This text is the primary in a collection of posts I am writing about operating varied SaaS merchandise and web sites for the final 8 years. I will be sharing a few of the points I’ve handled, classes I’ve realized, errors I’ve made, and possibly a couple of issues that went proper. Let me know what you assume!
Again in 2019 or 2020, I had determined to rewrite the complete backend for Block Sender, a SaaS software that helps customers create higher electronic mail blocks, amongst different options. Within the course of, I added a couple of new options and upgraded to way more trendy applied sciences. I ran the assessments, deployed the code, manually examined the whole lot in manufacturing, and aside from a couple of random odds and ends, the whole lot appeared to be working nice. I want this was the top of the story, however…
A number of weeks later, I used to be notified by a buyer (which is embarrassing in itself) that the service wasn’t working they usually had been getting a lot of should-be-blocked emails of their inbox, so I investigated. Many instances this difficulty is because of Google eradicating the connection from our service to the person’s account, which the system handles by notifying the person through electronic mail and asking them to reconnect, however this time it was one thing else.
It seemed just like the backend employee that handles checking emails towards person blocks saved crashing each 5-10 minutes. The weirdest half – there have been no errors within the logs, reminiscence was advantageous, however the CPU would sometimes spike at seemingly random instances. So for the following 24 hours (with a 3-hour break to sleep – sorry clients 😬), I needed to manually restart the employee each time it crashed. For some cause, the Elastic Beanstalk service was ready far too lengthy to restart, which is why I needed to do it manually.
Debugging points in manufacturing is all the time a ache, particularly since I could not reproduce the problem domestically, not to mention determine what was making it. So like several “good” developer, I simply began logging the whole lot and waited for the server to crash once more. Because the CPU was spiking periodically, I figured it wasn’t a macro difficulty (like once you run out of reminiscence) and was in all probability being brought on by a selected electronic mail or person. So I attempted to slender it down:
- Was it crashing on a sure electronic mail ID or kind?
- Was it crashing for a given buyer?
- Was it crashing at some common interval?
After hours of this, and observing logs longer than I might care to, finally, I did slender it all the way down to a selected buyer. From there, the search area narrowed fairly a bit – it was almost definitely a blocking rule or a selected electronic mail our server saved retrying on. Fortunately for me, it was the previous, which is a far simpler downside to debug on condition that we’re a really privacy-focused firm and do not retailer or view any electronic mail knowledge.
Earlier than we get into the precise downside, let’s first speak about one in every of Block Sender’s options. On the time I had many shoppers asking for wildcard blocking, which might enable them to dam sure varieties of electronic mail addresses that adopted the identical sample. For instance, in the event you needed to dam all emails from advertising and marketing electronic mail addresses, you may use the wildcard advertising and marketing@* and it could block all emails from any handle that began with advertising and marketing@.
One factor I did not take into consideration is that not everybody understands how wildcards work. I assumed that most individuals would use them in the identical means I do as a developer, utilizing one * to signify any variety of characters. Sadly, this specific person had assumed you wanted to make use of one wildcard for every character you needed to match. Of their case, they needed to dam all emails from a sure area (which is a local function Block Sender has, however they have to not have realized it, which is a complete downside in itself). So as an alternative of utilizing *@instance.com, they used **********@instance.com.
POV: Watching your customers use your app…
To deal with wildcards on our employee server, we’re utilizing the Node.js library matcher, which helps with glob matching by turning it into an everyday expression. This library would then flip **********@instance.com into one thing like the next regex:
/[sS]*[sS]*[sS]*[sS]*[sS]*[sS]*[sS]*[sS]*[sS]*[sS]*@instance.com/i
You probably have any expertise with regex, you realize that they’ll get very sophisticated in a short time, particularly on a computational stage. Matching the above expression to any affordable size of textual content turns into very computationally costly, which ended up tying up the CPU on our employee server. Because of this the server would crash each couple of minutes; it could get caught making an attempt to match a posh common expression to an electronic mail handle. So each time this person obtained an electronic mail, along with the entire retries we in-built to deal with momentary failures, it could crash our server.
So how did I repair this? Clearly, the fast repair was to seek out all blocks with a number of wildcards in succession and proper them. However I additionally wanted to do a greater job of sanitizing person enter. Any person may enter a regex and take down the complete system with a ReDoS assault.
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Dealing with this specific case was pretty easy – take away successive wildcard characters:
block = block.exchange(/*+/g, '*')
However that also leaves the app open to different varieties of ReDoS assaults. Fortunately there are a selection of packages/libraries to assist us with these varieties as effectively:
Utilizing a mix of the options above, and different safeguards, I have been capable of forestall this from occurring once more. However it was an excellent reminder you could by no means belief person enter, and you need to all the time sanitize it earlier than utilizing it in your software. I wasn’t even conscious this was a possible difficulty till it occurred to me, so hopefully, this helps another person keep away from the identical downside.
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