Posts Tagged ‘Google Docs’

Back in March I wrote about a few security issues with Google Docs while keeping some details private.

Google Security and the Google Docs product management team engaged me immediately after the issues became public, and kept me well informed of their findings through several days of productive exchange of ideas. I’m used to getting the silent treatment when reporting security issues, so I’d like to credit Google for keeping the lines of communications open.

I had been “on the road” since then and decided to take time off from blogging.  Now that I’m back home, I’d like to close these issues before writing about a few other (non-Google) security & privacy concerns I have in mind.

So without further delay let’s revisit the three Docs issues based on my emails with Google back in late March and early April.  I understand that Google have made changes to remediate part or all of these issues, according to their own risk determination.

1. No protection for embedded images

This issue was about the lack of protection (authentication) for images embedded in a document, and an image’s continued existence on Google’s servers after its containing document has been deleted.  The lack of authentication means that the image URL could be accessed by 3rd parties without the document’s owner consent.

Google correctly noted that the image URL would have been known only to those with previous access to the image, and someone with such access could have saved the image anyway, and perhaps disclosed the saved image with unauthorized persons.

However, from a privacy perspective, there is a crucial difference between a “saved” image being disclosed, and one being served directly by Google Docs: evidence of ownership.

Let’s examine how a typical Docs image URL is constructed:

docs.google.com/File?id=dtfqs27_1f3vfmkcz_b   (an image stored at Google)

The bolded portion of the URL (“dtfqs27”) seems to uniquely identify the resource owner (in this case, me).  Documents and images created by the same account will have this same ID as part of the URL.

Embedding “personally linkable” IDs in URLs is poor practice and has wide-ranging privacy implications on on its own — more on this later.  Yet we’re going twice further here by: 1) associating the ID with a document resource; and 2) making the entire URL publicly accessible.  This is a form of Insecure Direct Object Reference, a common security issue which I’ll have to say more about in the coming days.

Contrived scenario:

I share a picture of my company’s ultra-secret new tablet with a potential supplier.   An employee of said supplier saves the picture and wants to sell it to AdeInsider.com, a rumor-site tracking my inventions.   They accuse the employee of just making it all up in Photoshop.  So the employee shares the link instead, e.g.:


Since there is no authentication, AdeInsider.com can now widely publish that link, and point out to their readers that the image on my blog has the same unique identifier, thus positively determining ownership. Instant privacy breach.  (Instead of a secret gadget, imagine compromising pictures, etc.)  My only recourse is to get Google support to remove the image, since I can’t immediately do it myself by deleting the containing document.  But any action on my part would have been too late, anyway.

As I noted in a previous post, I can only recommend defense-in-depth.   In this case the lack of authentication — which appears benign by itself due to randomness in the URL — might cause a serious privacy breach due to another issue (leak of what is essentially personally identifying information.)


Tagging resources with IDs potentially linked to personal information is unfortunately a widespread practice, with Facebook being a big example.  Like Google Docs, images uploaded to Facebook are tagged with the user’s ID, are accessible without authentication, and subject to the same privacy flaw.  It’s trivial to map Facebook IDs to real names.  From a privacy perspective, ID tagging might in some cases be more problematic than tracking cookies.

2. File revision flashback

I’m not going to add much more to this issue except to note that privacy breaches can occur due to designed behavior having non-intuitive implications to regular users — the old Microsoft Fast Save feature comes to mind, as well as a number of accidental disclosures involving PDF.  The fact that someone can fiddle with an embedded image’s URL (normally buried in HTML) to get previous revisions is not obvious to your typical Docs user.

Google has added useful entries in their Help files and there are now explicit controls in the diagram tool.

3. I’ll help myself to your Docs, thanks

I reported that in some cases, a person removed from a shared document could add himself back to a document’s shared list without the owner’s permission or knowledge.  This issue obviously garnered the most attention and as it turned out, was much more complex than I originally thought.

Google clarified that this behavior is proper when a document has the “invitations may be used by anyone” option enabled.  The purpose of this option is to allow forwarding of invitations (e.g., for mailing lists), and essentially works by making the document public.

After Google’s clarification, I checked through my test documents, and sure enough, this option was enabled on them, explaining the behavior.  There was only one problem:  I had explicitly disabled this option when creating my test documents, yet somehow these documents became publicly accessible!

After additional analysis at the time, my findings indicated that:

– A race condition existed due to the way the document sharing control GUI was implemented.  Most of the time, the Docs sharing control worked fine.  However, in some cases the control could fail in three distinct ways: a) the “invitations may be used by anyone” option visibly re-enabled itself after being disabled, immediately prior to the user clicking “submit”; b) the option remained disabled on screen, but was incorrectly submitted as enabled; c) the GUI completely failed and became non-responsive (which is actually fine since that’s fail-secure.)

I was able to record screencasts of each failure type and submitted them for Google’s review.

