When viewing code snippets in Trello you may notice some of the text is highlighted, these texts represent bidirectional characters. Bidirectional characters are used to specify the order that characters should be rendered, for example, to support right-to-left languages (e.g. Arabic, Urdu, etc.). However, these characters could also be used maliciously to make a code snippet appear to do one thing, but actually do another. For this reason, we highlight bidirectional characters so you can identify them in code snippets in Trello.

You can find more information about this here: https://ja.confluence.atlassian.com/display/SECURITY/CVE-2021-42574+-+Unrendered+unicode+bidirectional+override+characters+in+Cloud+sites

Web 上の双方向文字の警告

次のようなツールチップを表示できます。

双方向文字はテキストのレンダリング順序を変更するため、悪意のあるコードを隠すために使用される可能性があります。

モバイル アプリ上の双方向文字の警告