While useful it needs a big red warning to potential leakers. If they were personally served documents (such as via email, while logged in, etc) there really isn't much that can be done to ascertain the safety of leaking it. It's not even safe if there are two or more leakers and they "compare notes" to try and "clean" something for release.
The watermark can even be contained in the wording itself (multiple versions of sentences, word choice etc stores the entropy). The only moderately safe thing to leak would be a pure text full paraphrasing of the material. But that wouldn't inspire much trust as a source.
Oof, that's a great point. We briefly touched on this a few weeks ago, but from the angle of canary tokens / tracking pixels [1].
Security-wise, our main concern is protecting people who read suspicious documents, such as journalists and activists, but we do have sources/leakers in our threat model as well. Our docs are lacking in this regard, but we will update them with information targeted specifically to non-technical sources/leakers about the following threats:
- Digital watermarking (what you're pointing out here)
- Fingerprinting (camera, audio, stylometry)
- Canary tokens (not metadata per se, but still a de-anonymization vector)
If you come in FOSDEM next week, we plan to talk about this subject there [2].
The goal here isn't to provide a false sense of security, nor frighten people. It's plain old harm reduction. We know (and encourage) sources to share documents that can help get a story out, but we also want to educate them about the circumstances in which they may contain their PII, so that they can make an informed choice.
This doesn't seem to be designed for leakers, i.e. people sending PDF's -- it's specifically for people receiving untrusted files, i.e. journalists.
And specifically about them not being hacked by malicious code. I'm not seeing anything that suggests it's about trying to remove traces of a file's origin.
I don't see why it would need a warning for something it's not designed for at all.
It would be natural for a leaker to assume that the PDF contains something "extra" and to try and and remove it with this method. It may not occur to them that this something extra could be part of the content they are going to get back.
I seem to remember Yahoo finance (I think it was them, maybe someone else) introducing benign errors into their market data feeds, to prevent scraping.
This lead to people doing 3 requests instead of just 1, to correct the errors, which was very expensive for them, so they turned it off.
I don't think watermarking is a winning game for the watermarker, with enough copies any errors can be cancelled.
> I don't think watermarking is a winning game for the watermarker, with enough copies any errors can be cancelled.
This is a very common assumption that turns out to be false.
There are Tardos probabilistic codes (see the paper I linked) which have the watermark scale as the square of the traitor count.
For example, with a watermark of just 400 bits, 4 traitors (who try their best to corrupt the watermark) will stand out enough to merit investigation and with 800 bits be accused without any doubt. This is for a binary alphabet, with text you can generate a bigger alphabet and have shorter watermarks.
These are typically intended for tracing pirated content, so they carry the so-called Marking Assumption (if given two or more versions of a piece of content, you must choose one. A pirate isn't going to corrupt or remove a piece of video, that would be unsuitable for leaking). So it would likely be possible to get better results with documents, may require larger watermarks to get such traitors reliably.
Why not leak a dataset of N full text paraphrasings of the material, together with a zero-knowledge proof of how to take one of the paraphrasings and specifically "adjust" it to the real document (revealed in private to trusted asking parties)? Then the leaker can prove they released "at least the one true leak" without incriminating themselves. There is a cryptographic solution to this issue.
Heh, I've seen this a bunch of times and it's of interest to me, but honestly? It's sooooo limiting by being an interface without a complementary command line tool. Like, I'd like to put this into some workflows but it doesn't really make sense to without using something like pyautogui. But maybe I'm missing something hidden in the documentation.
There is indeed a dangerzone-cli tool¹, and it should be made more visible. We plan on updating/consolidating our docs in the foreseeable future, to make things clearer.
Also, plans are here to make it possible to use dangerzone as a library, which should help use cases like the one you mention.
Not much further than their documentation, friend! But thanks for finding that, that's actually super helpful! I hope somebody puts in a pr for updating the documentation to make it clear what functionality their tool has.
