fix: protect special characters in math from markdown table parsers

Wiki.js uses markdown-it-attrs which interprets curly braces inside
inline math ($...$) as attribute directives, stripping them from the
formula. Additionally, markdown table parsers split cells at both `|`
and `&` characters, breaking formulas containing those symbols.

This fix replaces `{`, `}`, `|`, and `&` inside math expressions with
Unicode Private Use Area placeholders during markdown parsing, then
restores them before passing to KaTeX/MathJax for rendering.

- `<E000>` / `<E001>`: temporary replacements for `{` / `}`
- `<E002>`: temporary replacement for `|` (table cell delimiter)
- `<E003>`: temporary replacement for `&` (table cell delimiter in
  multiline tables, used by LaTeX cases/arrays)

The placeholder approach was chosen over HTML escaping because it
preserves LaTeX environments like `\begin{array}` that were broken
by the previous `{{}}` escaping method.

Fixes #1581
Fixes #1462

Co-authored-by: Claude <noreply@anthropic.com>
AI-model: kimi-for-coding/k2p6

client/components/editor/common/katex.js

@@ -1,3 +1,22 @@
+// Unicode Private Use Area characters to temporarily replace special
+// characters during markdown parsing:
+// - braces: prevent markdown-it-attrs from interpreting them as attribute
+//   delimiters.
+// - pipe: prevent markdown table parser from interpreting them as cell
+//   delimiters.
+const BRACE_OPEN_PLACEHOLDER = '\uE000'
+const BRACE_CLOSE_PLACEHOLDER = '\uE001'
+const PIPE_PLACEHOLDER = '\uE002'
+const AMPERSAND_PLACEHOLDER = '\uE003'
+
+export function restoreBraces (str) {
+  return str
+    .replaceAll(BRACE_OPEN_PLACEHOLDER, '{')
+    .replaceAll(BRACE_CLOSE_PLACEHOLDER, '}')
+    .replaceAll(PIPE_PLACEHOLDER, '|')
+    .replaceAll(AMPERSAND_PLACEHOLDER, '&')
+}
+
 // Test if potential opening or closing delimieter
 // Assumes that there is a "$" at state.src[pos]
 function isValidDelim (state, pos) {
@@ -27,6 +46,8 @@ function isValidDelim (state, pos) {
 }
 
 export default {
+  restoreBraces,
+
   katexInline (state, silent) {
     let start, match, token, res, pos
 
@@ -84,11 +105,13 @@ export default {
       token.content = state.src
         // Extract the math part without the $
         .slice(start, match)
-        // Escape the curly braces since they will be interpreted as
-        // attributes by markdown-it-attrs (the "curly_attributes"
-        // core rule)
-        .replaceAll("{", "{{")
-        .replaceAll("}", "}}")
+        // Replace curly braces with temporary placeholders to prevent
+        // markdown-it-attrs from interpreting them as attribute delimiters.
+        .replaceAll('{', BRACE_OPEN_PLACEHOLDER)
+        .replaceAll('}', BRACE_CLOSE_PLACEHOLDER)
+        // Replace pipe with temporary placeholder to prevent markdown
+        // table parser from interpreting it as a cell delimiter.
+        .replaceAll('|', PIPE_PLACEHOLDER)
     }
 
     state.pos = match + 1
@@ -133,15 +156,22 @@ export default {
       }
     }
 
-    state.line = next + 1
-
-    token = state.push('katex_block', 'math', 0)
-    token.block = true
-    token.content = (firstLine && firstLine.trim() ? firstLine + '\n' : '') +
-    state.getLines(start + 1, next, state.tShift[start], true) +
-    (lastLine && lastLine.trim() ? lastLine : '')
-    token.map = [ start, state.line ]
-    token.markup = '$$'
-    return true
-  }
+  state.line = next + 1
+
+  token = state.push('katex_block', 'math', 0)
+  token.block = true
+  token.content = ((firstLine && firstLine.trim() ? firstLine + '\n' : '') +
+  state.getLines(start + 1, next, state.tShift[start], true) +
+  (lastLine && lastLine.trim() ? lastLine : ''))
+    // Replace curly braces with temporary placeholders to prevent
+    // markdown-it-attrs from interpreting them as attribute delimiters.
+    .replaceAll('{', BRACE_OPEN_PLACEHOLDER)
+    .replaceAll('}', BRACE_CLOSE_PLACEHOLDER)
+    // Replace pipe with temporary placeholder to prevent markdown
+    // table parser from interpreting it as a cell delimiter.
+    .replaceAll('|', PIPE_PLACEHOLDER)
+  token.map = [ start, state.line ]
+  token.markup = '$$'
+  return true
+}
 }

client/components/editor/editor-markdown.vue

@@ -296,6 +296,53 @@ DOMPurify.addHook('uponSanitizeElement', (elm) => {
 // HELPER FUNCTIONS
 // ========================================
 
