Security researchers have found that it is possible to alter a digitally signed PDF without invalidating its signatures. To demonstrate it, they produced a fake document “refund order” of $1,000,000,000,000 dollars, with a valid signature from Amazon. This sparked my attention, since I was quite sure that they didn’t use some sort of quantum device to break the cryptography involved in the signing process. So what exactly is going on?
The researchers claim to found at least three different ways to, in their words:
… use an existing signed document (e.g., amazon.de invoice) and change the content of the document arbitrarily without invalidating the signatures. Thus, we can forge a document signed by email@example.com to refund us one trillion dollars.
That’s not good news if you take into account that the main purpose of digitally signing a document is, well, prevent unauthorized changes in that document. The good news is that you can update your software to fix this flaws because of this research; the main PDF readers companies were given time to fix the issues. The bad news is that if you rely on the signature verification for any sensitive process, you likely want to go back and see if you were using vulnerable software previously and check that documents were correctly validated. I’m thinking about government institutions, banks, insurance companies and so on.
The implications are yet to be seen and probably won’t even be fully known.
There are three classes of attacks that work on different software. I’ll try to go into each one from what I could tell from reading the research.