Oops... Code Execution and Content Spoofing: The First Comprehensive Analysis of OpenDocument Signatures

OpenDocument is one of the major standards for interoperable office documents. Supported by office suites like Apache OpenOffice, LibreOffice, and Microsoft Office, the OpenDocument Format (ODF) is available for text processing, spreadsheets, and presentations on all major desktop and mobile operating systems.

When it comes to governmental and business use cases, OpenDocument signatures can protect the integrity of a document's content, for example, for contracts, amendments, or bills. Moreover OpenDocument signatures also protect document's macros. Since the risks of using macros in documents is well-known, modern office applications only enable their execution if a trusted entity signs the macro code. Thus, the security of ODF documents often depends on the correct signature verification.

In this paper, we conduct the first comprehensive analysis of OpenDocument signatures and reveal numerous severe threats. We identified five new attacks and evaluated them against 16 office applications on Windows, macOS, Linux, iOS, Android, and two online services. Our investigation revealed 12 out of 18 applications to be vulnerable for macro code execution, although the application only executes macros signed by trusted entities. For 17 of 18 applications, we could spoof the content in a signed ODF document while keeping the signature valid and trusted. Finally, we showed that attackers possessing a signed ODF could alter and forge the signature creation time in 16 of 18 applications.

Our research was acknowledged by Microsoft, Apache OpenOffice, and LibreOffice during the coordinated disclosure.