DoS Captcha Preview Not showing

Hi snoonanCLG​ ,

Hope you will be doing fine.

Here are my 2 preliminary observations.

=======Observation 1========

Have you tested the CAPTCHA preview using both Default and Custom response types?

Please navigate to:

Security → DoS Protection → Protection Profiles → → Application Security → CAPTCHA Settings

For both First Response Type and Failure Response Type, temporarily change the setting from Custom to Default and click Show.

If the preview displays correctly with the Default response but remains blank with the Custom response, this would indicate that the CAPTCHA engine itself is functioning properly and the issue is likely related to the custom CAPTCHA HTML/template configuration.

F5 documents that custom CAPTCHA response bodies utilize HTML and support %DOSL7.captcha.*% variables, so it would be worth reviewing the custom response page for any formatting, syntax, or variable-related issues.

If both Default and Custom previews are blank, then we may need to investigate a broader GUI, browser, or software-related issue rather than the custom template itself.

As an F5 ASM engineer, I would consider this one of the highest-value isolation tests because it immediately tells you whether you're debugging the CAPTCHA engine or the custom CAPTCHA page.

======Observation 2========

Could you please help verify whether the issue is browser-specific?

Since the exception appears to be occurring within the BIG-IP Configuration Utility JavaScript, it would be helpful to test the CAPTCHA preview using different supported browsers and browser modes.

Please try the following:

Google Chrome

Microsoft Edge

Mozilla Firefox

Incognito/Private browsing mode

Clear the browser cache before testing, if possible

Based on the error related to contentDocument, I would specifically recommend comparing the behavior between Chrome/Edge and Firefox.

For example:

If the CAPTCHA preview is blank in Firefox but displays correctly in Chrome or Edge, this may indicate a browser compatibility or Configuration Utility UI rendering issue rather than a problem with the ASM/AWAF CAPTCHA engine itself.

If the behavior is consistent across all browsers, we can continue investigating other potential causes.

Additionally, F5's 17.5.1.3 compatibility documentation lists supported Configuration Utility browsers, including:

Firefox 140+

Chrome 138+

Edge 137+

Could you please share the results of the above tests, along with the browser versions used and any screenshots if available? This will help us narrow down the root cause and determine the next troubleshooting steps.

Along with the above 2 observations checks,

Could you please verify the DoS profile configuration from the tmsh command line to ensure the profile configuration is healthy and to identify any CAPTCHA-related settings that may be contributing to the issue?

Please execute the following commands and share the output:

1. Review the Complete DoS Profile Configuration

tmsh list security dos profile /Common/_ddos_policy all-properties

If the output is too large, please focus on the CAPTCHA-related configuration using:

2. Inspect CAPTCHA-Specific Configuration

tmsh list security dos profile /Common/_ddos_policy | grep -i -A20 -B10 captcha

3. Check for Custom CAPTCHA Response Configuration

tmsh list security dos profile /Common/_ddos_policy one-line | grep -i captcha

These commands will help us determine:

Whether CAPTCHA is configured as expected within the DoS profile

If any custom CAPTCHA response pages are configured

Whether there are any unusual or unexpected CAPTCHA-related settings in the profile configuration

If the issue is related to the profile configuration itself rather than the GUI preview functionality

Once you have collected the outputs, please share them for review. The exact information returned will depend on how the profile has been configured, but it should provide valuable insight into the current CAPTCHA configuration and help guide the next steps in troubleshooting.