yaCAPTCHA is a CAPTCHA plugin for WordPress that helps you block comment spam 
from automated bots. In order to post comments, users will have to write down 
the characters that are part of an image. Since it is relatively hard for 
automated programs to figure out those characters, this will help prevent 
comment spam from those programs.
<h4>Requirements</h4>
<ul>
<li>WordPress 1.5 or higher.</li>
<li>PHP 4.1.0 or higher with GD2 library support.</li>
<li>Theme must support the ‘comment_form’ action.</li>
<li>PHP sessions enabled and properly configured.</li>
</ul>
In case you are wondering, most hosting providers should have those basic 
requirements in place.
<h4>Strengths vs other solutions</h4>
<ul>
<li>Easy to install</li>
<li>Does not require Javascript to work</li>
<li>Broad WordPress version support</li>
</ul>
<h3>Thanks</h3>
Thanks to Kruglov Sergei for creating KCAPTCHA, his pretty good CAPTCHA. You 
can visit KCAPTCHA website <a href="http://www.captcha.ru/en/kcaptcha/" title="KCAPTCHA
website">here</a>.

yaCAPTCHA

yaCAPTCHA is a CAPTCHA plugin for WordPress that helps you block comment spam from automated bots.

Mike W

Read the <a href="http://ro.uow.edu.au/cgi/viewcontent.cgi?article=4088&context=eispapers" rel="nofollow ugc">SCIENTIFIC RESEARCH</a> by the University of Wollongong, Australia. Spoiler: the results are around page 20.
You’ve been there, you have installed 20 or more CAPTCHA plugins for WordPress to see them ultimately fail in even slowing down spam bots. We know, we have been there too. I used to run a 
a very high-profile BuddyPress community website that lost its moderators and administrators because of the web-spam issue. We tried everything, Aksmet, ReCaptcha, and nothing seemed to work. 
This is because spam bots are reading images with OCR technology. With only 3 minutes of setup on your site, their accuracy at solving your CAPTCHAs are a stunning 92%. This made me think outside the box and answer my own question: What 
can I do to make it virtually impossible for a computer to know what a human would know.
This lead to the first-ever text-based 3-D CAPTCHA and it was born right here on WordPress.org. The 
technology is so incredibly simple it is mind-blowing — render a transparent image of text and lay lines on top of it. Sounds too easy right? It is for a human. You can see this image and 
can immediately see what’s missing. A computer however would have to employ reasoning engines to determine what is missing. Our CAPTCHA has been broken by OCR bots programmed specifically 
for the easiest setting, but only at a 27% success rate. Most other plugins you will find on WordPress.org has a 70-99% success rate!
The same <a href="http://ro.uow.edu.au/cgi/viewcontent.cgi?article=4088&context=eispapers" rel="nofollow ugc">university study</a> that broke our captcha admitted, 
ramping up the settings will make it ‘ UNBREAKABLE ‘. While everyone is trying to complicate their CAPTCHAs with images, flash puzzles, and coloring it like a 2 year old with 
eight-inch markers, we decided to make an image provoke deduction skills.
This plugin no longer offers the CAPTCHA programming itself and the core image generation is hosted by <a href="http://goldsborowebdevelopment.com/" rel="nofollow ugc">Goldsboro Web Development</a> and you are granted 
FREE API access through via <a href="http://spam.goldsborowebdevelopment.com" rel="nofollow ugc">MyGWD Spam Database</a>. This means you are not required to have GD or Freetype installed. 
This was a decision made to ensure compatibility and to maintain rights to the future development of the platform. By passing the CAPTCHA code to our server, allowing us to process it and 
display the CAPTCHA in an iFrame on your site, we are able to ensure that 100% of WordPress administrators with up-to-date servers are able to use this stunning CAPTCHA. Also, we are able to grab, 
monitor, and detect when bots are attacking, find their routines and even stop them from using your precious bandwidth.
The service provided by <a href="http://goldsborowebdevelopment.com/" rel="nofollow ugc">Goldsboro Web Development</a> is free and no API key or code is ever required. At any time you can upgrade to Pro, in which you will be issued a license key. The price 
we charge for Pro is just enough to cover our basic costs, $1.99/mo (basically you’re buying us a coffee). Your price does not increase due to volume.
We didn’t aim to just band-aid the spam issue, we aspire to permanently fix it. To know what I mean, you really have to download the plugin and see for yourself all the features we added to make 
securing your site and reporting spamers as easy as possible! Here is a teaser, we automatically query the abuse email for a spamming IP and pre-populate an Abuse Report email so all you have to do is hit 
“Send”.
This plugin is revolutionary in the sense that it employs the most simple technique to solve a very complex problem. Deduction. You do it every day, and don’t even realize it. Every time 
you feel for your keys in your pocket, or swerve to avoid a pot-hole, you have employed deduction to realize something is missing. This is a human only feature that isn’t available to any other 
animal or computer. By laying lines over a transparent word, a human can easily see the word by deducing from what is missing. A computer simply looks for edges, and all they find is lines!
Super Captcha uses cloud processing by working in cooperation with your website to identify, stop, and ban spammers. The best way to flag a spammer is repeated failure of a CAPTCHA, but we understand 
humans make mistakes too, so we don’t kick off a ban from one or even 5 failures. A spam bot however will visit multiple Super Captcha enabled websites and will try and spam each one. Users that 
fail the captcha on multiple websites, are indeed banned. When your site helps us pop a spammer, 10 others do the same. Over the past year, we have identified over 11,000,000 spam bots! We have 
also determined some other interesting data, such as more than 70% of these bots are from China, 10% from Russia, and 10% from the USA.
Why are we doing this? I’ve seen some really awesome WordPress communities out there and am a WordPress fan myself, but what gets me cracking on this code in the mornings is the memory of some of these 
awesome sites closing down because a spam bot was paid to run over this site, blasting their links every where so the page owner could gain 0.0001% in page ranking, leaving the admin with hours of work 
cleaning up after the bot. Many admins just give up hope and shut the site down – I did once, but I hope this never has to happen again because of a spam bot!
This CAPTCHA is pre-programmed for the following forms:
<ul>
<li>WordPress Login</li>
<li>WordPress Registration</li>
<li>Buddypress Login</li>
<li>Buddypress Registration</li>
<li>Comment Forms</li>
<li>WordPress MU Blog Creation form</li>
<li>BuddyPress Blog Creation Form</li>
</ul>
LEGAL: <a href="https://goldsborowebdevelopment.com/privacy-policy/" rel="nofollow ugc">GWD Privacy Policy</a> | <a href="https://goldsborowebdevelopment.com/legal/terms-of-service/" rel="nofollow ugc">GWD Terms of Service</a> | <a href="https://goldsborowebdevelopment.com/legal/acceptable-use-policy/" rel="nofollow ugc">GWD Acceptable Use Policy</a>
SERVER REQUIREMENTS:
<ul>
<li>PHP v5.3.0 or greater (Highly recommended)</li>
<li>WordPress 4.0 or greater (Previous versions untested)</li>
</ul>
<h3>DEVELOPERS</h3>
Please read our forums on WordPress.org for integration development, hooks, and more. If you need help integrating this plugin into your plugin please ask on our forums.

Super CAPTCHA Security Suite – Hardened 3D CAPTCHA

Scientifically proven by research from the University of Wollongong as the most secure text-based 3D CAPTCHA available -- on the most simple settings.

yaCAPTCHA vs Super CAPTCHA Security Suite – Hardened 3D CAPTCHA