[follow up] Hardening Zinc’s Core HTTP Server

Here is yet another update:

The instances

– On Amazon AWS:

– On Microsoft Azure:

have now been running for 18+ days.

Having observed the overall amounts of critical resources inside the Pharo images, things look good.

Seaside WASSession cleanup goes slowly but does work over time.

Eventually I will stop at least the Azure instance because it costs me personal money.

Here is an update on the status of both instances.

A couple of people tried fiddling with invalid input which did result in exceptions, but everything was handled correctly so that the server kept on functioning.

There were no spurious crashes.

There was one attack by Levente Uzonyi that was successful though. 

He used the fact that Seaside sessions are long lived (cached for a certain time) combined with the fact that the Reddit.st app kept one GLORP database connection through P3 open to PostgreSQL and fired off a small denial of service (DOS) attack. Eventually either the VM ran out of space in the ExternalSemaphoreTable, making Socket creation, either for database connections or for handling new HTTP request impossible or the database’s own connection limit was reached. At that point the HTTP server or the Pharo VM stopped functioning.

Although Levente used a sequential number of requests, a concurrent number of request could cause similar problems (related, but differently).

In looking at what he reported it also became clear that I accidentally left a debugging exception handler active, which is not good in a production image.

Both instances are now redeployed with the following changes:

– the Reddit.st code was changed to not keep the database connection open all the time, but instead connect/disconnect per request. this might be a bit slower, but it conserves resources much better and solves the original issue Levente reported


– a new feature was added to Zinc HTTP Components’ ZnManagingMultiThreadedServer (the default) to enforce a maximum number of concurrent connections being allowed (the limit is 32 by default, but changeable if you know what you are doing). when the limit is reached, 503 Service Unavailable responses are sent to the excess clients and the connection is closed. this should help protect against concurrent connection DOS attacks


– the deploy script was changed to use the more primitive WAErrorHandler


Thanks again to Levente for taking the time to try an attack and for reporting it clearly.


First announce


Zinc HTTP Components [https://github.com/svenvc/zinc] has been a part of Pharo since version 1.3 (2011). It is an open-source framework to deal with the HTTP networking protocol, modelling all aspects involved. It also offers both client and server functionality.

The reliability of the code base has improved steadily over the years, thanks to virtually all Pharo developers using it, directly or indirectly. Over the summer a number of issues that popped up after Pharo 9 was released were resolved.

The robustness of the core HTTP server is one important aspect. To put this quality further to the test, I deployed two servers with the same demo Seaside application, Reddit.st, open to the internet, without any further protections.

– On Amazon AWS:

– On Microsoft Azure:

The application’s source code can be found at [https://github.com/svenvc/Reddit]. For the technically curious there are also deploy instructions at [https://github.com/svenvc/Reddit/blob/main/DEPLOY.md]. The demo app itself is described in an older article [https://medium.com/@svenvc/reddit-st-in-10-cool-pharo-classes-1b5327ca0740]. Note that, by definition, there is no HTTPS/TLS variant.

If you manage to break this server with (a) malicious request(s) in such a way that you can explain what you did for others to confirm your approach, you not only help me/us improve the code, but earn eternal fame as well 😉


PS: I hope I won’t regret this, I am looking for constructive criticism.

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