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I'm a computer security professional, most interested in cybercrime and computer forensics. I'm also on Twitter @bond_alexander All opinions are my own unless explicitly stated.

Friday, April 29, 2011

Firefox 4 Browser Forensics, Part 4

When I started this series, I had no idea it would go on this long. There are more forensic artifacts in FF4 than I thought. It seems like every time I turn around I find another database to mine for artifacts. For example, I was just about to start tearing apart cookies.sqlite when I saw there was a search.sqlite. It didn't turn out to have any significant artifacts, just the listing of search engines in the quick search box. Much more interesting are browser cookies.

Cookies
Browser cookies are a notable and well-known source of browsing history. In IE, each cookie is a separate text file, but in Firefox, they're stored in yet another sqlite database: cookies.sqlite. This database has just one table, moz_cookies, with several columns:
  • id: an index
  • name: the variable being stored
  • value: the value of "name"
  • host: the website the cookie is for
  • pathmain: the path the cookie is valid for.
  • expiry: when the cookie should be purged
  • lastAccessed: when the website last accessed the cookie in PRTime
  • isSecure: is HTTPS required to access the cookie?
  • isHttpOnly: Can the only be accessed by HTTP, or can other methods (Javascript) access it?
  • baseDomain: The site's base domain, without www or other subdomain
  • creationTime: when the cookie was created in PRTime

When you're drawing conclusions from cookies, remember to take into account the browser's cookie control settings as well as the cookie timestamps. Like browsing history, cookies can tell you when the user viewed a website and can be a source for usernames and passwords for insecure websites. However, if the user clears cookies regularly, the amount of data will be limited. It also doesn't mean the user visited the site directly, as many advertising cookies (such as doubleclick.net) will be downloaded for related sites.

Saved Sessions
One useful aspect of Firefox is it will automatically save the currently open browser tabs so you can reopen it the next session. The browser tabs are saved in sessionstore.js in Javascript object notation. These aren't just the raw URLs, possible fields include page title, referrer, formdata, and cookies. Any GET variables in the URL are preserved as well. As a result, if the user was logged into a website, they may still be be logged in when the saved session is restored. If sessionstore.js has been damaged or deleted, it may be recoverable in the backup, sessionstore.bak. A quick test shows that sessionstore.bak isn't always a duplicate of sessionstore.js, opening a new browser session overwrote sessionstore.js but not sessionstore.bak, so you may be able to recover two different browser sessions under some conditions.

Bookmark backups
In the profile folder there's a folder called bookmarkbackups, which contains a series of JSON files storing the last 10 backups of the user's bookmarks. The filenames are in the format of bookmarks-YYYY-MM-DD. These backup files include the bookmarks the user explicitly makes, but also Firefox's "Smart Bookmarks" which include items of forensic value like "Most Visited", "Recently Bookmarked", and "Recently Tagged". These backups also include timestamps in PRTime for when the bookmark was added and last modified. As always in forensics, backups like these can provide valuable insight into detecting antiforensics (such as deleting bookmarks) and in behavior over time.

Downloads
downloads.sqlite is where Firefox 3+ stores information relating to downloaded files. There's just one table, moz_downloads, but it has quite a few useful artifacts.
  • id: an index
  • name: the local filename of the download
  • source: the remote filename and path being downloaded
  • target: where it's being downloaded to
  • tempPath: if the file is complete, it will be blank. If not, it's where the incomplete file is being stored before moving to target
  • startTime: Time the download started, in PRTime
  • endTime: Time the download finished, also PRTime
  • state: state of download, encoded as an integer 
  • referrer: the page containing the link to the file
  • entityID: a value used for resuming downloads
  • currBytes: Number of bytes downloaded.
  • maxBytes: Total file size.
  • mimeType: MIME file type.
  • preferredApplication: From the download dialogue box, if the user clicks run, this stores the program that will open the downloaded file. If the user clicks save, this will be blank.
  • preferredAction: Action to take when download is complete. Default is 0, just save the file.
  • autoResume: Can the download be resumed if broken?
extensions.ini records what Firefox extensions are installed, which may be useful if, for example, hacker tools like Hackbar or anonymizers like FoxyProxy are installed.

Form history
formhistory.sqlite is another good source for artifacts, although unfortunately it doesn't track what website the form was used on.

  • id: Another numeric index
  • fieldname: The field that contained value. This may be an HTML field in a website's form, or it may be a Firefox field, like searchbar-history.
  • value: What was typed into fieldname. In addition to search history, this is a good source for email addresses and usernames connected to the user.
  • timesUsed: How many times the user has typed value into fieldname.
  • firstUsed: The first time the user typed value into fieldname, in PRTime as always.
  • lastUsed: The last time the user typed value into fieldname, in PRTime as always.
  • guid: A global id for the formhistory, in case syncing is enabled.


Cache
The cache exists in two folders, for Windows 7 they are Users/[user]/AppData/Local/Mozilla/Firefox/Profiles/[profile name]/Cache and Users/[user]/AppData/Local/Mozilla/Firefox/Profiles/[profile name]/OfflineCache. The offline cache is used when Firefox is in offline mode, but it's the standard cache which will be more likely to be used for forensics. The cache can be viewed through the browser by navigating to about:cache, or it can be viewed directly on the file system. On the filesystem, the actual cache files are stored in a set of directories and subdirectories with names in hex. The files can be located by using the _CACHE_MAP_ and _CACHE_00X_ files. A full writeup on how the cache works has been done by Symantec's response team here. As far as I can tell, the cache scheme hasn't changed since then. This layout is rather inconvenient to navigate by hand, so an automated tool like Firefox's cache browser or other tool is definitely the way to proceed here. Looking at about:cache, it appears Firefox stores useful information like the full URL cached (including GET parameters), cached file size, number of times the cached file was accessed, last time the file was modified and time the cached file expires, full hex dump of the file, and the full HTTP request issued to access it. Since these are individual files on the hard drive, the standard Modified-Accessed-Created timestamps can provide additional information.

Alright, we're finally through the forensic artifacts available in Firefox 4! There'll be just one entry left, anti-forensics in Firefox 4.

2 comments:

  1. Thanks for taking the time to do such a great overview of FF4!

    ReplyDelete
  2. You're welcome, I'm glad it's been useful.

    ReplyDelete