For some reason, printing 1 page of an Excel or Word document to a PDF often gets up to around 4MB in size. Passing it through this compresses it quite well.
I often view PDFs in Drive, and it's definitely not just displaying the document with the native web browser. It is rendered with their "Drive renderer", whatever that is. They don't even display a simple .txt file natively in the browser.
They have some kind of virus scanner for files you open via a share link. Not sure about the ones you have stored on your own drive unshared.
But probably the main security here is just using the chrome pdf viewer instead of the adobe one. Which you can do without google drive. The browser PDF viewers ignore all the strange and risky parts of the PDF spec that would likely be exploited.
Is there some reason why just viewing the PDF with a FLOSS, limited PDF viewer (e.g. atril) would not accomplish the same level of safety? What can a "dangerous PDF" do inside atril?
(Hi, disclaimer: I'm one of the current dangerzone maintainers)
That's a good question :-)
Opening PDFs, or images, or any other document directly inside your machine, even with a limited PDF viewer, potentially exposes your environment to this document.
The reason is that exploits in the image/font/docs parsing/rendering libraries can happen and are exploited in the wild. These exploits make it possible for an attacker to access the memory of the host, and in the worse case allow code execution.
Actually, that's the very threat Dangerzone is designed to protect you from.
We do that by doing the docs to pixel conversion inside a hardened container that uses gVisor to reduce the attack surface ¹
One other way to think about it is to actually consider document rendering unsafe. The approach Dangerzone is taking is to make sure the environment doing the conversion is as unprivileged as possible.
In practice, an attack is still possible, but much more costly: an attacker will be required to do a container escape or find a bug in the Linux kernel/gVisor in addition to finding an exploit in document rendering tools.
Not impossible, but multiple times more difficult.
A crafted PDF can potentially exploit a bug in atril to compromise the recipient's computer since writing memory-safe C is difficult. This approach was famously used by a malware vendor to exploit iMessage through a compressed image format that's part of the PDF standard:
This is why Firefox chose to implement a custom PDF reader in pure JS for better sandboxing leveraging the existing browser JS sandboxing.
As a side effect, it's been a helpful JS library for embedding PDFs on websites.
The Chrome PDF parser, originating from Foxit (now open-sourced as PDFium), has been the source of many exploits in Chrome itself over the years.
(Hi, disclaimer: I'm one of the current dangerzone maintainers)
You are correct: that's basically what Dangerzone is doing!
The challenges for us are to have a sandbox that keeps being secure and make it possible for non-tech folks (e.g. journalists) to run this in their machines easily.
About the sandbox:
- Making sure that it's still updated requires some work: that's testing new container images, and having a way to distribute them securely to the host machines ;
- In addition to running in a container, we reduce the attack surface by using gVisor¹ ;
- We pass a few flags to the Docker/Podman invocation, effectively blocking network access and reducing the authorized system calls ;
Also, in our case the sandbox doesn't mount the host filesystem in any way, and we're streaming back pixels, that will be then written to a PDF by the host (we're also currently considering adding the option to write back images instead).
The other part of the work is to make that easily accessible to non-tech folks. That means packaging Podman on macOS/Windows, and providing an interface that works on all major OSes.
Shameless self promotion: preview.ninja is a site I built that does this and supports 300+ file formats. I'm currently weekend coding version 2.0 which will support 500+ formats and allow direct data extraction in addition to safe viewing.
It is a passion project and will always be free because commercial CDR[1] solutions are insanely expensive and everyone should have access to the tools to compute securely.
To review documents received from a hostile and dishonest actor in litigation I used disposable VMs in qubes on a computer with a one way (in only) network connection[1], while running the tools (e.g. evince) in valgrind and with another terminal watching attempted network traffic (an approach that did detect attempted network callbacks from some documents but I don't think any were PDFs).
This would have been useful-- but I think I would have layered it on top of other isolation.
