Thursday, November 2, 2017

Forensics Quickie: Identifying "Clear Recent History" Settings for an Old Version of FireFox

FORENSICS QUICKIES! These posts will consist of small tidbits of useful information that can be explained very succinctly.

I saw this tweet from @phillmoore recently.

Archived tweet here.

Further down in this thread, we also see that the target browser is a ~5 year old version of Firefox. I've written in the past about verifying program behavior using source code. In fact, the previous post uses Firefox as an example. Given the information that we have -- and even though this is somewhat of a fringe case -- let's run through how to get the answer. Remember: it's one thing to know that an artifact exists; it's another to know how to find, understand, and make use of it

So, from start to finish, let's delve into how you'd go about answering the question Phill posted.

Scenario
You want to determine where an older version of Firefox stores the settings for the "Clear Recent History" dialog.

The Solution
First, we need to identify a version of Firefox that is about 5 years old. Mozilla hosts a page that lists all past releases of Firefox and links each to their respective release notes -- including the dates of release. Clicking around a bit, I found that Firefox version 17.0 was about 5 years old (Nov 2012).

With that out of the way, I needed to locate the source code and compiled executables archive for past releases. Googling brought me to Mozilla's "Downloading Source Archives" page. There are tons of ways to download the source, but I wanted a quick and easy way, so I navigated to: https://archive.mozilla.org/pub/.

Following down the index, I found what I needed at:
  • https://archive.mozilla.org/pub/firefox/releases/17.0/source/
    • firefox-17.0.source.tar.bz2
and
  • https://archive.mozilla.org/pub/firefox/releases/17.0/win32/en-US/
    • Firefox Setup 17.0.exe

Now that I have the v17 source code and installer, I can start testing. I'm targeting the "Clear Recent History" dialog, so I install Firefox v17 on a clean VM, and use the "Firefox" tab within the browser to navigate to History > Clear Recent History...

Default "Clear Recent History" Dialog for Firefox v17.

I'd like to find where the logic for this dialog is located within the source code. To make that easier, I need to grab a fairly unique string from this dialog -- one that won't come back with a lot of hits across many different source files. "Time range to clear" seems unique enough, so let's go with that.

There are many different ways to go about searching the extracted source code, but since I'm on Windows and want to run a quick and dirty search across many files, I'll just use AstroGrep.

Using AstroGrep to recursively search Firefox source code for a unique string.

I provide my search path that contains the source code, define my search string, and perform the search recursively across all file types. The results show two files that contain my unique string. The sanitize.dtd file sounds interesting, so let's open that one up.

Contents of "sanitize.dtd" showing our unique string along with some other context.

The first hit for our unique string can be seen at line 12. By looking around this area, we can gather some clues in order to pivot to other files that have more meat to them. Particularly interesting are lines 5 and 6. I'm looking for the settings within the dialog titled "Clear Recent History," so let's run an AstroGrep search for "sanitizeDialog2."

Using AstroGrep to recursively search Firefox source code for "sanitizeDialog2."

Again, the search string is unique enough to cut down on the amount of results we need to review. The file named sanitizeDialog.js seems to be what we're looking for here.

Contents of "sanitizeDialog.js" showing our searched string.

Line 64 shows our searched string. It also looks like we have something more than just localization and property data in this file. Browsing through this file would probably be a good idea.

Contents of "sanitizeDialog.js" showing the sanitize() function.

Line 100 contains the beginning of the sanitize() function and references the updatePrefs() function. A few lines down, we see what that's all about.

Contents of "sanitizeDialog.js" showing the updatePrefs() function.

The updatePrefs() function provides even more clues. It gets the timespan that the user sets within the dialog and hints at what we saw on line 105: the prefDomain of "privacy.cpd." We see "downloads" and "history" tacked on to that string, which leads me to believe that we're getting really close.

An AstroGrep search for "privacy.cpd" finally leads us to our destination.

Using AstroGrep to recursively search Firefox source code for "privacy.cpd."

The very first result, firefox.js, shows a few lines that not only contain "privacy.cpd," but also some familiar labels. These labels more or less line up with the checkboxes we saw in our "Clear Recent History" dialog at the beginning of this post. It gets even more interesting as we review the contents of firefox.js.

Contents of "firefox.js" showing the default "clear history" values and time span values.

As we can see, the source code is set up to check some of the items in the dialog by default. There are some other interesting lines here, but we'll get to that in a minute. What really caught my eye was line 495. Having done some research on Firefox proxy settings in the past, I knew that those settings were stored in the prefs.js file located in Firefox profiles. Couple that with the location of the firefox.js file within the source code folder structure (C:\4n6k\firefox-17.0.source\mozilla-release\browser\app\profile), and there's a really good chance all of what we need is in our profile's prefs.js. (which is typically located at C:\Users\4n6k\AppData\Roaming\Mozilla\Firefox\Profiles\<RandomChars>.default).

We can test this theory by performing a set of actions (as a normal user would) and documenting the results.

