Tip: Fight against SPAM in comments

Sunday, 29 October 2023
Écrit par
Grégory Soutadé

What a surprise when you manage a server and see in the morning your mailbox full of mails telling there is an issue ! It starts like a bad day... First thing : connect to the server and blacklist the attacker IP. This one was not from China or Russia, but from Germany ! It tries to do code and SQL injection on all web pages, with a delay between each bunch of requests to remain undercover. Fortunately, all dynamic web pages in my server are behind a login form. Public part use only statically generated HTML pages. This is done by my static blog generator Dynastie. It's a 10 years old project written in Python/Django. If I have to write it again, I would use Python templates and not XML, but this one has been especially written for my needs and perfectly fit it. So, even if the IHM is very basic, the rendering is aux petits oignons.

One of great feature (not available in other static generators), except dynamic post management, is dynamic comment support. Unfortunately, a website offering public comments without registration is a target for spammers. My automatic comment filtering works well since 2014, but has been bypassed this week. Here is how I fixed it.

As robot doesn't load CSS and JavaScript resources, we can play with hidden fields in the comment's form and do checks on webserver side. So, I added an hidden field which is filled by Javascript when user press on "Comment" button. Value set is a timestamp + a magic number that is then checked by server. So, if the spammer doesn't run Javascript, it'll be blocked ! For sure, this trick is very easy to break and a spammer can easily bypass it with a smart/targeted robot or by doing manual SPAM. In this case, the only solutions is a complex captcha/registration and/or manual comment validation. But it requires more complex modules and work from both parts (user and webmaster), which is overkill for small a website.

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