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Attacks, Vulnerabilities

What is CAPTCHA and How does it work?

It is a CAPTCHA that decides whether a client who is trying to gain access to a service or data is really a bot. While these tests can assist with halting vindictive bot action, they are a long way from secure.

What is CAPTCHA and How does it work?

An overview of CAPTCHA

A CAPTCHA test is used to identify whether an internet user is a human or a bot. CAPTCHA is a full form for "Completely Automated Public Turing Test to Distinguish Robots from People." On the Internet, CAPTCHA and reCAPTCHA tests are frequently encountered. Such experiments are one way to monitor bot migration, however the approach has certain drawbacks.

Despite the fact that CAPTCHAs are intended to impede mechanized bots, CAPTCHAs are themselves robotized. They're customized to spring up in specific puts on a site, and they consequently pass or bomb clients


History of CAPTCHA

Let’s find out when, how, and how CAPTCHA came into being. To surprise you, it dates back to the time when the Internet was evolving, and search engines were not that advanced at that point in time. 

Around 1997, AltaVista (a primitive search engine of that decade) was having a tough time fixing the high number of auto URL assets that were hampering its website ranking process severely.

To solve this issue, the then chief scientist of AltaVista, Andrei Broder, created an algorithm that later became famous as CAPTCHA. The algorithm was capable of introducing a series of auto-generated and random images with printed text just before the website. 

The algorithm was based on the hypothesis that if these surge URLs are created by a bot or computer, CAPTCHA can stop them from visiting the website as they won’t be able to identify the images. As humans can easily identify the image, they won’t have any issues accessing the website.

Broder and his team worked extensively on the algorithm and got the patent in 2001. Despite the commendable efforts of Broder, the algorithm still had the scope for improvement, which was seized by a team of scientists in 2003.

The team included Nicholas J. Hopper, Luis von Ahn, J. Langford, and M. Blum. These people were the top talents at popular organizations like IBM and Carnegie Mellon University.

The team officially named the algorithm as CAPTCHA, which denotes Completely Automated Public Turing Test. Its core aim was to separate computers and humans without any human intervention.

The core of this algorithm is the use of AI to figure out to which extent a computer can match human intelligence. The Turing test, a critical aspect of CAPTCHA, was formed by a notorious scientist, Alan Turing. 

In 2016, Jason Polakis, a professor specializing at Computer Science, worked on the difficulty level of CAPTCHA and figured out that the algorithm is so complex that even humans can fail to recognize the images. That makes it tougher for bots as well as and computers. Hence, it’s considered a viable tool to control undesirable website access.

How does CAPTCHA work?

Customers should separate letters in exemplary CAPTCHAs, which are as yet being used on certain online areas today. The letters are misshaped with the target that bots are not committed to have the choice to recollect them. To coast through the evaluation, clients need to interpret the twisted substance, type the right letters into a plan field, and present the development. On the off chance that the letters don't work with, clients are actuated to attempt once more. Such tests are standard in login structures, account information exchange structures, online audits, and web business checkout pages.

The thought is that a PC program, for example, a bot will be not prepared to decipher the twisted letters, while an individual, who knows about seeing and interpreting letters in a wide extent of settings – distinctive printed styles, diverse penmanship styles, and so forth – can all around remember them.

The best that different bots will truly have to do is input some irregular letters, making it really dubious that they will finish the assessment. In this way, bots bomb the test and are hindered from partner with the site or application, while people can keep on utilizing it like common.

Progressed bots can utilize AI to see these harmed letters, so such CAPTCHA tests are being supplanted with more inconsistent tests. Google reCAPTCHA has urged distinctive different tests to figure out human clients from bots.

Since the introduction of CAPTCHA, AI-based bots have been developed. Customary CAPTCHAs with tests composed in course of action accreditation are more interesting to these bots. Considering this new turn of events, fresher CAPTCHA methods depend on extra shocking tests. For portrayal, reCAPTCHA demands you to tap on a particular spot and deferral until the look at is finished.

What are CAPTCHAs used for?

At the point when online applications request client input, CAPTCHAs are usually utilized. Accept that you're maintaining an online business and you need to give your clients the choice of leaving item surveys in a remarks area. For now, you should guarantee that the entries are genuinely from your customers or, possibly, from human site guests. You'll spend a generous segment of your time going over frequently delivered spam comments – and in the most dire outcome imaginable, you'll team up with your enemy.

You may decrease the danger of this occurrence by consolidating a CAPTCHA into your site, which expects clients to demonstrate that they are human prior to presenting a remark. Manual human tests may now be found in pretty much every region where human customers ought to be recognized from bots. In contrast with online charts or web affiliations, for example, web crawler affiliations, this fortifies choice systems for email affiliations, warning, affiliations, and social affiliations.

Advantages and Disadvantages

Using CAPTCHA is preferred because: 

  • It’s highly worthwhile to discourage spam and unwanted access 
  • It can stop fake and ill-intended website access or usage 
  • CAPTCHAs are an easy way to improve the website’s accessibility
  • For human minds, making sense of CAPTCHA is easy. 

