In have a web form on my website, and someone keeps sending annoying spam through it. I know Captcha can block this, but why do people spam my site - what's their benefit?
How do they access my web form? Is there any script?
And how can I identify who I'm up against?
They spam your (mine or everybody's) website because in the end it pays. Maybe because your visitors click on the URL, maybe because their own website gets a higher ranking on Google (because of many websites linking to theirs). It is not because it is your website, it is because they found a way to inject information easily (through a script or so).
Because their cost is extremely low (usually all systems they use are not owned by themselves, but compromised systems all around the world). They need only a handful of paying visitors in their own shop for maybe every 1000k+ spam messages they leave. Think about spam in global scale, not precisely your website. Yours just happens to be in their path.
For every more or less standard website (for example, based on phpBB, Joomla, Drupal, Scuttle, ...) if there are many installs on the Internet, it pays to develop a script to spam. Google will find the vulnerable sites for you. Browse through your webserver logs, and pay attention to referrers that mention Google. Study the query that got them to your website. You'll be surprised what search terms they've used to find your content.
The only reason why spam exists is because in the end it pays good money (at least for someone).
I think jippie's answer of why spammers spam web forms could use a supplement. (And btw, if I'm not correct about this, someone please set me straight.)
With most website forms, the submitted information gets sent to the administrator, and that's that. However, some form submissions (such as comments, like what we're doing here), end up getting posted automatically to the website (at least, that is, if they're unmoderated comments). I guess that's where the gold is for the spammers, allowing them to post backlinks to their websites. Unfortunately, their spam scripts don't go to the trouble of knowing which forms are worth spamming, and which aren't.
That said, I wonder if someone can explain to me why my web form is getting spammed right now, yet the only links included in the submissions are email links?
In short: try to analyze traffic. Who is doing that? If it just one IP address - add this IP address to a black list and rest in peace. reCAPTCHA is your friend.
Spammers spam because they are paid to do so and spamming is a remunerative exercise.
Your web form must be publicly available so there is no problem in finding the same either manually or through a script. However, if you have not ensured sufficient spam protection mechanisms, your website is also a soft target.
You can identify the spammer by going through the logs that can reveal the IP address and other crucial info. Just block the IP addresses and introduce moderation of comments with Captcha on.
It seems you have done neither so far.
To help identify where the traffic originated from, look through your log files and look up those IP addresses on http://www.geobytes.com/iplocator.htm
This will tell you the origin of the attacks so tha tyou can see if it's from one attacker in Ukraine, or fifty attackers from all over the world.
There are several easy ways to protect against this.
whatever system that you are using make sure that all updates and patches are installed ASAP since that's probably how they found you and got in.
If your firewall supports GEO blocking and you only want traffic from the United States you could block the rest of the world, or just block the countries that are giving you problems.
ALWAYS require CAPTCHA since the spamming industry pays BIG. Check out the interview on the show screensavers with egod http://youtu.be/cDlmtGiE--w spammer nets $1,000 a day. Unfornately it's a business model just like telemarketing.