+// Unicode Private Use Area characters to temporarily replace special
+// characters inside math expressions:
+// - pipe (|): prevent markdown table parser from interpreting them as cell
+//   delimiters.
+// - ampersand (&): prevent markdown-it-multimd-table from interpreting them
+//   as cell delimiters in multiline tables.
+const PIPE_PLACEHOLDER = '\uE002'
+const AMPERSAND_PLACEHOLDER = '\uE003'
+
+/**
+ * Replace pipe and ampersand characters inside inline ($...$) and block
+ * ($$...$$) math expressions with placeholders to prevent markdown table
+ * parsers from splitting formulas containing | (e.g., |x|) or &
+ * (e.g., \begin{cases} ... & ... \\ ... \end{cases}).
+ */
+function protectMathPipes (text) {
+  let result = ''
+  let i = 0
+  while (i < text.length) {
+    // Check for block math ($$...$$)
+    if (text.slice(i, i + 2) === '$$') {
+      const end = text.indexOf('$$', i + 2)
+      if (end !== -1) {
+        result += text.slice(i, end + 2)
+          .replace(/\|/g, PIPE_PLACEHOLDER)
+          .replace(/&/g, AMPERSAND_PLACEHOLDER)
+        i = end + 2
+        continue
+      }
+    }
+    // Check for inline math ($...$)
+    if (text[i] === '$' && text[i + 1] !== '$') {
+      const end = text.indexOf('$', i + 1)
+      if (end !== -1) {
+        result += text.slice(i, end + 1)
+          .replace(/\|/g, PIPE_PLACEHOLDER)
+          .replace(/&/g, AMPERSAND_PLACEHOLDER)
+        i = end + 1
+        continue
+      }
+    }
+    result += text[i]
+    i++
+  }
+  return result
+}
+
 // Inject line numbers for preview scroll sync
 let linesMap = []
 function injectLineNumbers (tokens, idx, options, env, slf) {
@@ -328,7 +375,7 @@ const macros = {}
 md.inline.ruler.after('escape', 'katex_inline', katexHelper.katexInline)
 md.renderer.rules.katex_inline = (tokens, idx) => {
   try {
-    return katex.renderToString(tokens[idx].content, {
+    return katex.renderToString(katexHelper.restoreBraces(tokens[idx].content), {
       displayMode: false, macros
     })
   } catch (err) {
@@ -341,7 +388,7 @@ md.block.ruler.after('blockquote', 'katex_block', katexHelper.katexBlock, {
 })
 md.renderer.rules.katex_block = (tokens, idx) => {
   try {
-    return `<p>` + katex.renderToString(tokens[idx].content, {
+    return `<p>` + katex.renderToString(katexHelper.restoreBraces(tokens[idx].content), {
       displayMode: true, macros
     }) + `</p>`
   } catch (err) {
@@ -453,7 +500,9 @@ export default {
       linesMap = []
       // this.$store.set('editor/content', newContent)
       this.processMarkers(this.cm.firstLine(), this.cm.lastLine())
-      this.previewHTML = DOMPurify.sanitize(md.render(newContent), {
+      // Protect pipe characters inside math expressions before markdown parsing
+      const protectedContent = protectMathPipes(newContent)
+      this.previewHTML = DOMPurify.sanitize(md.render(protectedContent), {
         ADD_TAGS: ['foreignObject'],
         HTML_INTEGRATION_POINTS: { foreignobject: true }
       })

