How CAPTCHA works
A website or the security service in front of it decides whether a request looks risky. This decision may use IP reputation, request timing, browser signals, cookies and interaction patterns. It is one part of broader bot detection.
For a visible CAPTCHA, you select images, enter text or complete another task. Invisible versions may assign a risk score in the background and show a puzzle only for some requests. A successful check usually creates a short-lived token that the website verifies before accepting the action.
Common CAPTCHA types
- Checkbox: asks you to confirm that you’re human, then may open another challenge.
- Image or text: requires matching objects or reading distorted characters.
- Audio: provides spoken characters as an accessibility alternative.
- Invisible: evaluates signals in the background and challenges only higher-risk requests.
Modern systems often combine these checks with browser fingerprinting rather than judging the answer alone.
Why CAPTCHAs appear with proxies
A proxy changes the IP address a website sees, but it doesn’t correct inconsistent headers, browser signals or behavior. Challenges may become more frequent when:
- Requests arrive too quickly. Bursts can trigger rate limiting or a risk check.
- An IP has poor reputation. Shared addresses may carry activity from many unrelated users.
- The session changes unexpectedly. Switching exit IPs between loading and submitting a form can change the risk context.
- The client looks incomplete. Missing cookies, disabled JavaScript or unusual request headers can differ from normal browser traffic.
Solving a puzzle doesn’t ensure access. The website may still reject the request if other signals remain suspicious.
How to respond responsibly
- Pause repeated requests. Apply backoff if the site returns 429 Too Many Requests.
- Keep authorized flows consistent. Preserve cookies and one exit IP while completing a multi-step form or session.
- Check your client. Confirm that it handles JavaScript, redirects and cookies as the site expects.
- Use approved access methods. Prefer an official API when available and follow the site’s terms and robots.txt.
FAQ
Why do I get CAPTCHA when using a proxy?
Usually, the website has rated the request as higher risk. The proxy’s IP reputation may contribute, but request speed, cookies, headers and browser signals also matter.
Can rotating proxies prevent CAPTCHAs?
No. Rotation can distribute independent requests across IPs, but changing addresses too often can disrupt sessions and trigger more checks.
Is CAPTCHA the same as a 403 error?
No. A CAPTCHA is a verification step and may appear in a page returned with status 200. A 403 Forbidden response means the responding server or intermediary, such as the website, CDN, firewall or proxy, understood the request but refuses to fulfill it.