([1] constructed from a media converter pair, a fiber splitter to bring the link up on the tx side, and some off the shelf software for multicast file distribution).
While useful it needs a big red warning to potential leakers. If they were personally served documents (such as via email, while logged in, etc) there really isn't much that can be done to ascertain the safety of leaking it. It's not even safe if there are two or more leakers and they "compare notes" to try and "clean" something for release.
https://en.wikipedia.org/wiki/Traitor_tracing#Watermarking
https://arxiv.org/abs/1111.3597
The watermark can even be contained in the wording itself (multiple versions of sentences, word choice etc stores the entropy). The only moderately safe thing to leak would be a pure text full paraphrasing of the material. But that wouldn't inspire much trust as a source.
Oof, that's a great point. We briefly touched on this a few weeks ago, but from the angle of canary tokens / tracking pixels [1].
Security-wise, our main concern is protecting people who read suspicious documents, such as journalists and activists, but we do have sources/leakers in our threat model as well. Our docs are lacking in this regard, but we will update them with information targeted specifically to non-technical sources/leakers about the following threats:
- Metadata (simple/deep)
- Redactions (surprisingly easy to get wrong)
- Physical watermarking (e.g., printer tracking dots)
- Digital watermarking (what you're pointing out here)
- Fingerprinting (camera, audio, stylometry)
- Canary tokens (not metadata per se, but still a de-anonymization vector)
If you come in FOSDEM next week, we plan to talk about this subject there [2].
The goal here isn't to provide a false sense of security, nor frighten people. It's plain old harm reduction. We know (and encourage) sources to share documents that can help get a story out, but we also want to educate them about the circumstances in which they may contain their PII, so that they can make an informed choice.
[1]: https://social.freedom.press/@dangerzone/115859839710582670
[2]: https://fosdem.org/2026/schedule/event/JZ3F8W-dangerzone_ble...
(Dangerzone dev btw)
This doesn't seem to be designed for leakers, i.e. people sending PDF's -- it's specifically for people receiving untrusted files, i.e. journalists.
And specifically about them not being hacked by malicious code. I'm not seeing anything that suggests it's about trying to remove traces of a file's origin.
I don't see why it would need a warning for something it's not designed for at all.
It would be natural for a leaker to assume that the PDF contains something "extra" and to try and and remove it with this method. It may not occur to them that this something extra could be part of the content they are going to get back.
5 replies →
I seem to remember Yahoo finance (I think it was them, maybe someone else) introducing benign errors into their market data feeds, to prevent scraping. This lead to people doing 3 requests instead of just 1, to correct the errors, which was very expensive for them, so they turned it off.
I don't think watermarking is a winning game for the watermarker, with enough copies any errors can be cancelled.
> I don't think watermarking is a winning game for the watermarker, with enough copies any errors can be cancelled.
This is a very common assumption that turns out to be false.
There are Tardos probabilistic codes (see the paper I linked) which have the watermark scale as the square of the traitor count.
For example, with a watermark of just 400 bits, 4 traitors (who try their best to corrupt the watermark) will stand out enough to merit investigation and with 800 bits be accused without any doubt. This is for a binary alphabet, with text you can generate a bigger alphabet and have shorter watermarks.
These are typically intended for tracing pirated content, so they carry the so-called Marking Assumption (if given two or more versions of a piece of content, you must choose one. A pirate isn't going to corrupt or remove a piece of video, that would be unsuitable for leaking). So it would likely be possible to get better results with documents, may require larger watermarks to get such traitors reliably.
2 replies →
> The only moderately safe thing to leak would be a pure text full paraphrasing of the material. But that wouldn't inspire much trust as a source.
Isn't this what newspapers do?
Why not leak a dataset of N full text paraphrasings of the material, together with a zero-knowledge proof of how to take one of the paraphrasings and specifically "adjust" it to the real document (revealed in private to trusted asking parties)? Then the leaker can prove they released "at least the one true leak" without incriminating themselves. There is a cryptographic solution to this issue.
