What is RECAPTCHA and How it works?
                      WHAT IS 
RECAPTCH
A AND 
HOW IT 
WORKS?
What is 
reCAPTCHA?
• reCAPTCHA is a free service from Google 
that helps protect websites from spam 
and abuse. 
• A “CAPTCHA” is a turning test to tell 
human and bots apart. 
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SOURCE: GOOGLE.COM
Block automated 
software
• It is easy for humans to solve, but hard 
for “bots” and other malicious software 
to figure out. 
• By adding reCAPTCHA to a site, you can 
block automated software while helping 
your welcome users to enter with ease. 
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SOURCE: GOOGLE.COM
How does a 
CAPTCHA work?
• Classic CAPTCHAs, which are still in use 
on some web properties today, involve 
asking users to identify letters. 
• The letters are distorted so that bots are 
not likely to be able to identify them. 
• To pass the test, users must interpret the 
distorted text, type the correct letters 
into a form field, and submit the form.
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SOURCE: GOOGLE.COM
Idea behind 
CAPTCHA
• The idea is that a computer program 
such as a bot will be unable to interpret 
the distorted letters, while a human 
being, who is used to seeing and 
interpreting letters in all kinds of 
contexts different fonts, different 
handwritings, etc.  will usually be able to 
identify them.
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SOURCE: WWW.CLOUDFLARE.COM
Machine learning
• Advanced bots can use machine learning 
to identify these distorted letters, so 
these kinds of CAPTCHA tests are being 
replaced with more complex tests. 
• Google reCAPTCHA has developed 
several other tests to sort out human 
users from bots.
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SOURCE: WWW.CLOUDFLARE.COM
reCAPTCHA tests
• Google has expanded the functionality of 
reCAPTCHA tests so that they no longer 
must rely on the old style of identifying 
blurry or distorted text. Other types of 
reCAPTCHA tests include:
• Image recognition
• Checkbox
• General user behavior assessment (no 
user interaction at all)
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SOURCE: WWW.CLOUDFLARE.COM
How does an image 
recognition reCAPTCHA 
test work?
• For an image recognition reCAPTCHA 
test, typically users are presented with 9 
or 16 square images. 
• The images may all be from the same 
large image, or they may each be 
different. 
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SOURCE: WWW.CLOUDFLARE.COM
How do reCAPTCHA tests 
with a single checkbox 
work?
• Some reCAPTCHA tests simply prompt 
the user to check a box next to the 
statement, "I'm not a robot." 
• However, the test is not the actual 
action of clicking the checkbox it's 
everything leading up to the checkbox 
click.
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SOURCE: WWW.CLOUDFLARE.COM
How does reCAPTCHA work 
without any user 
interaction?
• The latest versions of reCAPTCHA are 
able to take a holistic look at a user's 
behavior and history of interacting with 
content on the Internet. 
• Most of the time, the program can 
decide based on those factors whether 
the user is a bot, without providing the 
user with a challenge to complete. 
• If not, then the user will get a typical 
reCAPTCHA challenge.
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SOURCE: WWW.CLOUDFLARE.COM
What triggers a 
CAPTCHA test?
• Some web properties just automatically 
have CAPTCHAs in place as a proactive 
defense against bots. 
• Other times, a test may be triggered if 
user behavior seems to resemble a bot's 
behavior: if users request webpages or 
click hyperlinks at a far higher rate than 
average, for instance.
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SOURCE: WWW.CLOUDFLARE.COM