Fixing Security Bugs

A bug has been reported as security-sensitive in Bugzilla and received a security rating.

If this bug is private - which is most likely for a reported security bug - the process for patching is slightly different than the usual process for fixing a bug.

Here are security guidelines to follow if you’re involved in reviewing, testing and landing a security patch. See Security Bug Approval Process for more details about how to request sec-approval and land the patch.

Keeping private information private

A security-sensitive bug in Bugzilla means that all information about the bug except its ID number are hidden. This includes the title, comments, reporter, assignee and CC’d people.

A security-sensitive bug usually remains private until a fix is shipped in a new release, and after a certain amount of time to ensure that a maximum number of users updated their version of Firefox. Bugs are usually made public after 6 months and a couple of releases.

From the moment a security bug has been privately reported to the moment a fix is shipped and the bug is set public, all information about that bug needs to be handled carefully in order to avoid an unmitigated vulnerability becoming known and exploited before we release a fix (0-day).

During a normal process, information about the nature of bug can be accessed through:

  • Bug comments (Bugzilla, GitHub issue)

  • Commit message (visible on Bugzilla, tree check-ins and test servers)

  • Code comments

  • Test cases

  • Bug content can potentially be discussed on public IRC/Slack channels and mailing list emails.

When patching for a security bug, you’ll need to be mindful about what type of information you share and where.

In commit messages

People are watching code check-ins, so we want to avoid sharing information which would disclose or help finding a vulnerability too easily before we shipped the fix to our users. This includes:

  • The nature of the vulnerability (overflow, use-after-free, XSS, CSP bypass…)

  • Ways to trigger and exploit that vulnerability - In commit messages, code comments and test cases.

  • The fact that a bug / commit is security-related:

    • Trigger words in the commit message or code comments such as “security”, “exploitable”, or the nature of a security vulnerability (overflow, use-after-free…)

    • Security approver’s name in a commit message.

  • The Firefox versions and components affected by the vulnerability.

  • Patches with an obvious fix.

In Bugzilla and other public channels

In addition to commits, you’ll need to be mindful of not disclosing sensitive information about the bug in public places, such as Bugzilla:

  • Mention the bugs in comment of the private bug instead.

  • Do not comment sensitive information in public related bugs.

  • Also be careful about who you give bug access to: double check before CC’ing the wrong person or alias.

  • As of recently, you may now add public bugs in the “duplicate”, “depends on”, “blocks”, “regression”, “regressed by”, or “see also” section. Bugzilla will only reveal those relationships to people with editbugs permission or access to the security bug.

On IRC, Slack channels, GitHub issues, mailing lists: If you need to discuss about a security bug, use a private channel (protected with a password or with proper right access management)

During Development

Testing security bugs

Pushing to Try servers requires Level 1 Commit access but content viewing is publicly accessible.

As much as possible, do not push to Try servers. Testing should be done locally before checkin in order to prevent public disclosing of the bug.

Because of the public visibility, pushing to Try has all the same concerns as committing the patch. Please heed the concerns in the Landing your patch (with or without sec-approval) section before thinking about it, and check with the security team for an informal “sec-approval” before doing so.

Do not push the bug’s own vulnerability testcase to Try.

If you need to push to Try servers, make sure your tests don’t disclose what the vulnerability is about or how to trigger it. Do not mention anywhere it is security related.

Obfuscating a security patch

If your security patch looks obvious because of the code it contains (e.g. a one-line fix), or if you really need to push to Try servers, consider integrating your security-related patch to non-security work in the same area. And/or pretend it is related to something else, like some performance improvement or a correctness fix. Definitely don’t include the bug number in the commit message. This will help making the security issue less easily identifiable. (The absolute ban against “Security through Obscurity” is in relation to cryptographic systems. In other situations you still can’t rely on obscurity but it can sometimes buy you a little time. In this context we need to get the fixes into the hands of our users faster than attackers can weaponize and deploy attacks and a little extra time can help.)

Requesting sec-approval

See Security Bug Approval Process for more details

Landing your patch (with or without sec-approval)

Before asking for sec-approval or landing, ensure your patch does not disclose information about the security vulnerability unnecessarily. Specifically:

  1. The patch commit message and its contents should not mention security, security bugs, or sec-approvers. Note that you can alter the commit message directly in phabricator, if that’s the only thing you need to do - you don’t need to amend your local commit and re-push it. While comprehensive commit messages are generally encouraged; they should be omitted for security bugs and instead be posted in the bug (which will eventually become public.)

  2. Separate out tests into a separate commit. Do not land tests when landing the patch. Remember we don’t want to 0-day ourselves! This includes when pushing to try.

    • Tests should only be checked in later, after an official Firefox release that contains the fix has been live for at least four weeks. For example, if Firefox 53 contains a security issue that affects the world and that issue is fixed in 54, tests for this fix should not be checked in until four weeks after 54 goes live.

      The exception to this is if there is a security issue that doesn’t affect any release branches, only mozilla-central and/or other development branches. Since the security problem was never released to the world, once the bug is fixed in all affected places, tests can be checked in to the various branches.

    • There are two main techniques for remembering to check in the tests later:

    1. clone the sec bug into a separate “task” bug that is also in a security-sensitive group to ensure it’s not publicly visible called something like “land tests for bug xxxxx” and assign to yourself. It should get a “sec-other” keyword rating.

      Tip: In phabricator, you can change the bug linked to a commit with tests if the tests were already separate, while keeping the previously granted review, meaning you can just land the patch when ready, rather than having your reviewer and you have to remember what this was about a month or two down the line.

    2. Or, set the “in-testsuite” flag to “?”, and later set it to “+” when the tests get checked in.

Landing tests

Tests can be landed once the release containing fixes has been live at least 4 weeks.

The exception is if a security issue has never been shipped in a release build and has been fixed in all development branches.

Making a security bug public

This is the responsibility of the security management team.


  • Do not disclose any information about the vulnerability before a release with a fix has gone live for enough time for users to update their software.

    • This includes code comments, commit messages, tests, public communication channels.

  • If any doubt: ‘’’request sec-approval? ‘’’

  • If any doubt: needinfo security folks.

  • If there’s no rating, assume the worst and treat the bug as sec-critical.

Documentation & Contacts