Have you ever wondered why some websites ask you to tick a checkbox to confirm you’re not a bot? Or why sometimes CAPTCHA codes appear as images you’re asked to enter? We have answers to all these questions and more in this article, where we’ll explain what CAPTCHA means, how the security checks work, and tell you about the different types of CAPTCHA. You’ll also discover how fake CAPTCHA attacks work and how you can protect yourself from them with Avira Free Security.
What is a CAPTCHA and how does it work?
CAPTCHA stands for Completely Automated Public Turing Test to tell Computers and Humans Apart. In plain English, a CAPTCHA verifies whether you’re a real person or an automated computer program (a bot) that might have bad intentions — such as to spread spam or carry out a cyberattack.
There are also different types of CAPTCHA test. Whereas in some cases you only need to tick a checkbox, in others you may need to solve a simple challenge. Here, the security mechanism verifies if the user is a human through a challenge-response test where they’re asked to solve a challenge and send back the response.
The challenges are generated automatically using a random number generator in accordance with predefined rules. The response is then subjected to a validation process in which algorithms are used to analyze the response and determine the probability that it was generated by a human. If the challenge cannot be solved, the corresponding action is usually blocked.
CAPTCHAs are designed to be relatively easy for humans to solve, but difficult for bots. However, this is changing primarily through the use of artificial intelligence (AI) and machine learning, meaning many challenges no longer pose a major barrier for bots. In addition to visible CAPTCHAs, there are also CAPTCHAs that work invisibly in the background — something we’ll explore more in the section on the different types of CAPTCHA.
What are CAPTCHAs used for?
CAPTCHA verification is used on websites to prevent bots from interfering with or misusing website functionality.
CAPTCHAs are primarily intended to:
- Prevent the spread of spam (such as in reviews, comments and posts, or via contact forms)
- Prevent automated account creation
- Prevent multiple voting, manipulation of online surveys, and competition fraud
- Prevent automated threats (like brute-force attacks, dictionary attacks, account takeover attacks, and credential stuffing)
- Prevent ticket fraud through mass purchases by black marketeers
- Prevent web scraping (automated extraction of website content)
What triggers a CAPTCHA test?
Whereas older, simpler CAPTCHA systems display tests by default during security processes (such as when you register, submit a form, log in, or reset your password), the latest CAPTCHA systems work invisibly in the background, using various mechanisms to detect automated access.
A visible CAPTCHA is only displayed if the system deems an additional security check necessary.
Risk-based CAPTCHA systems start with a risk assessment based on various technical and behavioral parameters. If the calculated risk score exceeds a defined threshold, this can trigger a security check.
The following events can trigger a CAPTCHA check:
- A suspicious IP address
- Unusual ways of interacting with the website (such as unnaturally fast keyboard inputs, lack of mouse movement, frequent webpage refreshes, or repeated login attempts)
- Use of a VPN, proxy server, or the Tor Browser
- Unusual device configuration and browser fingerprint
- Missing information to identify you, your device, or session (such as blocked cookies)
- JavaScript turned off
Types of CAPTCHA and examples
As mentioned , there are both simple and modern types of CAPTCHA. Traditional versions such as distorted text images, where you have to recognize letters and numbers, are among the basic methods. Modern solutions analyze user behavior and technical parameters in the background or use other mechanisms, sometimes going about their work all without visible challenges. Since attackers are always developing new ways to bypass security mechanisms, providers must continuously improve their systems — creating a never-ending race between CAPTCHA solving services and CAPTCHA systems.
Text CAPTCHAs (CAPTCHA codes)
Text-based CAPTCHAs consist of a random sequence of letters and/or numbers, which is how they get their name. They are often placed on colored or patterned backgrounds, distorted with graphic elements, crossed out, or warped to make it difficult for bots to read them automatically — all great security measures in theory, but even humans often find it difficult to decipher the character strings correctly.
Image CAPTCHAs
Building on comparatively simple text-based CAPTCHAs, image-based CAPTCHAs represent the next stage of evolution in CAPTCHA development. Instead of entering a CAPTCHA code, the user must select from a series of photos those that contain a specific object — such as a traffic light or car. In some cases, the user even has to rotate an image the right way round (called a rotate CAPTCHA).
Puzzle CAPTCHAs
With these, you need to position puzzle pieces correctly using a slider or by dragging and dropping the pieces to complete the image. They’re usually more intuitive and user-friendly than traditional CAPTCHA codes.
Audio CAPTCHAs
Audio-based CAPTCHAs were developed for visually impaired users. With these CAPTCHAs, the audio challenge is generated dynamically and the displayed letters and numbers are distorted or played back with background noise. The downside is that they’re often so hard to understand that not only users with hearing impairments but also those with visual impairments find them a challenge to solve.
