Add-Ons Outage Post-Mortem Result

As I mentioned in my previous post, we’ve been conducting a post-mortem on the add-ons outage. Sorry this took so long to get out; we’d hoped to have this out within a week, but obviously that didn’t happen. There was just a lot more digging to do than we expected. In any case, we’re now ready to share the results. This post provides a high level overview of our findings, with more detail available in Sheila Mooney’s incident report and Matt Miller & Peter Saint-Andre’s technical report.

Root Cause Analysis

The first question that everyone asks is “how did you let this happen?” At a high level, the story seems simple: we let the certificate expire. This seems like a simple failure of planning, but upon further investigation it turns out to be more complicated: the team responsible for the system which generated the signatures knew that the certificate was expiring but thought (incorrectly) that Firefox ignored the expiration dates. Part of the reason for this misunderstanding was that in a previous incident we had disabled end-entity certificate checking, and this led to confusion about the status of intermediate certificate checking. Moreover, the Firefox QA plan didn’t incorporate testing for certificate expiration (or generalized testing of how the browser will behave at future dates) and therefore the problem wasn’t detected. This seems to have been a fundamental oversight in our test plan.

The lesson here is that: (1) we need better communication and documentation of these parts of the system and (2) this information needs to get fed back into our engineering and QA work to make sure we’re not missing things. The technical report provides more details.

Code Delivery

As I mentioned previously, once we had a fix, we decided to deliver it via the Studies system (this is one part of a system we internally call “Normandy”). The Studies system isn’t an obvious choice for this kind of deployment because it was intended for deploying experiments, not code fixes. Moreover, because Studies permission is coupled to Telemetry, this meant that some users needed to enable Telemetry in order to get the fix, leading to Mozilla temporarily over-collecting data that we didn’t actually want, which we then had to clean up.

This leads to the natural question: “isn’t there some other way you could have deployed the fix?” to which the answer is “sort of.” Our other main mechanisms for deploying new code to users are dot releases and a system called “Balrog”. Unfortunately, both of these are slower than Normandy: Balrog checks for updates every 24 hours whereas Normandy checks every 6. Because we had a lot of users who were affected, getting them fixed was a very high priority, which made Studies the best technical choice.

The lesson here is that we need a mechanism that allows fast updates that isn’t coupled to Telemetry and Studies. The property we want is the ability to quickly deploy updates to any user who has automatic updates enabled. This is something our engineers are already working on.

Incomplete Fixes

Over the weeks following the incident, we released a large number of fixes, including eight versions of the system add-on and six dot releases. In some cases this was necessary because older deployment targets needed a separate fix. In other cases it was a result of defects in an earlier fix, which we then had to patch up in subsequent work. Of course, defects in software cannot be completely eliminated, but the technical report found that at least in some cases a high level of urgency combined with a lack of available QA resources (or at least coordination issues around QA) led to testing that was less thorough than we would have liked.

The lesson here is that during incidents of this kind we need to make sure that we not only recruit management, engineering, and operations personnel (which we did) but also to ensure that we have QA available to test the inevitable fixes.

Where to Learn More

If you want to learn more about our findings, I would invite you to read the more detailed reports we produced. And as always, if those don’t answer your questions, feel free to email me at

Eric is CTO of the Firefox team at Mozilla.

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