Adversaries may avoid CAPTCHA challenges by mimicking human behavior. Automation tests, such as CAPTCHA, are typically used by defenders to detect and prevent bot and spam traffic from accessing protected webpages. CAPTCHAs are often placed before login screens or form fields. Traditional CAPTCHAs ask the user to decipher a string of characters, type the characters into a box, and submit the form. reCAPTCHA asks the user to identify images, click a checkbox, or evaluate the user’s behavior from mouse movement and device cookies.(Citation: CAPTCHAS Cloudflare) Adversaries may attempt to avoid CAPTCHA challenges by constructing a bot that mimics human behavior through browser automation. The automated browser may mimic a human by [Masquerading](https://attack.mitre.org/techniques/T1036) a [Browser Fingerprint](https://attack.mitre.org/techniques/T1036/012) and through attributes such as session history, IP reputation, and the use of browser APIs.(Citation: CAPTCHA Bypass Olostep) For example, adversaries may use Selenium browsers to simulate a human user. In the Selenium browsers, JavaScript is injected into the website’s Document Object Model (DOM). The injected script may modify browser attributes such as Audio Context and Voice engines, graphic rendering, installed fonts, and browser navigator objects.(Citation: Sentinel CAPTCHA Bypass)
Employ network appliances and endpoint software to filter ingress, egress, and lateral network traffic. This includes protocol-based filtering, enforcing firewall rules, and blocking or restricting traffic based on predefined conditions to limit adversary movement and data exfiltration. This mitigation can be implemented through the following measures: Ingress Traffic Filtering: - Use Case: Configure network firewalls to allow traffic only from authorized IP addresses to public-facing servers. - Implementation: Limit SSH (port 22) and RDP (port 3389) traffic to specific IP ranges. Egress Traffic Filtering: - Use Case: Use firewalls or endpoint security software to block unauthorized outbound traffic to prevent data exfiltration and command-and-control (C2) communications. - Implementation: Block outbound traffic to known malicious IPs or regions where communication is unexpected. Protocol-Based Filtering: - Use Case: Restrict the use of specific protocols that are commonly abused by adversaries, such as SMB, RPC, or Telnet, based on business needs. - Implementation: Disable SMBv1 on endpoints to prevent exploits like EternalBlue. Network Segmentation: - Use Case: Create network segments for critical systems and restrict communication between segments unless explicitly authorized. - Implementation: Implement VLANs to isolate IoT devices or guest networks from core business systems. Application Layer Filtering: - Use Case: Use proxy servers or Web Application Firewalls (WAFs) to inspect and block malicious HTTP/S traffic. - Implementation: Configure a WAF to block SQL injection attempts or other web application exploitation techniques.
Use intrusion detection signatures to block traffic at network boundaries.
MITRE ATT&CK® is a registered trademark of The MITRE Corporation. © 2026 The MITRE Corporation. This work is reproduced and distributed with the permission of The MITRE Corporation.