Also see Google's official description of how Safe Browsing works in Chrome.

Firefox has had support for Google's Safe Browsing since 2005 when it started as a stand-alone Firefox extension. At first it was only available in the USA, but it was opened up to the rest of the world in 2006 and moved to the Google Toolbar. It then got integrated directly into Firefox 2.0 before the public launch of the service in 2007.

Many people seem confused by this phishing and malware protection system and while there is a pretty good explanation of how it works on our support site, it doesn't go into technical details. This will hopefully be of interest to those who have more questions about it.

Browsing Protection

The main part of the Safe Browsing system is the one that watches for bad URLs as you're browsing. Browsing protection currently protects users from:

If a Firefox user attempts to visit one of these sites, a warning page will show up instead, which you can see for yourself here:

The first two warnings can be toggled using the browser.safebrowsing.malware.enabled preference (in about:config) whereas the last one is controlled by browser.safebrowsing.phishing.enabled.

List updates

It would be too slow (and privacy-invasive) to contact a trusted server every time the browser wants to establish a connection with a web server. Instead, Firefox downloads a list of bad URLs every 30 minutes from the server ( and does a lookup against its local database before displaying a page to the user.

Downloading the entire list of sites flagged by Safe Browsing would be impractical due to its size so the following transformations are applied:

  1. each URL on the list is canonicalized,
  2. then hashed,
  3. of which only the first 32 bits of the hash are kept.

The lists that are requested from the Safe Browsing server and used to flag pages as malware/unwanted or phishing can be found in urlclassifier.malwareTable and urlclassifier.phishTable respectively.

If you want to see some debugging information in your terminal while Firefox is downloading updated lists, turn on browser.safebrowsing.debug.

Once downloaded, the lists can be found in the cache directory:

  • ~/.cache/mozilla/firefox/XXXX/safebrowsing/ on Linux
  • ~/Library/Caches/Firefox/Profiles/XXXX/safebrowsing/ on Mac
  • C:\Users\XXXX\AppData\Local\mozilla\firefox\profiles\XXXX\safebrowsing\ on Windows

Resolving partial hash conflicts

Because the Safe Browsing database only contains partial hashes, it is possible for a safe page to share the same 32-bit hash prefix as a bad page. Therefore when a URL matches the local list, the browser needs to know whether or not the rest of the hash matches the entry on the Safe Browsing list.

In order resolve such conflicts, Firefox requests from the Safe Browsing server ( all of the hashes that start with the affected 32-bit prefix and adds these full-length hashes to its local database. Turn on browser.safebrowsing.debug to see some debugging information on the terminal while these "completion" requests are made.

If the current URL doesn't match any of these full hashes, the load proceeds as normal. If it does match one of them, a warning interstitial page is shown and the load is canceled.

Download Protection

The second part of the Safe Browsing system protects users against malicious downloads. It was launched in 2011, implemented in Firefox 31 on Windows and enabled in Firefox 39 on Mac and Linux.

It roughly works like this:

  1. Download the file.
  2. Check the main URL, referrer and redirect chain against a local blocklist (urlclassifier.downloadBlockTable) and block the download in case of a match.
  3. On Windows, if the binary is signed, check the signature against a local whitelist (urlclassifier.downloadAllowTable) of known good publishers and release the download if a match is found.
  4. If the file is not a binary file then release the download.
  5. Otherwise, send the binary file's metadata to the remote application reputation server (browser.safebrowsing.downloads.remote.url) and block the download if the server indicates that the file isn't safe.

Blocked downloads can be unblocked by right-clicking on them in the download manager and selecting "Unblock".

While the download protection feature is automatically disabled when malware protection (browser.safebrowsing.malware.enabled) is turned off, it can also be disabled independently via the browser.safebrowsing.downloads.enabled preference.

Note that Step 5 is the only point at which any information about the download is shared with Google. That remote lookup can be suppressed via the browser.safebrowsing.downloads.remote.enabled preference for those users concerned about sending that metadata to a third party.

Types of malware

The original application reputation service would protect users against "dangerous" downloads, but it has recently been expanded to also warn users about unwanted software as well as software that's not commonly downloaded.

These various warnings can be turned on and off in Firefox through the following preferences:

  • browser.safebrowsing.downloads.remote.block_dangerous
  • browser.safebrowsing.downloads.remote.block_dangerous_host
  • browser.safebrowsing.downloads.remote.block_potentially_unwanted
  • browser.safebrowsing.downloads.remote.block_uncommon

and tested using Google's test page.

If you want to see how often each "verdict" is returned by the server, you can have a look at the telemetry results for Firefox Beta.


One of the most persistent misunderstandings about Safe Browsing is the idea that the browser needs to send all visited URLs to Google in order to verify whether or not they are safe.

While this was an option in version 1 of the Safe Browsing protocol (as disclosed in their privacy policy at the time), support for this "enhanced mode" was removed in Firefox 3 and the version 1 server was decommissioned in late 2011 in favor of version 2 of the Safe Browsing API which doesn't offer this type of real-time lookup.

Google explicitly states that the information collected as part of operating the Safe Browsing service "is only used to flag malicious activity and is never used anywhere else at Google" and that "Safe Browsing requests won't be associated with your Google Account". In addition, Firefox adds a few privacy protections:

  • Query string parameters are stripped from URLs we check as part of the download protection feature.
  • Cookies set by the Safe Browsing servers to protect the service from abuse are stored in a separate cookie jar so that they are not mixed with regular browsing/session cookies.
  • When requesting complete hashes for a 32-bit prefix, Firefox throws in a number of extra "noise" entries to obfuscate the original URL further.

On balance, we believe that most users will want to keep Safe Browsing enabled, but we also make it easy for users with particular needs to turn it off.

Learn More

If you want to learn more about how Safe Browsing works in Firefox, you can find all of the technical details on the Safe Browsing and Application Reputation pages of the Mozilla wiki or you can ask questions on our mailing list.

Google provides some interesting statistics about what their systems detect in their transparency report and offers a tool to find out why a particular page has been blocked. Some information on how phishing sites are detected is also available on the Google Security blog, but for more detailed information about all parts of the Safe Browsing system, see the following papers: