In the ever-evolving digital landscape, safeguarding online security is paramount. CAPTCHAs stand as a crucial defence against automated bot attacks. As CAPTCHAs advance, so do attackers, refining methods to breach these security barriers. This time, we dissect tactics attackers employ to bypass CAPTCHAs, scrutinize common defence mechanisms, and explore how GeeTest CAPTCHA provides heightened security for its clients.

How Attackers Defeat CAPTCHA?

Bypass via API/Protocol

Attackers exploit vulnerabilities in API/protocol interactions, and common strategies include:

  • Reverse Engineering the Authentication Process: Unraveling the CAPTCHA verification process by studying the code and algorithm used.
  • Encoding and Encrypting Key Parameters: After obtaining the CAPTCHA answer, encoding and encrypting critical parameters to complete the verification process.
  • Cracking Efficiency of Interface-Based Attacks: Exploiting weaknesses in the interface to crack CAPTCHAs efficiently.

Simulator Cracking

This method involves emulating browser environments using simulators.

Key points include:

  • Emulating the Browser Environment: Creating a simulated browser environment for CAPTCHA resolution.
  • Mouse Simulation for CAPTCHA Completion: Simulating mouse movements and clicks for CAPTCHA interaction.
  • Limitations and Efficiency of Simulator-Based Attacks: Understanding the constraints and effectiveness of attacks based on simulators.

Machine Learning-Based Attacks

Attacks leveraging machine learning involve:

  • Deep Learning Models for CAPTCHA Recognition: Training models to recognize and classify CAPTCHA images.
  • Generating Synthetic Training Data: Creating synthetic data to improve model accuracy.
  • Evading CAPTCHA through Model-Based Attacks: Evading CAPTCHA defenses using trained models.

CAPTCHA Solving Services

Here, attackers leverage external services for CAPTCHA resolution:

  • Crowdsourcing Human Solvers: Employing humans to solve CAPTCHAs.
  • Outsourcing CAPTCHA Challenges to Low-Cost Labor Markets: Utilizing low-cost labour markets for solving CAPTCHAs.
  • Bypassing CAPTCHA using Automated Solving Services: Leveraging automated services for CAPTCHA bypass.

Additional Insight: Attackers often leverage Optical Character Recognition (OCR), CAPTCHA farms, and machine learning attacks to automate CAPTCHA bypass processes.

What Makes a CAPTCHA Secure in 2023?

Advanced Image Obfuscation Techniques

Secure CAPTCHAs utilize advanced image obfuscation techniques:

  • Adversarial Noise Injection: Introducing random noise to distort images.
  • Pattern Distortion and Transformation: Distorting patterns to hinder pattern-matching algorithms.
  • Random Background Generation: Creating unpredictable backgrounds for added complexity.

Contextual and Behavioral Analysis:

Security measures extend to behaviour analysis:

  • Incorporating User Behavior Analysis: Analyzing user behaviour patterns for anomaly detection.
  • Detecting Human-like Interactions: Identifying patterns resembling human interactions.
  • Adaptive CAPTCHA Challenges Based on User Profiles: Tailoring challenges based on individual user profiles.

Multi-Factor Authentication Integration

Advanced CAPTCHAs incorporate biometrics and multi-factor authentication:

  • Voice-Based CAPTCHAs: Implementing voice and speech for authentication.

CAPTCHA Response Time Analysis

Security measures extend to analyzing response times:

  • Time-Based CAPTCHA Challenges: Introducing time constraints for completion.
  • Detecting Anomalous Response Times: Identifying abnormal response times for additional security.
  • Real-Time Monitoring and Adaptive CAPTCHA Generation: Dynamically generating CAPTCHAs based on real-time monitoring.

Insight: CAPTCHA security in 2023 relies on image obfuscation, behaviour analysis, and real-time response monitoring.