– Compounding the issue, a different GUI problem could hide the fact that a deleted “sharee” has added himself back to a document.

In Google Docs there are several areas where a document’s sharing status can be seen, including from the main screen’s “folder view”, from the left-nav of the main screen, and from a document’s sharing dialog.  When a sharee deleted a document (breaking the share) then immediately added himself back, the main folder view and left-nav will show that the document is no longer being shared when in fact it still is.

So weaknesses in the Google Docs user interface implementation could cause private documents invitations to be wrongly permissioned as public, and furthermore, deleted share participants could add him/herself back to documents without  document’s owner noticing.

What are essentially simple UI flaws (which arguably should have been caught by developers and/or QA) now have security and privacy implications.   This “escalation” is an inherent risk with collaborative applications, especially “cloud” applications which have world-shareable features.

I must state, the likelihood of a direct breach due to wrong permissioning is low.  However, as Issue #1 demonstrated, even seemingly minor flaws could lead to privacy leaks.  Indeed, documents incorrectly permissioned in this way are subject to the same evidence of ownership leak as the images in Issue #1.

From what I could tell, Google quickly implemented changes to fix part if not all of these issues.  I have no visibility regarding how many documents were incorrectly marked public.  Readers with highly sensitive documents should periodically review their sharing controls.

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[Update 11/13: Please see my follow-up to these issues.]

[Update 3/26: I’m now in contact with Google Security.]

[Update 3/28: I’m aware of Google’s official response to the issues raised in this blog.  I am continuing to share my findings with Google Security and appreciate the excellent feedback they are providing me.  It would be premature for me to provide further comment at this time. ]

If you can see the image below, you’ve just hacked Google Docs:

The above image should not be accessible to you.  It’s supposed to be embedded solely within a protected Google Docs document, which I have not shared. In fact, I’ve actually deleted that document.  It shouldn’t even exist anymore.  Yet here you are, viewing my precious picture in all its glory, nakedly served by Google servers,  outside of the protective Docs environment.

What went wrong?  In light of the recent Google Docs privacy glitch, let’s take a look at three privacy issues highlighting problems with the way documents are shared:

1. No protection for embedded images

When you embed (“insert”) an image from your computer into a Google Document, that image is “uploaded” onto Google servers and assigned an id.   From then on, the image is accessible via a URL.  For example, the URL for the above image is:


However, unlike the containing document, embedded images are not protected by the sharing controls.  That means anyone with access to the URL can view the image.  If you’ve shared a document containing embedded images with someone, that person will always be able to view those images.  Even after you’ve stopped sharing the document.  Or as the image above demonstrates, even after you’ve deleted the document.

That’s counter-intuitive behavior for most users.   If you embed an image into a protected document, you’d expect the image to be protected too.  If you delete a document, you’d expect any embedded resources to be deleted also. The end result is a potential privacy leak.

2. File revision flashback

It’s 4am and you’re been working all night on a document.   This document contains a Docs diagram, blueprinting that million-dollar-idea you have in your head.

You want to share this document with potential suppliers, but you don’t want to reveal all of your secrets just yet.   So you diligently redact the diagram, removing all the sensitive parts of the blueprints.  Satisfied that your idea is safe, you share the document (view-only).

Next thing you know, your idea has been stolen.  A Chinese company quickly ships knockoffs based on your complete blueprints.  What happened?

Unknown to you, anyone you shared the document with can view any version of any diagram embedded in the document.  The fact that you’ve deleted sensitive parts of the diagram doesn’t matter, because the viewer can see the older versions.

How?  Quite easy.  In Google Docs, a diagram is a set of instructions that’s rasterized into an image (in PNG format).  Each time you modify a diagram, a new raster image is created, but the old versions remain accessible via a URL, in the format:


To view any previous version, just change the “rev=” number above.

This problem is reminiscent of the old Microsoft Word Fast Save issue, and will have similar privacy implications if not changed.

3. I’ll help myself to your Docs, thanks

So you learned your lesson from above, and stopped sharing your documents.  You’ve kicked everyone out from your Docs.  This negates the purpose of Docs somewhat, but you’d rather be safe than sorry.

Working solo, you happily add new ideas to your secret document, patting yourself on the back before you go on a well-deserved vacation.

Too bad while you’re sipping piña coladas on the beach, those same suppliers you’ve just kicked out have added themselves back to your Docs and stealing your new ideas!  What?

It’s true.  Even if you unshare a document with a person, that person can in certain cases still access your document without your permission, a serious breach of privacy.  For now I’m withholding the mechanics of when/why/how this happens, pending further research and feedback from Google if any.


These findings are based upon my investigations stemming from Issue #1 above.  I disclosed this particular issue to Google on March 18.  I tend to follow rfpuppy’s Full Disclosure Policy and so waited five business days for Google to comment.  I’ve yet received any response from Google other than the usual automated, canned reply (which I don’t consider a real response.)

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