While these advantages of CAPTCHAs are impressive, it has significant disadvantages that can’t be overlooked. Have a look at the key disadvantages of CAPTCHA. 

  • Don’t consider CAPTCHA as a foolproof tool as it’s not. It’s most effective for spam. The rest of its applications are flawed and highly restricted. 
  • Solving CAPTCHA needs time and effort, which is not feasible for every website user. Some might even drop the idea of visiting a website if it uses CAPTCHA. Hence, it might lead to a certain drop in website traffic. If the sole purpose of having a website is to generate leads and traffic, CAPTCHA shouldn’t be used. 
  • CAPTCHAs are not suitable for visually impaired users. So, you won’t be able to target such users if you use CAPTCHA.

Examples of type CAPTCHA

Text-based, picture-based, and sound-based CAPTCHAs are the three types of CAPTCHAs available today.


The most standard kind of check is text CAPTCHAs. These CAPTCHAs can fuse prominent articulations or explanations, similarly as uncommon digits and letters blends. Some substance-based CAPTCHAs break down different kinds of capitalization.

These characters are shown in an odd style by the CAPTCHA, requiring translation. Characters that are scaling, incensed, or turned would all have the option to be coordinated with malevolence. It may moreover join suitable segments like tone, foundation wobbliness, lines, winds, or spots just as covering characters. Despite the way that it may be difficult to understand for individuals, this opening plans for bots doing lacking substance affirmation computations.

Strategies for making text-based CAPTCHAs include:

  • Gimpy

The gimpy selects a handful of emotionally charged words from a rundown of 850 words and conveys them in an unusual manner.

Gimpy captcha
  • EZ-Gimpy

It's an assortment of Gimpy that just utilizes a single word.

ez-Gimpy captcha
  • Gimpy-r

This picks reassuring letters, then bends and embellishes them with foundation discontent.

Gimpy-r captcha
  • Simard's HIP

This method picks alphabets and numerals at random and then alters them with curves and shadings.

Simard's HIP captcha


Manual human tests utilizing pictures are developed on in a split second clear graphical components instead of a vexing strategy including digits and letters. At last, a few photographs of ordinary things are compared. The customer should feature which photographs have all the earmarks of being the most significant or show which ones tackle a semantic issue.

Google, then again, utilizes Google Street View CAPTCHAs that expect clients to enter a street address or a road sign into the material box.

Most clients can address an image based CAPTCHA very quickly. Regardless, a PC program's capacity to acquire an addressed picture, then, at that point request it's anything but, and afterward work out near one is restricted partly. Thusly, picture-based CAPTCHAs give preferred security over text-based cycles.



Manual human tests are a sort of development that permits individuals to get to obstructed sites. These CAPTCHAs are as often as possible utilized related to message based and picture based CAPTCHAs. Customers ought to expect a progression of moving characters or numbers in a decent CAPTCHA.

Bots can't separate crucial characters from establishment shock in these CAPTCHAs. Concerning bots, these mechanical gatherings, like substance-based CAPTCHAs, can be difficult for individuals to fathom.


Math or verbal problems

A CAPTCHA framework that also satisfies the needs of the purportedly weakened utilizations science concerns or problems is used to bypass spambots. When necessary, a screen reader may be used to examine an assignment like the one below, implying that it can also be used by clients with non-visual yield contraptions.

These mathematical aspects aren't too complex to interpret, but the problem is that they don't solve a really progressive balance for PCs, which are designed to coordinate numbers. This form of CAPTCHA is frequently coupled with various types of text scorn, making it nearly impossible to interpret for screen viewers. On the off chance that the outcome is a word as opposed to a number, or if a solitary digit of the outcome should be contribution because of some lucky new turn of events (for instance, discover 7 x 7 and just enter the principal digit of the outcome in the compartment), it is intrinsically harder for applications. The CAPTCHA game methodology would be 4) if the outcome was 49.

CAPTCHAs are also used in the same way as enrollment tries are. They incorporate exercises errands and general information requests. Frequently, and with a clear connection to the specific site. Before moving on to the next level of a conversation regarding SMF (Simple Machines Forum) programming, the visitor must respond to two tasks regarding the topic.

Math or verbal problems

What is reCAPTCHA?

As an option to conventional CAPTCHAs, reCAPTCHA is a free instrument that assists with Google offers. Shortly after its inception, Google purchased reCAPTCHA from some scientists at Carnegie Mellon University in 2009.

reCAPTCHA is a more advanced version of the standard CAPTCHA tests. Some reCAPTCHAs, like CAPTCHA, require consumers to submit images of text that PCs have difficulty interpreting. Unlike traditional CAPTCHAs, reCAPTCHA gets its content from real images: photographs of street addresses, text from printed books, text from historical newspapers, and so on.


After some time, Google has improved the us

ability of reCAPTCHA tests so that they no longer need to rely on the previous approach for seeing hazy or destroyed content. Various reCAPTCHA tests are used to combine information:

  • Picture acknowledgment
  • Checkbox
  • General client conduct evaluation (no client association by any means)

What are the disadvantages of using a CAPTCHA?

