15 ms·

New PDFex attack can exfiltrate data from encrypted PDF files

72 points by lelf 7y ago

WhiteOwlLion 7y ago
I just remove the password with a command line tool without needing to know the password.
- lxgr 7y ago
  This tool likely only works for "passwordless" PDF encryption/protection, which restricts some aspect of using the document (printing, exporting single pages etc). This is the domain of DRM/whitebox cryptography, and leaving aside all discussions about the feasibility of such efforts, PDF's implementation of it can safely be assumed to be broken – the master key used can be found in several open source PDF tools' source code and on Stack Overflow, among other places. I assume the attacks presented in the paper are about the other type of PDF encryption, which requires a password to even display the protected content, which could definitely be done in a way not vulnerable to the attacks described.
  - poizan42 7y ago
    The "master key" is just the empty password which means that the effective key is all padding. This is documented in section 7.6.3.3 of ISO 32000-1:2008 (available for free from Adobe's website). There was never any real attempt to keep it secret.
- bonyt 7y ago
  The PDF format supports actual encryption, too. https://www.pdflib.com/pdf-knowledge-base/pdf-password-security/encryption/ https://www.pdflib.com/pdf-knowledge-base/pdf-password-secur...
mistrial9 7y ago
spies! everywhere! honestly, as someone who cares about typography and culture in print, the constant and relentless "spies everywhere" statements from the security community look more and more like a sickness, not a defense. PDF was designed for print integrity, with images and typesetting, and I am grateful for it everyday. Please calm your alarmist content, and yes, security is necessary.. a good servant and a bad master, I would say.
- kevin_thibedeau 7y ago
  Then they added JavaScript and made it Turing complete, completely negating the effort to eliminate the general computability of Postscript.
  - otakucode 7y ago
    Not to mention the capability to execute any arbitrary executable.
ropiwqefjnpoa 7y ago
Who would have imagined we'd have "branded" exploits in the future?
- deleted 7y ago
  [deleted]
- skavi 7y ago
  Vulnerabilities have been branded for some time now. https://github.com/hannob/vulns https://github.com/hannob/vulns
  - ropiwqefjnpoa 7y ago
    I still consider 2011 "the future"
- bxlxixt 7y ago
  William Gibson, Neuromancer, 1984: "Data transfer from Bockris Systems GmbH, Frankfurt, advises, under coded transmission, that content of shipment is Kuang Grade Mark Eleven penetration program."
andrekandre 7y ago
from the list, looks like preview.app/skim is the least affected... i wonder if that’s because it’s just lacking dynamic features of the others, or it’s just coded better, or it’s because macOS security defaults...
userbinator 7y ago
Good riddance, the only thing I've ever seen PDF encryption used for is DRM. In my mind, anything that breaks DRM is a bonus.
- JaimeThompson 7y ago
  They are also used to stop quotes and documents from being changed once they are sent to others.
  - munk-a 7y ago
    There are other better ways to accomplish that like lodging a document with a third party to ensure a lack of changes... regarding quoting... preventing text capture is a futile effort that really needs to stop existing - it's a pain to deal with and provides no real security since, much like snapchat - you can never counter someone taking a picture of the screen.
    - JaimeThompson 7y ago
      Yes there are better ways but for a lot of small businesses those better ways simply do not get paid. It isn't how it should be and it will take a long time for things to change.
    - nitrogen 7y ago
      you can never counter someone taking a picture of the screen. Except with digital watermarks. Photocopiers generally recognize things like currency and refuse to copy at high fidelity. The PS3 I believe would refuse to play ripped cinema soundtracks all the way through, using audio watermarking. It's conceivable that cameras could be required to do the same thing for digital restriction watermarks.
      - ptx 7y ago
        Even if they conspire with every camera manufacturer, they can't prevent someone from just retyping the text they want to quote. On a mechanical typewriter if necessary. I guess they could try to do a denial of service attack on the typist by deliberately overusing letters from the same vertical keyboard row to wear out the finger.
moftz 7y ago
FIRST Robotics Competition used to distribute the rulebook for the season in an encrypted PDF. The password and unencrypted copies were released after the live kickoff stream. Ten years ago we'd be busting ass trying to crack the PDF but techniques weren't great then.
- AlphaWeaver 7y ago
  This is still how the rules are distributed today. I wonder if we'll see teams using this technique this year. Edit: It looks like this technique wouldn't be useful, since its an exfiltration technique and not a flaw in the encryption algorithm.
- pietroglyph 7y ago
  FRC was the thing I immediately thought about when I saw this (they still do PDF encryption), but the techniques here don't really apply. I'm pretty sure that you could just try to brute force it, as the passwords as usually dictionary-based with substitutions (e.g. R0bot$InSp&ce). Edit: After taking a much more in-depth look at it, all of the past game manuals' passwords are too complex to crack in a reasonable time (even with a dictionary). With a top 1000 word list dictionary with substitutions of 3-letter words like the above there are (conservatively) 1x10^20 combinations. You can't crack the PDF user key directly either, because recent game manuals use 256-bit AES keys.
reilly3000 7y ago
I've always operated under the assumption that nothing in a PDF is secure, and that the only safe way to redact something from one is to print it and rescan it. Convince me otherwise?
- bepvte 7y ago
  I believe that if you edit the pdf so that the thing you are redacting is no longer in the pdf file, it is safely redacted. Printing a pdf and rescanning a pdf breaks text selection and screen readers, and basically makes it into a fancy PNG.
  - munk-a 7y ago
    There is a bunch of data in PDFs that may have previously been visible but is no longer. I know that there are also fun meta-data attributes embedded on a per-page and per document basis - and also that PDFs can be portfolio documents where such a document will often contain no longer referenced PDFs that became irrelevant to the PDF. The file format is pretty crazy.
- captn3m0 7y ago
  There are a few redaction tools that do exactly this, but in software. You get a PNGish version, which you redact and combine back to PDF
taxidump 7y ago
A company I was a contractor for would send payment invoices with a password. The password was the last four digits of the ID number specific to the contractor. Our CRM we all used had every contractors ID number in their profile publicly available. When I asked what the point was I was told upper management required a password and this was the easiest.
xvilka 7y ago
At the same time no open source PDF reader can handle all cases of embedded animation and PDF forms[1]. [1] https://gitlab.freedesktop.org/poppler/poppler/issues?label_name%5B%5D=forms https://gitlab.freedesktop.org/poppler/poppler/issues?label_...
mettamage 7y ago
Ehm, what is an exfiltration channel? I know what a sidechannel is, but I haven't heard of exfiltration channel.
- otakucode 7y ago
  Just a channel that is used for exfiltration. It's not a specific sort of thing.
MartinMond 7y ago
PSPDFKit CTO here. TL;DR on the PDFex vulnerability: The file format can carry both encrypted and unencrypted parts, which allows "injecting" malicious payload (embedded JavaScript, PDF forms that submit to remote URLs, etc) into an encrypted PDF that then can be used to exfiltrate the encrypted PDF parts, after the user decrypted the file (= entered the password). We're currently investigating the vulnerability in detail and plan to ship an update ASAP that will allow our customers to mitigate it. I'll be monitoring this thread to answer any questions.

