Story Beyond the Eye: Glyph Positions Break PDF Text Redaction
Maxwell Bland, Anushya Iyer, Kirill Levchenko

TL;DR
This paper reveals that many PDF redactions are insecure due to character positioning leaks, allowing the recovery of redacted information, and provides open-source fixes and notifications to relevant organizations.
Contribution
It uncovers a vulnerability in common PDF redactions caused by character positioning and offers open-source algorithms to improve redaction security.
Findings
Many redactions leak character position information
Redacted text can be effectively recovered in real-world PDFs
Open source tools can mitigate the leakage
Abstract
In this work we find that many current redactions of PDF text are insecure due to non-redacted character positioning information. In particular, subpixel-sized horizontal shifts in redacted and non-redacted characters can be recovered and used to effectively deredact first and last names. Unfortunately these findings affect redactions where the text underneath the black box is removed from the PDF. We demonstrate these findings by performing a comprehensive vulnerability assessment of common PDF redaction types. We examine 11 popular PDF redaction tools, including Adobe Acrobat, and find that they leak information about redacted text. We also effectively deredact hundreds of real-world PDF redactions, including those found in OIG investigation reports and FOIA responses. To correct the problem, we have released open source algorithms to fix trivial redactions and reduce the amount…
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Taxonomy
TopicsAdvanced Malware Detection Techniques · Digital and Cyber Forensics · Internet Traffic Analysis and Secure E-voting