  • Awful customer experience

A CAPTCHA test can encroach upon the movement of what customers are endeavoring to do, giving them a negative point of view on their experience on the web property, and provoking them giving up the webpage page all around sometimes.

  • Not usable for obviously blocked individuals

The issue with CAPTCHAs is that they rely upon visual insight. This makes them practically incomprehensible, for people who are really outwardly debilitated, yet for anyone with truly hindered vision.

  • These tests can be deceived by bots

As portrayed above, CAPTCHAs are not totally bot-proof and shouldn't be relied on for bot the chiefs.

Can CAPTCHAs stop bots?

The quantity of connection required for the site is considerably decreased when a CAPTCHA is employed to keep automated spams out while enabling people to pass. Administrators of sites with original material won't have to check submissions on a frequent basis.

Diverse CAPTCHA suppliers are attempting to compensate for AI degrees of progress by making the tests verifiably more severely orchestrated. Manual human tests finally become unsolvable, paying little notice to how long it requires.

How attackers defeat CAPTCHAs

The existing breed of cybercriminals and hackers is so skilled that they will have no hard time bypassing a CAPTCHA. As long as techniques used to bypass CAPTCHA are concerned, most hackers take the help of ML to defeat CAPTCHAs. The algorithm can predict the CAPTCHA pattern and empower a bot to bypass CAPTCHA.

Other than this, the deep learning model is another viable weapon for hackers. The model involves pre-installing a huge number of CAPTCHAs and solving it beforehand. This is done to predict the pattern and learn ways to decode them.

Bypassing CAPTCHA

The above CAPTCHA bypassing techniques are advanced and suitable for someone having high technical competency. What about general users? How can they bypass CAPTCHA?

Well, it’s possible with the help of many solutions like using a browser add-on that installs in a blink of an eye and automatically starts blocking CAPTCHAs as soon as the end-user accesses a website. Consider-worthy add-ons are Rumola and AntiCaptcha. 

We tested AntiCaptcha and figured out that it proffers a high-end CAPTCHA solver for Firefox and Chrome browsers. When activated, the solver will automatically spot a CAPTCHA and solve it, so that website visitors don’t have to do that. What’s worth noting here is that it can help visually impaired people as well.

Rumola works on Safari, Chrome, and Firefox with the same ease and perfection. Both the browser extension and bookmarklet tools are offered. However, one must understand that using 3rd party tools can put data at risk. Add-on programs and the plugin can log your details and expose them to others. So, it’s highly recommended to refer to the logging policy of the tool you’re going to use, adopt adequate security measures, and keep the data password protected.

Turing test and CAPTCHA

A Turing test assesses a PC's capacity to imitate human discourse. In 1950, Alan Turing, a pioneer in the control, proposed the Turing test. The Turing test is "passed" if a PC program's exhibition is unclear from that of a human all through the test - on the off chance that it's anything but a human would. A Turing test isn't tied in with tracking down the right courses of action; it's about how "human" the suitable reactions appear, whether or not they're right or mistaken.

A CAPTCHA isn't a Turing test, despite the fact that it isn't a "Open Turing test" - it isn't at the stage where anybody can tell if a human client is actually a computer program (a bot) or not, rather than attempting to determine whether a computer is human. To do this, a CAPTCHA must present a simple job that people can perform while PCs struggle. Seeing text and images follows these rules for the most part.

How to Keep CAPTCHA Codes Secure?

If, by all means, you wish to employ CAPTCHA to secure a website, you need to learn ways to keep the CAPTCHA code secured. As long as CAPTCHA is protected, the website is protected. We present you with the most common and viable ways to secure CAPTCHA codes.

  • Protect the image by doing random distortion. Make sure the image should be slightly distorted before it’s presented to an end-user. This way, it gains immunity against automated attacks.
  • Use matchless CAPTCHAs so that hackers can’t predict the algorithm you’re using or the pattern you’re following in CAPTCHA creation. Also, make sure that you don’t use the same sort of CAPTCHA for all the websites you own. This increases the predictability that works in the favor of hackers. Deploy an algorithm that can easily change the CAPTCHA, create unique codes, and never follow the same pattern.
  • Never use common expressions or equations, as hackers won’t have a tough time bypassing them. 1+1, 1*1, or 1-1 are not the kind of CAPTCHA that is considered safe and sound. Having such a CAPTCHA is equivalent to having no CAPTCHA.
  • Pay attention to the script security as it’s important to make sure that images are non-readable for computers and bots. Some of the best practices to make it include CAPTCHA answer transfer in the form of plain text only and not using open-source CAPTCHA scripts.
  • Combine multiple techniques for CAPTCHA. Don’t always follow the pattern of reading text or image selection. Try to combine two techniques like text reading and audio recording or image selection and audio description. This makes CAPTCHA complex and secure.
  • Using antivirus software: It is a good move to secure CAPTCHA codes as the tool will keep malicious content at bay.


What is the Turing test?
What are the different types of CAPTCHA?
Why is CAPTCHA used?
What is CAPTCHA?
How are CAPTCHA and reCAPTCHA related to artificial intelligence (AI) projects?
How are Turing tests relevant to CAPTCHA tests?

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February 26, 2024
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