server/modules/rendering/markdown-core/renderer.js

@@ -19,6 +19,53 @@ const quoteStyles = {
   Swedish: '””’’'
 }
 
+// Unicode Private Use Area characters to temporarily replace special
+// characters inside math expressions:
+// - pipe (|): prevent markdown table parser from interpreting them as cell
+//   delimiters.
+// - ampersand (&): prevent markdown table parser from interpreting them
+//   as cell delimiters in multiline tables.
+const PIPE_PLACEHOLDER = '\uE002'
+const AMPERSAND_PLACEHOLDER = '\uE003'
+
+/**
+ * Replace pipe and ampersand characters inside inline ($...$) and block
+ * ($$...$$) math expressions with placeholders to prevent markdown table
+ * parsers from splitting formulas containing | (e.g., |x|) or &
+ * (e.g., \begin{cases} ... & ... \\ ... \end{cases}).
+ */
+function protectMathPipes (text) {
+  let result = ''
+  let i = 0
+  while (i < text.length) {
+    // Check for block math ($$...$$)
+    if (text.slice(i, i + 2) === '$$') {
+      const end = text.indexOf('$$', i + 2)
+      if (end !== -1) {
+        result += text.slice(i, end + 2)
+          .replace(/\|/g, PIPE_PLACEHOLDER)
+          .replace(/&/g, AMPERSAND_PLACEHOLDER)
+        i = end + 2
+        continue
+      }
+    }
+    // Check for inline math ($...$)
+    if (text[i] === '$' && text[i + 1] !== '$') {
+      const end = text.indexOf('$', i + 1)
+      if (end !== -1) {
+        result += text.slice(i, end + 1)
+          .replace(/\|/g, PIPE_PLACEHOLDER)
+          .replace(/&/g, AMPERSAND_PLACEHOLDER)
+        i = end + 1
+        continue
+      }
+    }
+    result += text[i]
+    i++
+  }
+  return result
+}
+
 module.exports = {
   async render() {
     const mkdown = md({
@@ -50,6 +97,8 @@ module.exports = {
       await renderer.init(mkdown, child.config)
     }
 
-    return mkdown.render(this.input)
+    // Protect pipe characters inside math expressions before markdown parsing
+    const protectedInput = protectMathPipes(this.input)
+    return mkdown.render(protectedInput)
   }
 }

server/modules/rendering/markdown-katex/renderer.js

@@ -3,6 +3,25 @@ const chemParse = require('./mhchem')
 
 /* global WIKI */
 
+// Unicode Private Use Area characters to temporarily replace special
+// characters during markdown parsing:
+// - braces: prevent markdown-it-attrs from interpreting them as attribute
+//   delimiters.
+// - pipe: prevent markdown table parser from interpreting them as cell
+//   delimiters.
+const BRACE_OPEN_PLACEHOLDER = '\uE000'
+const BRACE_CLOSE_PLACEHOLDER = '\uE001'
+const PIPE_PLACEHOLDER = '\uE002'
+const AMPERSAND_PLACEHOLDER = '\uE003'
+
+function restoreBraces (str) {
+  return str
+    .replaceAll(BRACE_OPEN_PLACEHOLDER, '{')
+    .replaceAll(BRACE_CLOSE_PLACEHOLDER, '}')
+    .replaceAll(PIPE_PLACEHOLDER, '|')
+    .replaceAll(AMPERSAND_PLACEHOLDER, '&')
+}
+
 // ------------------------------------
 // Markdown - KaTeX Renderer
 // ------------------------------------
@@ -29,7 +48,7 @@ module.exports = {
       mdinst.inline.ruler.after('escape', 'katex_inline', katexInline)
       mdinst.renderer.rules.katex_inline = (tokens, idx) => {
         try {
-          return katex.renderToString(tokens[idx].content, {
+          return katex.renderToString(restoreBraces(tokens[idx].content), {
             displayMode: false, macros
           })
         } catch (err) {
@@ -44,7 +63,7 @@ module.exports = {
       })
       mdinst.renderer.rules.katex_block = (tokens, idx) => {
         try {
-          return `<p>` + katex.renderToString(tokens[idx].content, {
+          return `<p>` + katex.renderToString(restoreBraces(tokens[idx].content), {
             displayMode: true, macros
           }) + `</p>`
         } catch (err) {
@@ -135,11 +154,19 @@ function katexInline (state, silent) {
     return true
   }
 