Seems like a similar but less elegant solution as parsing and normalization to a “safe” subset but not just blasting it to pixels.
https://github.com/caradoc-org/caradoc
http://spw16.langsec.org/slides/guillaume-endignoux-slides.p...
Heh, I've seen this a bunch of times and it's of interest to me, but honestly? It's sooooo limiting by being an interface without a complementary command line tool. Like, I'd like to put this into some workflows but it doesn't really make sense to without using something like pyautogui. But maybe I'm missing something hidden in the documentation.
(Hi, dangerzone maintainer here)
There is indeed a dangerzone-cli tool¹, and it should be made more visible. We plan on updating/consolidating our docs in the foreseeable future, to make things clearer.
Also, plans are here to make it possible to use dangerzone as a library, which should help use cases like the one you mention.
¹ https://github.com/freedomofpress/dangerzone/blob/main/dange...
https://github.com/freedomofpress/dangerzone/blob/main/dange...
How hard did you look the other times?
Not much further than their documentation, friend! But thanks for finding that, that's actually super helpful! I hope somebody puts in a pr for updating the documentation to make it clear what functionality their tool has.
2 replies →
It seems to meant for end-users like journalists processing files individually like e-mail attachments.
It doesn't seem to be meant for usage at scale -- it's not for general-purpose conversion, as the resulting files are huge, will have OCR errors, etc.
I'm the target audience for this sort of tool. :)
A handy side use for this is compressing PDFs.
For some reason, printing 1 page of an Excel or Word document to a PDF often gets up to around 4MB in size. Passing it through this compresses it quite well.
Just ran a quick test:
- 1-page Excel PDF export: 3.7MB
- Processing with Dangerzone (OCR enabled): 131KB
I wonder if the Excel export is retaining a lot of document structure in the event that it's imported back into Excel again at a later point.
I personally just upload them to google drive. It would be a serious pwn if they could somehow still do a compromise through google drive.
Does google drive apply any transformation over the PDF, or are you effectively loading the same document in your browser on the round trip?
I often view PDFs in Drive, and it's definitely not just displaying the document with the native web browser. It is rendered with their "Drive renderer", whatever that is. They don't even display a simple .txt file natively in the browser.
They have some kind of virus scanner for files you open via a share link. Not sure about the ones you have stored on your own drive unshared.
But probably the main security here is just using the chrome pdf viewer instead of the adobe one. Which you can do without google drive. The browser PDF viewers ignore all the strange and risky parts of the PDF spec that would likely be exploited.
1 reply →
Do you have any specifics on what Drive does? Any examples of it fixing embedded virii? Or is this blind assumption?
I assume they mean "upload to drive and use the web based reader to view the PDF," not "upload to drive and download it again"
5 replies →
Firefox has a builtin PDF reader, PDF.js, that resides inside of the Javascript sandbox. In theory, it's as safe as loading a webpage.
So not actually all that safe since sandbox escapes happen all the time. PDF.js has had many vulnerabilities as well
Is there some reason why just viewing the PDF with a FLOSS, limited PDF viewer (e.g. atril) would not accomplish the same level of safety? What can a "dangerous PDF" do inside atril?
(Hi, disclaimer: I'm one of the current dangerzone maintainers)
That's a good question :-)
Opening PDFs, or images, or any other document directly inside your machine, even with a limited PDF viewer, potentially exposes your environment to this document.
The reason is that exploits in the image/font/docs parsing/rendering libraries can happen and are exploited in the wild. These exploits make it possible for an attacker to access the memory of the host, and in the worse case allow code execution.
Actually, that's the very threat Dangerzone is designed to protect you from.
We do that by doing the docs to pixel conversion inside a hardened container that uses gVisor to reduce the attack surface ¹
One other way to think about it is to actually consider document rendering unsafe. The approach Dangerzone is taking is to make sure the environment doing the conversion is as unprivileged as possible.
