I’m having trouble with a HubSpot form on my website. Every time someone tries to submit it, I get this error message:
Blocked form submission to '/_hcms/forms/submissions/v3/public/submit/formsnext/multipart/4521887/b7e2dc45-8122-4f61-a654-f983be772c1d'
because the form's frame is sandboxed and the 'allow-forms' permission is not set.
I also see another message about active resource loading limits being reached when the tab is in the background. It says network requests get delayed until previous loading finishes or the tab comes back to foreground.
The JavaScript file is properly linked to the form, so I’m not sure what’s causing this issue. Has anyone encountered this before? The form worked fine until recently and I haven’t made any major changes to the page structure.
Had the exact same issue six months back - turned out our CDN was the culprit. It was auto-wrapping our content in extra iframes we didn’t know about, which triggered the sandbox error. Here’s what worked: check your page source in dev tools and look for unexpected iframe wrappers around the HubSpot code. Your hosting platform or security plugins might be adding these without telling you. We switched from the iframe embed to direct JavaScript and that fixed it. Also check if browser updates or extensions are causing problems - some ad blockers flag form submissions as sketchy and sandbox them automatically.
This usually happens when HubSpot forms get stuck in iframes with overly strict sandbox settings. Had the same problem last year when our security team changed our CSP policies. The issue’s almost always with how the parent page configures the iframe, not HubSpot’s actual code. Check if your form’s loading inside any iframe and make sure the sandbox permissions are set up right. Also look at your Content Security Policy headers - they might be blocking form submissions. That background tab throttling thing is separate but connected to how browsers manage resources. It can mess with form loading when people switch tabs while submitting. If you’re using a CMS or third-party platform that creates iframes automatically for embedded stuff, you’ll probably need to tweak those settings or reach out to their support for proper iframe setup.
yep, that sounds like a permissions issue with the iframe. try adding allow-forms to the sandbox attributes. i had the same prob after an update too, just make sure everything’s set up right!