Wednesday, November 08, 2006


"CAPTCHA" stands for ""Completely Automated Public Turing test to tell Computers and Humans Apart". You've likely seen them; web sites use CAPTCHAs to verify that a visitor is human (rather than a computer).

Here is an example of a CAPTCHA:

CAPTCHAs are an attempt to prevent computer-generated spam and other hijinx. Unfortunately, they cause several problems of their own:

  1. They decrease usability by adding to the amount of time and effort it takes for users to accomplish their goals.
  2. Some are difficult for humans to read.
  3. Some computer programs are better able to read them than humans.
Avoid CAPTCHAs if you can find an alternate way of preventing spam.


Chip said...

There was a guy at the first BarCamp Austin that was talking about one technique spammers use to get around captchas. Imagine a spam farm that has lots of open http sessions with sites that have captcha defenses. The farm is also a web site that advertise links to free porn. When people come to get the porn, the spam webserver shows them one of the captcha images and says "just tell me what it says to get your free porn." the spam-server takes the input and sends it on to the target site. ba-dow! more spam misery cuz you can't stop searching for free pictures of screech and mr sanchez.

Roger L. Cauvin said...

Very clever.