In practice, an attack is still possible, but much more costly: an attacker will be required to do a container escape or find a bug in the Linux kernel/gVisor in addition to finding an exploit in document rendering tools.
Not impossible, but multiple times more difficult.
¹ We covered that in more details in this article https://dangerzone.rocks/news/2024-09-23-gvisor/
It looks like atril is mostly written in C:
https://github.com/mate-desktop/atril
A crafted PDF can potentially exploit a bug in atril to compromise the recipient's computer since writing memory-safe C is difficult. This approach was famously used by a malware vendor to exploit iMessage through a compressed image format that's part of the PDF standard:
https://projectzero.google/2021/12/a-deep-dive-into-nso-zero...
This is why Firefox chose to implement a custom PDF reader in pure JS for better sandboxing leveraging the existing browser JS sandboxing. As a side effect, it's been a helpful JS library for embedding PDFs on websites.
The Chrome PDF parser, originating from Foxit (now open-sourced as PDFium), has been the source of many exploits in Chrome itself over the years.
Why not just open it inside of and print to a static image output within a fully sandboxed Docker container?
(Hi, disclaimer: I'm one of the current dangerzone maintainers)
You are correct: that's basically what Dangerzone is doing!
The challenges for us are to have a sandbox that keeps being secure and make it possible for non-tech folks (e.g. journalists) to run this in their machines easily.
About the sandbox:
- Making sure that it's still updated requires some work: that's testing new container images, and having a way to distribute them securely to the host machines ;
- In addition to running in a container, we reduce the attack surface by using gVisor¹ ;
- We pass a few flags to the Docker/Podman invocation, effectively blocking network access and reducing the authorized system calls ;
Also, in our case the sandbox doesn't mount the host filesystem in any way, and we're streaming back pixels, that will be then written to a PDF by the host (we're also currently considering adding the option to write back images instead).
The other part of the work is to make that easily accessible to non-tech folks. That means packaging Podman on macOS/Windows, and providing an interface that works on all major OSes.
¹ https://dangerzone.rocks/news/2024-09-23-gvisor/
Yep. A static image would be better, although I'd also prefer the option of getting a simple text file so that I can get the URLs out of hyperlinks.
Why not upload to Google docs and view there? Way less work.
You might not want to make this file, or the fact that you are in posession of this file known by law enforcement.
Why not DJVU with a high DPI instead of a PDF?
Shameless self promotion: preview.ninja is a site I built that does this and supports 300+ file formats. I'm currently weekend coding version 2.0 which will support 500+ formats and allow direct data extraction in addition to safe viewing.
It is a passion project and will always be free because commercial CDR[1] solutions are insanely expensive and everyone should have access to the tools to compute securely.
1. https://en.wikipedia.org/wiki/Content_Disarm_%26_Reconstruct...
Now teach this HR departments. They still ask for Word docs or PDF from untrusted people. ASCII text is frowned upon. Go figure.
The employment readyness check if you can trust a company.
It's a neat program, but what's the use for JPGs and PNGs?
There are some neat detection bypass / compromise methods using various image formats, including PNG [0] and SVG [1]!
I imagine that folks like journalists could have that type of attack in their threat model, and EFF already do a lot of great stuff in this space :)
0. https://isc.sans.edu/diary/31998
1. https://www.cloudflare.com/cloudforce-one/research/svgs-the-...
To review documents received from a hostile and dishonest actor in litigation I used disposable VMs in qubes on a computer with a one way (in only) network connection[1], while running the tools (e.g. evince) in valgrind and with another terminal watching attempted network traffic (an approach that did detect attempted network callbacks from some documents but I don't think any were PDFs).
This would have been useful-- but I think I would have layered it on top of other isolation.
([1] constructed from a media converter pair, a fiber splitter to bring the link up on the tx side, and some off the shelf software for multicast file distribution).
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