Defence Mechanisms Employed by General CAPTCHA Vendors

a. Text Distortion:

General CAPTCHA vendors use text distortion techniques:

  • CAPTCHAs often employ various techniques such as image distortion, noise addition, and character warping to make it difficult for OCR algorithms to accurately interpret the text.

b. Time Constraints:

Implementing time constraints serves as a defence mechanism:

  • Some CAPTCHA implementations incorporate time limits within which the user must complete the task. This counteracts the use of CAPTCHA farms by reducing the efficiency of manual solving. Attackers are less likely to use human labour when time constraints make it impractical.

c. Behaviour Analysis:

Behaviour analysis adds an extra layer of defence:

  • Advanced CAPTCHA systems may analyze user behaviour patterns, such as mouse movements and click patterns, to differentiate between human and automated interaction. This behaviour analysis adds an extra layer of defence by identifying suspicious activity and blocking potential attackers.

GeeTest's Advanced Security Strategies

GeeTest CAPTCHA is an innovative solution that goes beyond traditional CAPTCHA mechanisms to provide enhanced security for its clients. Some key features of GeeTest CAPTCHA include:

  • Adaptive security strategies:

Compared with the passive static security strategy of AI-powered CAPTCHA, GeeTest v4-Adaptive Verification adopts active and dynamic confrontation, providing a 7-layer dynamic security strategy which changes with the patterns that bot attack and transforms to 4374 security strategies per defence cycle, increasing 3.714 times the cost of cyber attackers.

  • 7-layer dynamic protection:

Layer 1: Dynamic Update of JS Obfuscation

JS Obfuscation strategies update periodically to increase the cost of reverse engineering

Layer 2: Parameter dynamic update

Dynamic parameters update periodically to increase the cost of API hacking

Layer 3: Global Risk Database

Layer 4: CAPTCHA type

Layer 5: CAPTCHA difficulty

Layer 6: Behavioral algorithm models

Improve the accuracy rate of recognizing suspicious behaviour traces. Machine learning models are trained and evolved regularly based on suspicious trace samples

Layer 7: Parameter encryption

Dynamic parameter encryption increases the cost of API hacking

  • Fully customized protection:

On-demand GeeTest Adaptive CAPTCHA allows clients to configure CAPTCHA challenge frequency, difficulty, and types for suspicious requests.

  • Seamless integration:

GeeTest Adaptive CAPTCHA offers three modes, Intelligent Mode, Invisible Mode, and Direct Platform Integration to make CAPTCHA service integrate with the customer's security system.

  • Uninterrupted user experience:

There are 9 CAPTCHA types with Adaptive CAPTCHA, including No CAPTCHA, Slide CAPTCHA, IconCrush CAPTCHA, Gobang CAPTCHA etc, which suit various security demands without interrupting the user experience. For end users: The product should be able to cover and be compatible with individual users when they use all products of this company from all end devices, and meanwhile, ensure the best response speed of user services.

  • Multilingual support and global deployment:

GeeTest adaptive CAPTCHA supports quick client responses assured via assigning users to the nearest server or clusters, meanwhile, Concise communication flow for easier integration with up to 78 languages support.

  • Upgrade to an intelligent and modular operating customer console:

There is a big difference in the demand for security for different enterprises. GeeTest customer dashboard through the analysis of user behaviour data, GeeTest CAPTCHA offers an analysis of customers' current business scenarios. There are 8 modules for features and services configuration; Tailored service and operation for various events; also equipped with Real-time risk detection and settings at a glance.

How GeeTest Prevents CAPTCHA Bypass

Regardless of the method used for CAPTCHA bypass, knowing the answers to the CAPTCHA resources is fundamental to all defence mechanisms. The two primary approaches are:

Exhaustive Decoding (Cracking):

  • Regular updates to the image dataset to counter brute force attacks.
  • Effective against high-intensity attacks using decoding platforms.

Model Defense:

  • Incorporating new categories to counter model attacks.
  • Attacking critical pixels to confuse recognition operations.

Both methods force hackers into the manual collection and decoding process, disrupting the automation of bypass attacks.


As CAPTCHA bypass techniques continue to evolve, it is crucial to stay one step ahead of attackers by employing robust defence mechanisms. While general CAPTCHA defences offer a baseline level of security, innovative solutions like GeeTest CAPTCHA provide enhanced protection for clients through anti-OCR techniques, interactive challenges, and behaviour monitoring. By implementing advanced CAPTCHA technologies, we can better defend against automated attacks and ensure a safer online environment for users and businesses alike.

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Hayley Hong

Content Marketing @ GeeTest