RadEditor - EditorFilters.StripCssExpressions tanks performance

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Last Updated: 18 Dec 2025 16:35 by ADMIN

Release 2026 Q1 (Feb)

Frank

Created on: 18 Dec 2025 14:23

Category: Editor

Type: Bug Report

RadEditor - EditorFilters.StripCssExpressions tanks performance

Dear support,

we've recently updated to the Telerik version 2025.4.1210.462 and are experiencing issues with the (relatively) new RadEditor filter EditorFilters.StripCssExpressions. In particular, setting the RadEditor.Content and rendering the RadEditor object drastically tanks performance in certain situations.

We're aware of the web.config setting to increase the Telerik.RegexMatchTimeout, but for a 79KiB content to be rendered basically instantaneously with the previously mentioned filter being explicitly disabled to take 23+ seconds to render if the filter is enabled is quite shocking to me.

Please find a simple sample page (including the content-value) attached.

We'd really like to use the new security features of the RadEditor, but given this performance impact, we'll be unable to ship this feature as-is.

(Off-topic: We're currently evaluating to re-enable the EditorFilters.StripJavaScriptUris which removed our base64-encoded images in the November release of Telerik, but got fixed with the December release.)

Thanks in advance for investigating this issue and I'll be awaiting your feedback.

Kind regards

Attached Files:

test.zip

1 comment

ADMIN

Rumen

Posted on: 18 Dec 2025 16:35

Hi Frank,

I have converted your ticket into a public bug report so it is visible to other customers.

Thank you for taking the time to report this issue and for providing a detailed sample page. It was extremely helpful in identifying the root cause.

We have investigated the performance issue related to the EditorFilters.StripCssExpressions filter, identified the underlying problem, and addressed it internally.

The fix will be included in the next official release, currently scheduled for January 2025.

In the meantime, if you need to use the security filters, you have a couple of workarounds:

Continue disabling the StripCssExpressions filter as you're currently doing
Consider processing base64 images server-side before setting the RadEditor content (extracting them as separate resources)

Regarding your off-topic note about EditorFilters.StripJavaScriptUris - affirmative the December release resolved the base64 image issue. Please let us know if you encounter any further problems after enabling it.

As a small token of our gratitude for reporting this issue and helping us improve the product, we have added Telerik points to your account.

Thank you again for your patience and for being a valued Telerik customer. Please don't hesitate to reach out if you have any further questions.

Regards,
Rumen
Progress Telerik

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