A vulnerability in OOXML signatures makes it easy to forge documents. Anyone who wants to securely send an important Word document digitally can protect it with a signature—or so it seems. However, as researchers from the Chair of Network and Data Security at the Horst-Görtz Institute for IT Security and the CASA Cluster of Excellence at Ruhr University Bochum and Mainz University of Applied Sciences have discovered, undetected tampering with the document is child’s play for attackers. Simon Rohlmann, Vladislav Mladenov, Christian Mainka, Daniel Hirschberger, and Jörg Schwenk at the renowned IT security conference“Usenix Security Symposium,”which will take place August 9–11, 2023, in California, USA.
“The purpose of a digital signature is to verify the integrity of a document,” explains Simon Rohlmann, who is now working at Mainz University of Applied Sciences. To do this, a signature is generated using a private key based on public-key algorithms, which can then be verified using a public key. The person sending the document can thus protect it from subsequent tampering while still making it accessible to others. Thanks to this secure cryptographic process, the recipient can also be confident that the document’s content is valid. However, the researchers have discovered a vulnerability that makes it easy to manipulate documents in Microsoft’s Office Open XML (OOXML): “We realized that documents are only partially signed. This could allow someone, for example, to add new content or hide signed content without anyone noticing,” explains Simon Rohlmann.
"Digital Signature for These Documents Practically Worthless"
The researchers have identified a total of five potential attack vectors made possible by structural inconsistencies in the Office system: The developers of the OOXML standard apparently decided to sign only parts of the document package, according to the researcher. “This renders the digital signature for these documents practically worthless. An attacker could, for example, use signed documents to make social engineering attacks appear particularly trustworthy, since the document contains a valid signature from a supervisor,” concludes Simon Rohlmann.
XML-based file formats affected by this issue have been in use by Microsoft since 2007. Users can usually recognize them by the “-X” suffix in the filename, such as file.docx or file.xlsx. Their main advantage is that, thanks to a compression technique, they require little storage space and—unlike their predecessors—are actually supposed to offer greater security. When the researchers first discovered the security vulnerabilities in 2022, they immediately notified Microsoft and the relevant standardization authority. However, despite repeated attempts by the researchers to contact the company, Microsoft did not resolve the issue promptly. Since last month, at least in the retail version of Microsoft Office 2021 (Version 2305 (Build 16501.20210)), only one of the five attack vectors—the Universal Signature Forgery (USF) attack—remains possible; all others have been fixed. “In the latest LTSC version of Microsoft Office 2021 (Version 2108 (Build 14332.20517)), the vulnerabilities have not yet been fixed,” said Rohlmann (as of Friday, June 16, 2023).
The idea to investigate this security vulnerability stemmed from the success of another research paper published by the team at the Chair of Network and Data Security in 2019: In that study, the Bochum researchers were able to demonstrate for the first time that many applications failed to detect the circumvention of digital signatures in PDF documents. Since then, the researchers have regularly focused on investigating signatures, which are becoming increasingly widespread, particularly in professional settings and in the context of government agencies. However, Simon Rohlmann cannot accurately assess the extent to which Microsoft Office’s signature features are used in this area.
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