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A Critical Oversight in PDF Conversion Puts Sensitive Data at Risk
A recently uncovered vulnerability in Microsoft 365’s file conversion functionality has spotlighted how even well-established platforms can harbor critical security flaws. The issue, now patched by Microsoft, enabled Local File Inclusion (LFI) attacks via the platform’s HTML-to-PDF conversion pipeline. Initially mistaken for a client-side issue, the flaw was eventually traced back to Microsoft’s own infrastructure, particularly within the Microsoft Graph API. This revelation not only underscores the risks inherent in automated document processing but also showcases the vital role that responsible disclosure plays in the broader cybersecurity ecosystem. For his findings, the security researcher earned a \$3,000 bounty from Microsoft after a four-month review.
How the Exploit Unfolded and What It Revealed
The vulnerability first came to light during a routine security assessment of a client’s application that utilized a feature to convert documents into PDFs and host them via SharePoint. While testing, the researcher found that local files on the server were being read during the HTML-to-PDF conversionâa serious red flag. Initially assumed to be a flaw in the clientâs own code, it was later revealed that their application merely acted as a wrapper for Microsoftâs official API services. This shift in focus led the researcher to directly probe Microsoft 365âs file handling systems, where the true extent of the flaw emerged.
Microsoftâs Graph API, known for offering file conversion support across formats like DOCX, CSV, and more, was unknowingly exposing a hidden and undocumented behavior: HTML-to-PDF conversions. Unlike other conversions, this method opened up a dangerous avenue for LFI attacks. By embedding <embed>, <object>, or <iframe> tags into the HTML file, attackers could instruct the system to pull in local files during conversionâfiles which were then included in the generated PDF. These could range from configuration files like web.config or win.ini, to database credentials and application source code.
The exploitation process was alarmingly simple:
- Upload a malicious HTML file through the Microsoft Graph API.
2. Request a PDF version of the file.
3. Download the PDF, which now includes the
This three-step procedure gave attackers potential access to extremely sensitive server data. Even more concerning, in complex multi-tenant environments, there existed a narrow but plausible risk of cross-tenant data leakage. Microsoft rated the vulnerability as âImportantâ and has since patched it. However, the case is a stark reminder of the hidden dangers in cloud document services, particularly those involving automated or undocumented behavior.
What Undercode Say:
A Textbook Case of Overlooked Risk in Enterprise APIs
This incident serves as a potent example of how implicit trust in vendor servicesâespecially cloud-based giants like Microsoftâcan introduce unseen vulnerabilities. While Microsoft Graph API is typically seen as secure, the existence of undocumented HTML-to-PDF conversion hints at gaps in internal documentation, testing, or both. Itâs a sobering reminder that even “black-box” services trusted by millions can harbor potentially devastating flaws.
HTML-to-PDF conversion may seem like a harmless feature, but it carries unexpected risks when it processes untrusted input. The use of embedding tags like <iframe> or <object> taps into the rendering engine’s behavior, coercing it into reading server-side files during the conversion process. The root cause isnât a bug in the traditional sense, but a side effect of an undocumented feature misbehaving under malicious input. It blurs the line between functionality and vulnerabilityâone of the trickiest areas in software security.
Whatâs especially striking is how minimal the barrier to entry was for exploitation. No advanced shell access, buffer overflows, or reverse engineering was needed. All it took was a manipulated HTML file and a legitimate API call. In environments where cloud services manage file uploads at scale, even a minor oversight like this can lead to far-reaching exposure, especially when coupled with weak file sanitization or inadequate validation.
From a design perspective, this should raise questions about how PDF rendering engines are sandboxed, how temporary files are managed, and whether tenants are adequately isolated in shared environments. It’s also a wake-up call for organizations relying on API-based integrations to reconsider their input sanitization layers, even when working with trusted platforms.
Microsoftâs relatively low bounty payout of \$3,000 may reflect internal criteria rather than the severity itself. In reality, the attack vector was subtle but powerful enough to compromise sensitive operational files. Its inclusion of configuration files, secrets, and possibly source code, could have enabled full server compromise under the right conditions. The fact that it was caught and reported responsibly is a win for the security community, but it should also inspire more robust auditing practices within Microsoft and similar SaaS platforms.
Finally, this is a reminder that not all security risks come from flashy exploits or zero-days. Sometimes, itâs the quiet, undocumented behaviorsâthose tiny cracks in otherwise massive systemsâthat hold the most dangerous implications. Organizations should be cautious not just about external threats, but about the invisible assumptions they make regarding trusted third-party services.
đ Fact Checker Results:
â Verified: Microsoft patched the HTML-to-PDF LFI vulnerability
â
Verified: Researcher used Microsoft Graph API to demonstrate the flaw
â Not Verified: No confirmed incidents of exploitation in the wild yet
đ Prediction:
This discovery is likely to push Microsoft and other cloud service providers to re-evaluate their documentation, especially around file conversion features. Future updates to Microsoft Graph and SharePoint APIs may include stricter sanitization, conversion restrictions, or warnings when handling HTML-based uploads. As more enterprises rely on automated document processing, hidden behaviors like these will become prime targets for red teaming and bug bounty hunters. Expect increased scrutiny on cloud-based file services and higher bounties for similar discoveries moving forward.
References:
Reported By: cyberpress.org
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