Researchers Demonstrate 2 New Hacks to Modify Certified PDF Documents

Cybersecurity scientists have disclosed two new assault strategies on licensed PDF files that could perhaps allow an attacker to change a document’s obvious articles by displaying destructive content material in excess of the certified material without the need of invalidating its signature.

“The attack notion exploits the adaptability of PDF certification, which allows signing or adding annotations to qualified files underneath unique permission concentrations,” claimed scientists from Ruhr-College Bochum, who have systematically analyzed the protection of the PDF specification in excess of the decades.

The findings were introduced at the 42nd IEEE Symposium on Protection and Privacy (IEEE S&P 2021) held this week.

The two attacks — dubbed Evil Annotation and Sneaky Signature attacks — hinge on manipulating the PDF certification course of action by exploiting flaws in the specification that governs the implementation of digital signatures (aka approval signature) and its a lot more versatile variant known as certification signatures.

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Certification signatures also allow for various subsets of modifications on the PDF document centered on the permission amount established by the certifier, including the skill to write text to distinct form fields, provide annotations, or even add a number of signatures.

The Evil Annotation Assault (EAA) functions by modifying a certified doc which is provisioned to insert annotations to involve an annotation containing malicious code, which is then sent to the victim. On the other hand, the idea driving the Sneaky Signature attack (SSA) is to manipulate the physical appearance by adding overlaying signature things to a document that enables filling out variety fields.

“By inserting a signature field, the signer can determine the specific posture of the subject, and moreover its visual appearance and information, the researchers said. “This adaptability is essential considering that just about every new signature could have the signer’s details. The data can be a graphic, a text, or a mixture of both of those. Even so, the attacker can misuse the overall flexibility to stealthily manipulate the doc and insert new content.”

In a hypothetical attack situation detailed by the teachers, a certifier generates a licensed deal with delicate information although enabling the solution to increase further signatures to the PDF deal. By getting advantage of these permissions, an attacker can modify the contents of the doc, say, to screen an Worldwide Bank Account Amount (IBAN) less than their management and fraudulently transfer resources, as the victim, unable to detect the manipulation, accepts the tampered deal.

15 of 26 PDF purposes evaluated by the scientists, counting Adobe Acrobat Reader (CVE-2021-28545 and CVE-2021-28546), Foxit Reader (CVE-2020-35931), and Nitro Professional, were being found susceptible to the EAA attack, enabling an attacker to improve the noticeable articles in the doc. Soda PDF Desktop, PDF Architect, and six other apps were being discovered as prone to SSA assaults.

Additional troublingly, the research revealed that it really is doable to execute large-privileged JavaScript code — e.g., redirect the person to a malicious web site — in Adobe Acrobat Pro and Reader by sneaking such code through EAA and SSA as an incremental update to the accredited doc. The weak point (CVE-2020-24432) was tackled by Adobe as element of its Patch Tuesday update for November 2020.

To fend off this kind of attacks, the researchers suggest prohibiting FreeText, Stamp, and Redact annotations as nicely as making sure that signature fields are established up at described locations in the PDF document prior to certification, together with penalizing any subsequent addition of signature fields with an invalid certification status. The researchers have also designed a Python-centered utility known as PDF-Detector, which parses qualified documents to highlight any suspicious components uncovered in the PDF document.

“Despite the fact that neither EAA nor SSA can alter the material itself – it generally remains in the PDF – annotations and signature fields can be employed as an overlay to add new information,” the scientists mentioned. “Victims opening the PDF are unable to distinguish these additions from frequent articles. And even even worse: annotations can embed significant privileged JavaScript code that is permitted to be added to particular certified documents.”

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