data creates inherent risks. Malformed PDF files could trigger vulnerabilities in the parser allowing attackers to execute malicious code on the server.

Integrating a PDF renderer with SharePoint means the renderer must interface with SharePoint’s security context to display documents. Attackers may attempt cross-site request forgery, cross-site scripting attacks, or attempt to leverage SharePoint permissions to access the PDF renderer in unauthorized ways.

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Additionally, PDF rendering software needs to be kept up-to-date via patches, especially regarding newly discovered parsing vulnerabilities. The parsing software may contain bugs that could be exploited before patches are available. Organizations must weigh the risks of enabling PDF rendering against the benefits. Ultimately the security of the PDF renderer and how exposed its attack surface is to users on the internet needs to be considered.

Securing the PDF Rendering Environment

Restricting and isolating the PDF rendering environment can reduce the risks of attacks against both the renderer and SharePoint. Various security configuration best practices should be followed:

Disable unsafe PDF features like actions, JavaScript, file attachments, external file references, and auto launching. These features extend the PDF attack surface and are often not needed for basic viewing.
Sandbox the PDF renderer using containerization or isolation tools like Docker to prevent the parser accessing sensitive OS or network resources.
Establish policies and procedures to keep the PDF renderer patched and up-to-date.
Limit access to the PDF renderer through IP allow listing, authenticated requests, or route traffic through a reverse proxy with WAF rules.

Best Practices for Secure Configuration

Properly integrating and configuring a PDF renderer with SharePoint’s security context can mitigate many of the additional risks.

Enforcing TLS for PDF Requests

SharePoint should require authentication and transport encryption using TLS 1.2+ for browser requests to rendered PDF resources. This prevents remote eavesdropping and man-in-the-middle attacks.

Using Dedicated PDF Rendering Servers

The PDF renderer should run on dedicated servers or containers isolated from the general SharePoint environment. This limits the blast radius if the renderer is compromised.

Automating Security Updates

Apply operating system and PDF renderer patches promptly using centralized patch management solutions. Automated patching ensures renderer security issues are addressed rapidly.

Monitoring for Suspicious PDF Rendering Activity

Analyze PDF renderer logs for spikes in errors, crashes, exploit-associated patterns, or other indicators of compromise. Alert personnel to investigate signs of possible attacks.

Example Code for Securely Enabling PDF Rendering

Reference code examples demonstrate implementing several of the defenses discussed previously.

IIS Configuration for Sandboxing PDF Renderer

<configuration>

  <system.webServer>
    <aspNetCore processPath="Renderer.exe" 
      arguments="--sandbox 1 --iptables on" 
      stdoutLogEnabled="true" 
      stdoutLogFile="\\logs\renderer-stdout.log"
      loggingEnabled="true" 
      forwardWindowsAuthToken="false">
    </aspNetCore>
  
  </system.webServer>

</configuration>

PowerShell Script for Applying Latest Security Patches

$updates = Get-WindowsUpdate

Foreach ($update in $updates) {

  if($update.Title -match "(Security Update|Defense)"){
    
    Install-WindowsUpdate -KBArticleID $update.KB 
  }
}

Script for Monitoring PDF Access Logs

$log = Get-Content \\logs\pdf-requests.log

$stat = $log | Group-Object {$_.StatusCode}

if($stat[500].Count -gt 100){

  Send-Alert "PDF renderer experienced $stat[500].Count errors today, please investigate!"
  
}

Testing and Validation

Verifying security controls and defenses through testing helps ensure robust protection:

Tools for Detecting PDF Vulnerabilities

Specialty static and dynamic analysis tools can detect flaws introduced in custom integrations with SharePoint or within the PDF parser itself during development.

Process for Performing Penetration Testing

Conduct recurring internal and independent third-party penetration tests on the PDF renderer deployment using the latest threats to validate the efficacy of all controls.

Validating Security Controls through Audit

Routinely audit security configurations like sandbox policies, patch levels, segmented domains, and other best practices to confirm continued enforcement.

Maintaining Protection Over Time

Do not deploy and forget. PDF threats continuously evolve and Zero Day vulnerabilities are uncovered. Maintain diligence:

Schedule recurring penetration testing every 6 months.
Monitor emerging PDF exploits in the wild through security bulletins.
Evolve controls to align with updated best practice guidance.