Test #01: Perform a default "Clear Recent History"

The first test was to open Firefox v17 on a VM that did not have Firefox already installed and clear the history using the default values within the dialog.

Here's what a default "Clear Recent History" action looks like on Firefox v17:

A default "Clear Recent History" action on Firefox v17. No settings were changed.

By default, the time range is set to "Last Hour," and the "Browsing & Download History," "Cookies," "Cache," and "Active Logins" checkboxes are selected. The "Offline Website Data" and "Site Preferences" checkboxes are not selected.
Note: prefs.js - Complete Re-write Upon Exit

Upon hitting the "Clear Now" button, Firefox (at least in the case of v17) must be closed in order for us to accurately test changes to the prefs.js file. The prefs.js file gets written to upon closing the application. And, per the warning at the top of the file, the entirety of the file will get re-written upon application exit. In other words, the born time and last written time get updated upon Firefox's exit; it is a brand new file. Every prefs.js file in these tests was acquired after application closure.

For reference, you can view the prefs.js files that existed before and after the default clear was performed.
[BEFORE]  [AFTER]

A quick comparison of the two prefs.js files using Beyond Compare shows very few changes. None are relevant to what we're testing.

Using Beyond Compare to compare two prefs.js files. No relevant differences are seen here.

Test #02: Perform a default "Clear Recent History" after form data input

In the previous test, the "Form & Search History" checkbox is grayed out. I wanted to test the default history clear while that checkbox was enabled, so I browsed to Twitter and logged in. That was enough to save some form data for the username field.

A default "Clear Recent History" action on Firefox v17 with form data present.

Note that I did not check the "Form & Search History" checkbox. It was checked by default after some form data was introduced. This variance did not show anything new; there was no relevant change in the prefs.js file.
[BEFORE]  [AFTER]

Using Beyond Compare to compare two prefs.js files. No relevant differences are seen here.

Test #03: Perform a "Clear Recent History" with all boxes selected + 2hr time span

For this test, a few websites were browsed, form data was input, all checkboxes were selected, and the "time range to clear" was changed from the default "1 hour" to 2 hours.

A modified "Clear Recent History" action on Firefox v17. All boxes are selected and time span is changed..

With these changes, we finally see some relevant items get written to the prefs.js file.
[BEFORE]  [AFTER]

Using Beyond Compare to compare two prefs.js files.Some relevant differences are detected.

We see the following get written:
  • user_pref("privacy.cpd.offlineApps", true);
  • user_pref("privacy.cpd.siteSettings", true);
  • user_pref("privacy.sanitize.timeSpan", 2);
Also note the "2" value after the timeSpan line. Remember: the value at that position is defined in the source code:
  • 0 - Clear everything
  • 1 - Last Hour
  • 2 - Last 2 Hours
  • 3 - Last 4 Hours
  • 4 - Today
Test #04: Perform a "Clear Recent History" with first box selected + 4hr time span

For this test, a few more websites were browsed, form data was input, all checkboxes were deselected except for the first one (unchecking everything will gray out the "Clear Now button"), and the "time range to clear" was changed from the default "1 hour" to 4 hours.

A modified "Clear Recent History" action on Firefox v17. Only one box is selected and time span is changed..

With these changes come more changes in the prefs.js file.
[BEFORE]  [AFTER]

Using Beyond Compare to compare two prefs.js files.More relevant differences are detected.
 
The key is to understand that if something deviates from the default "Clear Recent History" settings, there will be an entry for it in the prefs.js file. Note that this recent change caused entries to show up for cache, cookies, formdata, and sessions. This is because they have been switched to the opposite of what the default settings are set to. As an example, cache is checked in the default settings. When it is unchecked, it deviates from the default, and therefore, Firefox needs to make note of it in prefs.js. Likewise, "Offline Website Data" (aka offlineApps) is NOT checked in the default settings. When it is checked, it deviates from the default, and therefore, Firefox needs to make note of it in the prefs.js.

One final anecdote is that the checkboxes and time span within the "Clear Recent History" dialog do not get updated/saved until you click the "Clear Now" button (at least in the case of v17). That is, if you check/uncheck items or change the time span value and hit cancel, your settings will not be persistent -- neither in the current session nor the prefs.js file.

With that, we now have a better idea of how one of the many facets of Firefox operates. There is much, much more there (especially in pref.js), but testing specific functionality in depth goes a long way in understanding how things work.

If there's one takeaway from all of this, it's to make sure you're leveraging what is available. If you have source code to look through, identifying the behavior of a given application can be a walk in the park. 

...unless it's written in Perl.




-Dan Pullega (@4n6k)


References
1. Reference to Original Tweet 01
2. Forensics Quickie: Verifying Program Behavior Using Source Code (by 4n6k)
3. Reference to Original Tweet 02
4. Mozilla Firefox Release Notes Archive
5. Downloading Source Archives (Mozilla)
6. Firefox Source Code Archive
7. AstroGrep Homepage


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