72 points by lelf 7y ago

WhiteOwlLion 7y ago

I just remove the password with a command line tool without needing to know the password.

lxgr 7y ago
This tool likely only works for "passwordless" PDF encryption/protection, which restricts some aspect of using the document (printing, exporting single pages etc). This is the domain of DRM/whitebox cryptography, and leaving aside all discussions about the feasibility of such efforts, PDF's implementation of it can safely be assumed to be broken – the master key used can be found in several open source PDF tools' source code and on Stack Overflow, among other places. I assume the attacks presented in the paper are about the other type of PDF encryption, which requires a password to even display the protected content, which could definitely be done in a way not vulnerable to the attacks described.
- poizan42 7y ago
  The "master key" is just the empty password which means that the effective key is all padding. This is documented in section 7.6.3.3 of ISO 32000-1:2008 (available for free from Adobe's website). There was never any real attempt to keep it secret.
bonyt 7y ago
The PDF format supports actual encryption, too. https://www.pdflib.com/pdf-knowledge-base/pdf-password-security/encryption/ https://www.pdflib.com/pdf-knowledge-base/pdf-password-secur...

mistrial9 7y ago

spies! everywhere! honestly, as someone who cares about typography and culture in print, the constant and relentless "spies everywhere" statements from the security community look more and more like a sickness, not a defense. PDF was designed for print integrity, with images and typesetting, and I am grateful for it everyday. Please calm your alarmist content, and yes, security is necessary.. a good servant and a bad master, I would say.

kevin_thibedeau 7y ago
Then they added JavaScript and made it Turing complete, completely negating the effort to eliminate the general computability of Postscript.
- otakucode 7y ago
  Not to mention the capability to execute any arbitrary executable.

ropiwqefjnpoa 7y ago

Who would have imagined we'd have "branded" exploits in the future?

deleted 7y ago
[deleted]
skavi 7y ago
Vulnerabilities have been branded for some time now. https://github.com/hannob/vulns https://github.com/hannob/vulns
- ropiwqefjnpoa 7y ago
  I still consider 2011 "the future"
bxlxixt 7y ago
William Gibson, Neuromancer, 1984: "Data transfer from Bockris Systems GmbH, Frankfurt, advises, under coded transmission, that content of shipment is Kuang Grade Mark Eleven penetration program."

andrekandre 7y ago

from the list, looks like preview.app/skim is the least affected... i wonder if that’s because it’s just lacking dynamic features of the others, or it’s just coded better, or it’s because macOS security defaults...

userbinator 7y ago

Good riddance, the only thing I've ever seen PDF encryption used for is DRM. In my mind, anything that breaks DRM is a bonus.

