diff --git a/packages/ui-default/backendlib/markdown-it-xss.ts b/packages/ui-default/backendlib/markdown-it-xss.ts
index d9fbe90a8..c3033a8e4 100644
--- a/packages/ui-default/backendlib/markdown-it-xss.ts
+++ b/packages/ui-default/backendlib/markdown-it-xss.ts
@@ -151,7 +151,7 @@ export const xss = new FilterXSS({
   css: cssFilterOptions,
   safeAttrValue(tag, name, value) {
     if (name === 'id') return escapeAttrValue(`xss-id-${value}`);
-    if (name === 'class') return value.split(' ').filter((i) => whitelistClasses.includes(i)).join(' ');
+    if (name === 'class') return value.split(' ').filter((i) => whitelistClasses.includes(i) || i.startsWith('language-')).join(' ');
     return safeAttrValue(tag, name, value, CssFilter);
   },
 });