Render AI Output Safely in HTML and Spreadsheets

An AI assistant returns a supplier name. A dashboard displays it, a report embeds it and an export places it in a spreadsheet. Those destinations can interpret the same characters differently. A safe output path starts with the destination's rules, not a request that the model avoid dangerous text.

OWASP describes improper output handling as insufficient treatment of model responses before downstream use. The practical issue is the boundary where generated data acquires the ability to affect another system.

Keep that boundary in application code under normal review. The model can propose a value; the application decides its type and permitted use.

Ordinary text should remain ordinary text

If a dashboard only needs a sentence, render a sentence. For an ordinary HTML element, OWASP's XSS prevention guidance identifies textContent as a safe text sink. It does not interpret the supplied string as HTML. Framework escaping helps, but explicit HTML insertion can bypass it.

Do not add rich rendering because an assistant happens to return Markdown or tags. Decide which formatting the product needs. If rich HTML is required, use a maintained sanitisation approach appropriate to the allowed features, and avoid later changes that undermine its result.

URLs require a separate decision. A link label is text; the destination is a navigation instruction. Validate permitted schemes and, where the workflow needs it, permitted destinations. Output encoding depends on context: HTML text, attributes, JavaScript and URLs do not share one universal escaping function.

Make this visible in the interface contract. A field called summary_text should not sometimes contain a rendered card, sometimes a URL and sometimes executable markup. Separate fields let the application validate each purpose without guessing from the content.

Spreadsheet exports need an explicit type decision

A CSV file can look like plain data while the application opening it interprets particular cells as formulas. OWASP's CSV injection guidance warns that leading formula characters and spreadsheet-specific behaviour matter. Quoting a CSV field does not, by itself, stop formula interpretation, and saving and reopening a file can change how protective quoting is handled.

For each exported column, decide whether it contains text, numbers, dates or an intentionally generated formula. A supplier name or assistant-written explanation should remain text even if its first character resembles a formula. A legitimate negative amount must remain a number if the workbook needs arithmetic.

This makes blanket replacements a poor default. Removing every leading minus sign damages financial data. Applying a protective prefix to every cell can also change what another application imports. Design the export for its declared consumer and keep the original typed value separately from its presentation.

Where practical, a workbook format with deliberately assigned cell types gives more control than asking a CSV reader to infer everything. Check the chosen library's behaviour: a string beginning with = must actually be stored as text when that is the field's contract. A filename ending in .xlsx does not establish this on its own.

The spreadsheet prompt injection note covers instructions entering an assistant through a workbook. Export handling covers the later boundary where generated content reaches a spreadsheet application.

Keep a small destination test pack

Build a synthetic set of outputs that exercises the actual rendering paths. Include ordinary text, quotation marks, angle brackets, accented characters, line breaks, a long value and harmless formula markers such as =1+1. Add a normal negative amount in a numeric field.

For the browser view, inspect the resulting document structure. Text intended as text should remain a text value, with no new elements or event behaviour created from it. Check links separately, including a destination outside the permitted policy.

For exports, open the file in the spreadsheet application the business uses. Inspect both the displayed result and the underlying cell type or formula bar. Save, close and reopen a copy. If the workflow includes a second importer, test that step as well. A pass in one viewer says little about a different interpretation path.

Record the application and version used, the export library version and the expected result for each field. Use fake records so the test artefacts can be shared during review without carrying customer information.

Release one output path at a time

Start with a text-only view or a tightly specified export. Keep the previous renderer or export option available during a limited rollout, and compare results using the same synthetic pack.

If rich formatting creates an unexpected behaviour, disable that feature while retaining the underlying text. If a spreadsheet export cannot preserve the required types reliably, pause that export path and provide an approved alternative. Repair the application boundary; changing the assistant's wording is too fragile to serve as the control.

References

• OWASP, LLM05:2025 Improper Output Handling. Accessed 17 October 2026.
• OWASP, Cross Site Scripting Prevention Cheat Sheet. Accessed 17 October 2026.
• OWASP, CSV Injection. Accessed 17 October 2026.