JaimeThompson 7y ago
They are also used to stop quotes and documents from being changed once they are sent to others.
- munk-a 7y ago
  There are other better ways to accomplish that like lodging a document with a third party to ensure a lack of changes... regarding quoting... preventing text capture is a futile effort that really needs to stop existing - it's a pain to deal with and provides no real security since, much like snapchat - you can never counter someone taking a picture of the screen.
  - JaimeThompson 7y ago
    Yes there are better ways but for a lot of small businesses those better ways simply do not get paid. It isn't how it should be and it will take a long time for things to change.
  - nitrogen 7y ago
    you can never counter someone taking a picture of the screen. Except with digital watermarks. Photocopiers generally recognize things like currency and refuse to copy at high fidelity. The PS3 I believe would refuse to play ripped cinema soundtracks all the way through, using audio watermarking. It's conceivable that cameras could be required to do the same thing for digital restriction watermarks.
    - ptx 7y ago
      Even if they conspire with every camera manufacturer, they can't prevent someone from just retyping the text they want to quote. On a mechanical typewriter if necessary. I guess they could try to do a denial of service attack on the typist by deliberately overusing letters from the same vertical keyboard row to wear out the finger.

moftz 7y ago

FIRST Robotics Competition used to distribute the rulebook for the season in an encrypted PDF. The password and unencrypted copies were released after the live kickoff stream. Ten years ago we'd be busting ass trying to crack the PDF but techniques weren't great then.

AlphaWeaver 7y ago
This is still how the rules are distributed today. I wonder if we'll see teams using this technique this year. Edit: It looks like this technique wouldn't be useful, since its an exfiltration technique and not a flaw in the encryption algorithm.
pietroglyph 7y ago
FRC was the thing I immediately thought about when I saw this (they still do PDF encryption), but the techniques here don't really apply. I'm pretty sure that you could just try to brute force it, as the passwords as usually dictionary-based with substitutions (e.g. R0bot$InSp&ce). Edit: After taking a much more in-depth look at it, all of the past game manuals' passwords are too complex to crack in a reasonable time (even with a dictionary). With a top 1000 word list dictionary with substitutions of 3-letter words like the above there are (conservatively) 1x10^20 combinations. You can't crack the PDF user key directly either, because recent game manuals use 256-bit AES keys.

reilly3000 7y ago

I've always operated under the assumption that nothing in a PDF is secure, and that the only safe way to redact something from one is to print it and rescan it. Convince me otherwise?

bepvte 7y ago
I believe that if you edit the pdf so that the thing you are redacting is no longer in the pdf file, it is safely redacted. Printing a pdf and rescanning a pdf breaks text selection and screen readers, and basically makes it into a fancy PNG.
- munk-a 7y ago
  There is a bunch of data in PDFs that may have previously been visible but is no longer. I know that there are also fun meta-data attributes embedded on a per-page and per document basis - and also that PDFs can be portfolio documents where such a document will often contain no longer referenced PDFs that became irrelevant to the PDF. The file format is pretty crazy.
captn3m0 7y ago
There are a few redaction tools that do exactly this, but in software. You get a PNGish version, which you redact and combine back to PDF

taxidump 7y ago

A company I was a contractor for would send payment invoices with a password. The password was the last four digits of the ID number specific to the contractor. Our CRM we all used had every contractors ID number in their profile publicly available. When I asked what the point was I was told upper management required a password and this was the easiest.

xvilka 7y ago

At the same time no open source PDF reader can handle all cases of embedded animation and PDF forms[1]. [1] https://gitlab.freedesktop.org/poppler/poppler/issues?label_name%5B%5D=forms https://gitlab.freedesktop.org/poppler/poppler/issues?label_...

mettamage 7y ago

Ehm, what is an exfiltration channel? I know what a sidechannel is, but I haven't heard of exfiltration channel.

otakucode 7y ago
Just a channel that is used for exfiltration. It's not a specific sort of thing.

MartinMond 7y ago

PSPDFKit CTO here. TL;DR on the PDFex vulnerability: The file format can carry both encrypted and unencrypted parts, which allows "injecting" malicious payload (embedded JavaScript, PDF forms that submit to remote URLs, etc) into an encrypted PDF that then can be used to exfiltrate the encrypted PDF parts, after the user decrypted the file (= entered the password). We're currently investigating the vulnerability in detail and plan to ship an update ASAP that will allow our customers to mitigate it. I'll be monitoring this thread to answer any questions.