Logic CAPTCHAs
These CAPTCHAs consist of simple challenges, such as basic addition or knowledge questions, which are easy to answer. If they’re displayed as plain text, people with visual impairments can also solve them as screen readers can read the challenges — unlike graphical text or image CAPTCHAs — but they’re also easy for bots to solve.
Honeypot CAPTCHAs
Honeypot CAPTCHAs essentially lure bots into a trap using an invisible form field built into the website, which normal users can’t see. Bots, on the other hand, often fill in every field automatically, giving themselves away. This method is very user-friendly, but only offers protection from simple automated attacks.
Cryptographic CAPTCHAs (proof-of-work CAPTCHAs)
Alternative CAPTCHA systems like Friendly Captcha and mCaptcha represent the latest type of security check where your computer has to solve a cryptographic challenge (proof of work) behind the scenes. You don’t have to do anything yourself — the calculation is done invisibly and automatically in the browser. The server cryptographically validates the result, immediately detecting manipulated or fake requests. If the system detects signs of automated access, it increases the difficulty of the challenge. This makes it more time‑consuming to solve and reduces the economic viability of automated abuse.
Examples include Cloudflare’s Turnstile solution. It uses a hybrid approach in managed mode, offering multi-stage verification: The system checks the hardware and browser, and it analyzes user behavior within the widget. Non-interactive challenges (including proof-of-work calculations) are only initiated in the background if the system is unsure the user is legitimate. Only as a last resort is user interaction required, such as clicking a checkbox or button.
Behavioral CAPTCHAs
Behavioral CAPTCHAs are also invisible and analyze user interaction and other factors (referred to as signals) in the background. In the event of suspicious user behavior, the system may trigger an additional verification step and display a visible CAPTCHA. This makes these CAPTCHAs user-friendly and provides a high level of security. Well-known examples include Google’s reCAPTCHA v2 Invisible Badge and Intuition Machines’ hCaptcha Invisible.
Advanced solutions such as reCAPTCHA v3, reCAPTCHA Enterprise, and hCaptcha Enterprise take a risk-based approach, creating a risk score based on various signals. Based on these, the website (and not the system) decides on the next steps. Both hCaptcha and reCAPTCHA use sophisticated AI and machine learning models to evaluate device information and interaction patterns in real time.
Google reCAPTCHA
reCAPTCHA (short for Reverse CAPTCHA) is a CAPTCHA system developed in 2007 by Luis von Ahn, Manuel Blum, and Nicholas Hopper at Carnegie Mellon University. Since Google acquired it in 2009, the system has been continuously developed and expanded into one of the world’s most widely used CAPTCHA services.
There are three reCAPTCHA versions:
reCaptcha v1
The first version of reCAPTCHA was discontinued in 2018. It displayed a known check digit along with a word from scanned documents, which OCR software had trouble recognizing. If the control word was entered correctly, it was assumed that the second word was also likely transcribed correctly. In this way, the security check also served to digitize difficult-to-read texts — hence the name “reverse” CAPTCHA.
Later, the second word was replaced with photos from the Google Street View project. Users had to either read and enter house numbers or click on parts of an image showing specific objects. In this way, they helped to train and improve automatic object recognition.
reCAPTCHA v2
There are two variants of the second reCAPTCHA version. If a user is classified as suspicious, an additional image challenge may appear in both variants.
- Checkbox variant (No CAPTCHA reCAPTCHA): Users will only see a checkbox labeled “I’m not a robot” which they must confirm. Google analyzes how the user interacts with the checkbox and checks subtle behavioral patterns such as mouse movements and click patterns.
- Invisible badge variant: Here, an invisible widget is embedded into the page, which is activated via JavaScript — either by clicking a button (like “Submit”) or automatically after a specific interaction on the website. Interaction behavior is also checked here.
reCAPTCHA v3
With the third reCAPTCHA version, verification checks are also run completely invisibly in the background, but no user interaction is required. Google analyzes all the user’s behavior on the website (such as mouse movements, interaction patterns, and typing speed), takes into account cookies and previous page visits, and evaluates device and browser information as well as the reputation of the user’s IP address, calculating a risk score from this (0=very likely bot, 1=very likely human).
Based on this score, the website operator decides how the website should respond at certain thresholds — for example, allowing access, displaying an additional challenge, triggering further security mechanisms, or blocking the action. Users who are signed in with a Google account have a better score because they’re easier to identify.
reCAPTCHA v3 is regularly criticized because Google collects extensive user data for analysis and can track browsing behavior across different websites, raising data protection concerns (especially with regard to the GDPR — the EU’s General Data Protection Regulation).
