Seeing this right after finishing a webhook integration.
Collection of best practices for providing and consuming webhooks
31–40 of 42 posts
Re: Collection of best practices for providing and consuming webhooks
#32There are some interesting attack vectors to be aware of if you run a service where users can define webhooks, and your service will will call the user-defined webhooks to notify about certain system events. In my case, a monitoring service which can send notifications by calling user-defined webhook. * Timeouts: the user can set up a webhook receiver that takes very long to generate a response. Your service must be…
>Domains that resolve to private IPs: attacker could set up foo.com which resolves to a private IP
There's a clever extension to this attack; a naive way to mitigate it is to do a DNS resolution first to verify it's not a private IP and then do the actual request. An attacker can simply return a public IP on the first DNS resolution (with a 0 TTY) and then return a private IP on the second. This is called a "TOCTOU" (time-of-check time-of-use) vulnerability. I've written about this and other security best practices on my blog here - https://www.ameyalokare.com/technology/webhooks/2021/05/03/s...
I've also built an egress proxy that prevents such attacks here - https://github.com/juggernaut/webhook-sentry
Same caveat applies, use at your own risk :-)
Re: Collection of best practices for providing and consuming webhooks
#33If only there was a single standard for webhook subscription verification. Cloud services invent their own authentication protocols. Webhook verification in Dropbox is different from that in Trello. The lack of a single standard makes it hard to design a universal incoming webhook service (we faced this problem).
I love the idea of a standard but it gets colorful as you start digging in what the individual leaders are doing.
On the signature front, there's this but it's not enough: https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-mes...
Re: Collection of best practices for providing and consuming webhooks
#34There are some interesting attack vectors to be aware of if you run a service where users can define webhooks, and your service will will call the user-defined webhooks to notify about certain system events. In my case, a monitoring service which can send notifications by calling user-defined webhook. * Timeouts: the user can set up a webhook receiver that takes very long to generate a response. Your service must be…
Those are all great points! At Svix (we do webhooks as a service), we disallow redirects because of what you mentioned above, and because it's also just bad for performance (for both sides) and is most likely a configuration error anyway. Resolution and timeouts: the aiohttp library for Python is slightly better in terms of letting you configure these things, though it's better to just use a sending proxy that does i…
Disallowing redirects altogether is probably too big of a hammer. There are legit reasons to use redirects (like migration to new versions). A limit to the number of redirects seems ideal -- that's what Twilio does, for example.
Re: Collection of best practices for providing and consuming webhooks
#35There are some interesting attack vectors to be aware of if you run a service where users can define webhooks, and your service will will call the user-defined webhooks to notify about certain system events. In my case, a monitoring service which can send notifications by calling user-defined webhook. * Timeouts: the user can set up a webhook receiver that takes very long to generate a response. Your service must be…
>Domains that resolve to private IPs: attacker could set up foo.com which resolves to a private IP There's a clever extension to this attack; a naive way to mitigate it is to do a DNS resolution first to verify it's not a private IP and then do the actual request. An attacker can simply return a public IP on the first DNS resolution (with a 0 TTY) and then return a private IP on the second. This is called a "TOCTOU"…
Yeah, resolving twice is a really bad idea. A good rule of thumb for security: if you think you have a clever hack (e.g. checking DNS twice as a workaround to not being able to patch DNS resolution), it probably isn't so clever as you think.
Re: Collection of best practices for providing and consuming webhooks
#36Earlier quoted context omitted.
Those are all great points! At Svix (we do webhooks as a service), we disallow redirects because of what you mentioned above, and because it's also just bad for performance (for both sides) and is most likely a configuration error anyway. Resolution and timeouts: the aiohttp library for Python is slightly better in terms of letting you configure these things, though it's better to just use a sending proxy that does i…
Disallowing redirects altogether is probably too big of a hammer. There are legit reasons to use redirects (like migration to new versions). A limit to the number of redirects seems ideal -- that's what Twilio does, for example.
Migrating to a new service: you can just update the webhook URL. On the other hand, there are at least a couple of problems with allowing even one redirect: it opens a pandora's box of security implications, and it's a performance penalty that is paid both by the sender and the receiver on every webhook sent. Realistically, 3xx are most likely to be a mis-configurations (e.g. including a trailing slash where one shouldn't) so I think being noisy about it is a great idea.
Not saying that there aren't valid use-cases (e.g. maybe some sort of dynamic webhook receiving), I'm just saying that we made this choice given the above, and we would be willing to change it if it's ever a barrier for someone.
Re: Collection of best practices for providing and consuming webhooks
#37There are some interesting attack vectors to be aware of if you run a service where users can define webhooks, and your service will will call the user-defined webhooks to notify about certain system events. In my case, a monitoring service which can send notifications by calling user-defined webhook. * Timeouts: the user can set up a webhook receiver that takes very long to generate a response. Your service must be…
Great list. Closing layer 3* access (inbound firewall rule of deny-all) helps mitigate against the common webhook attack vectors as well (should still do all the L7 mitigation and best practices alluded to above for a multi-layered approach).
* examples:
https://netfoundry.io/zero-trust-webhook-security/ (blog)
https://youtu.be/TWohosldLP4 (video specifically for a Lambda function)
Re: Collection of best practices for providing and consuming webhooks
#38Earlier quoted context omitted.
Disallowing redirects altogether is probably too big of a hammer. There are legit reasons to use redirects (like migration to new versions). A limit to the number of redirects seems ideal -- that's what Twilio does, for example.
Migrating to a new service: you can just update the webhook URL. On the other hand, there are at least a couple of problems with allowing even one redirect: it opens a pandora's box of security implications, and it's a performance penalty that is paid both by the sender and the receiver on every webhook sent. Realistically, 3xx are most likely to be a mis-configurations (e.g. including a trailing slash where one shou…
>you can just update the webhook URL
Fair enough, but often consumers that use multiple vendors receive webhooks in the same service; think about going from /v1/webhooks -> /v2/webhooks, they'd have to change the URL for every vendor. Easier to redirect first then update the URLs later. I think it's a reasonable expectation that a HTTP client would honor redirects as long as the usage isn't malicious (like loops etc)
Re: Collection of best practices for providing and consuming webhooks
#39As this page makes very clear, it's actually pretty hard to make a robust webhooks implementation! What alternatives are there? I've looked at: * Publishing AWS EventBridge events to other accounts. * /events instead of webhooks https://blog.sequin.io/events-not-webhooks/ * ???
> What alternatives are there?
At the top of my head: Short Polling, long polling, websockets, response streaming, SSE, SNS, paging API, comet...
Re: Collection of best practices for providing and consuming webhooks
#40Earlier quoted context omitted.
It seems like webhooks have enough corner cases for the sender to require a specialized tool to protect itself from malicious users and to stay performant. Does anyone have suggestion for such tools/services that they might have used in production?
Outgoing webhooks dispatched from a Lambda seemed to solve most of the above problems for us
Using lambda doesn't protect you against any of these.
Timeout attacks: instead of failing you'll now be just be paying through the nose. And also with enough scale you can hit lambda limits and fail.
SSRF: you are still as vulnerable unless the lambda is outside your VPC (but then it's the VPC that solved it, not the lambda).