Downsides of CAPTCHA
Although CAPTCHAs offer protection from bots, they also have some downsides. Depending on the CAPTCHA system, these range from the aforementioned data protection concerns, errors, and problems with the tests, lack of accessibility to weakening effectiveness due to highly specialized circumvention techniques.
CAPTCHA solvers
CAPTCHA solvers are programs that solve CAPTCHAs using traditional algorithms or AI and machine learning. In combination with advanced automation tools, they also simulate human interaction patterns, such as mouse dynamics or typing rhythm, and use browser spoofing to bypass behavioral analysis and technical checks. CAPTCHA solvers are available as simple browser extensions for visually impaired users, more powerful add-ons, or standalone automation tools. These often use API interfaces to external solver services which provide the solution through specialized AI models or human solvers (see CAPTCHA farms).
A study by researchers at ETH Zurich shows that a specially trained AI system was able to successfully solve all tested reCAPTCHA v2 image challenges using an object recognition model (YOLO — or You Only Look Once). A study by the University of California proves that specialized AI systems now solve text CAPTCHAs with a success rate of 99.8 percent, whereas the success rate of human subjects on the first attempt is only between 50 and 84 percent, depending on the level of difficulty.
The technological arms race is therefore leading to a dead end with traditional CAPTCHAs: To effectively hinder AI systems, CAPTCHAs must be designed to be so complex that they’re almost impossible for humans to solve. To break this vicious cycle, modern behavior-based and cryptographic systems no longer depend on visible challenges but on background verification procedures. The aim is to raise the barriers to automated attacks by AI-supported bots to such an extent that their execution becomes economically unviable due to the massive additional technological or financial effort required.
CAPTCHA farms
CAPTCHA farms are organized services where people (often in low-wage countries) solve CAPTCHAs en masse in near real time. Criminals and spam operators use API interfaces to automatically forward their bots’ queries to solver services, which in turn distribute the tasks to their workforce. The solution tokens generated by the workers are then returned to the bot via the service provider. This allows even complex tasks to be tackled that purely software-based AI solvers often cannot yet reliably solve. In combination with IP obfuscation techniques, systems with risk-based assessments can also be deceived.
Poor usability and accessibilityTraditional CAPTCHAs can significantly impair the usability of a website, causing frustration and abandoned actions — especially if they’re difficult to solve. For people with visual or hearing impairments, they can even represent an insurmountable barrier. That’s why modern solutions perform CAPTCHA verification in the background without the user needing to do anything.
CAPTCHA issues
Occasionally, errors or problems can occur with CAPTCHAs. If you’re unable to solve a CAPTCHA test or you entered information incorrectly, you usually have the option to request a new challenge. To do that, you typically press or tap an update button (a small circular arrow icon). Sometimes you’ll also see a refresh button or a link to generate a new image or code.
There can be a range of reasons if a CAPTCHA doesn’t work, isn’t displayed, verification fails, or you get blocked. The most common reasons for CAPTCHA issues include an outdated browser or browser data, browser extensions such as content blockers, disabled cookies, disabled JavaScript, an active VPN, or a Tor connection.
How to fix CAPTCHA errors:
- Reload the page.
- Update your browser.
- Clear your browser cache and delete cookies.
- Temporarily disable browser extensions or use private/incognito mode in your browser.
- Use a different browser.
- If necessary, turn off VPN or proxy services, or close the Tor browser.
- Turn on JavaScript in your browser-settings.
- Allow the website to store third-party cookies.
Fake CAPTCHA scams
In some cases, cybercriminals use fake CAPTCHAs to spread malware. With CAPTCHA scams, for example, users are tricked by phishing attacks into visiting a fake website containing a manipulated CAPTCHA. Clicking the checkbox executes a function in the background that copies a malicious command to the computer’s clipboard.
The user is then asked to paste the command into the Windows Run window and execute it, which downloads and installs the malware. GenDigital reports that some CAPTCHAs are being abused as part of malware campaigns to trick users into downloading the malware Lumma Stealer. This malware is designed to steal sensitive information such as login credentials, cryptocurrency wallets, and other valuable data.
Protect yourself from fake CAPTCHA attacks
A cyberprotection solution like Avira Antivirus Security can help you protect yourself from malware spread through fake CAPTCHAs or other means. That’s because it includes antivirus and download protection so you can block malware. On top of that, you can check your computer for possible infections by running regular virus scans. You also get to enjoy many other features to strengthen the protection of your online security and privacy, including Avira Browser Safety to block infected websites and phishing sites, a VPN, and lots more.
Friendly Captcha is a registered trademark of Friendly Captcha GmbH
hCaptcha is a registered trademark of Intuition Machines, Inc.
reCAPTCHA is a registered trademark of Google LLC.
Turnstile is a registered trademark of Cloudflare, Inc.