-  if (!silent) {
-    token = state.push('katex_inline', 'math', 0)
-    token.markup = '$'
-    token.content = state.src.slice(start, match)
-  }
+    if (!silent) {
+      token = state.push('katex_inline', 'math', 0)
+      token.markup = '$'
+      token.content = state.src
+        .slice(start, match)
+        // Replace curly braces with temporary placeholders to prevent
+        // markdown-it-attrs from interpreting them as attribute delimiters.
+        .replaceAll('{', BRACE_OPEN_PLACEHOLDER)
+        .replaceAll('}', BRACE_CLOSE_PLACEHOLDER)
+        // Replace pipe with temporary placeholder to prevent markdown
+        // table parser from interpreting it as a cell delimiter.
+        .replaceAll('|', PIPE_PLACEHOLDER)
+    }
 
   state.pos = match + 1
   return true
@@ -187,9 +214,16 @@ function katexBlock (state, start, end, silent) {
 
   token = state.push('katex_block', 'math', 0)
   token.block = true
-  token.content = (firstLine && firstLine.trim() ? firstLine + '\n' : '') +
+  token.content = ((firstLine && firstLine.trim() ? firstLine + '\n' : '') +
   state.getLines(start + 1, next, state.tShift[start], true) +
-  (lastLine && lastLine.trim() ? lastLine : '')
+  (lastLine && lastLine.trim() ? lastLine : ''))
+    // Replace curly braces with temporary placeholders to prevent
+    // markdown-it-attrs from interpreting them as attribute delimiters.
+    .replaceAll('{', BRACE_OPEN_PLACEHOLDER)
+    .replaceAll('}', BRACE_CLOSE_PLACEHOLDER)
+    // Replace pipe with temporary placeholder to prevent markdown
+    // table parser from interpreting it as a cell delimiter.
+    .replaceAll('|', PIPE_PLACEHOLDER)
   token.map = [ start, state.line ]
   token.markup = '$$'
   return true

server/modules/rendering/markdown-mathjax/renderer.js

@@ -2,6 +2,25 @@ const mjax = require('mathjax')
 
 /* global WIKI */
 
+// Unicode Private Use Area characters to temporarily replace special
+// characters during markdown parsing:
+// - braces: prevent markdown-it-attrs from interpreting them as attribute
+//   delimiters.
+// - pipe: prevent markdown table parser from interpreting them as cell
+//   delimiters.
+const BRACE_OPEN_PLACEHOLDER = '\uE000'
+const BRACE_CLOSE_PLACEHOLDER = '\uE001'
+const PIPE_PLACEHOLDER = '\uE002'
+const AMPERSAND_PLACEHOLDER = '\uE003'
+
+function restoreBraces (str) {
+  return str
+    .replaceAll(BRACE_OPEN_PLACEHOLDER, '{')
+    .replaceAll(BRACE_CLOSE_PLACEHOLDER, '}')
+    .replaceAll(PIPE_PLACEHOLDER, '|')
+    .replaceAll(AMPERSAND_PLACEHOLDER, '&')
+}
+
 // ------------------------------------
 // Markdown - MathJax Renderer
 // ------------------------------------
@@ -38,7 +57,7 @@ module.exports = {
       mdinst.inline.ruler.after('escape', 'mathjax_inline', mathjaxInline)
       mdinst.renderer.rules.mathjax_inline = (tokens, idx) => {
         try {
-          const result = MathJax.tex2svg(tokens[idx].content, {
+          const result = MathJax.tex2svg(restoreBraces(tokens[idx].content), {
             display: false
           })
           return MathJax.startup.adaptor.innerHTML(result)
@@ -54,7 +73,7 @@ module.exports = {
       })
       mdinst.renderer.rules.mathjax_block = (tokens, idx) => {
         try {
-          const result = MathJax.tex2svg(tokens[idx].content, {
+          const result = MathJax.tex2svg(restoreBraces(tokens[idx].content), {
             display: true
           })
           return `<p>` + MathJax.startup.adaptor.innerHTML(result) + `</p>`
@@ -149,7 +168,15 @@ function mathjaxInline (state, silent) {
   if (!silent) {
     token = state.push('mathjax_inline', 'math', 0)
     token.markup = '$'
-    token.content = state.src.slice(start, match)
+    token.content = state.src
+      .slice(start, match)
+      // Replace curly braces with temporary placeholders to prevent
+      // markdown-it-attrs from interpreting them as attribute delimiters.
+      .replaceAll('{', BRACE_OPEN_PLACEHOLDER)
+      .replaceAll('}', BRACE_CLOSE_PLACEHOLDER)
+      // Replace pipe with temporary placeholder to prevent markdown
+      // table parser from interpreting it as a cell delimiter.
+      .replaceAll('|', PIPE_PLACEHOLDER)
   }
 
   state.pos = match + 1
@@ -198,9 +225,16 @@ function mathjaxBlock (state, start, end, silent) {
 
   token = state.push('mathjax_block', 'math', 0)
   token.block = true
-  token.content = (firstLine && firstLine.trim() ? firstLine + '\n' : '') +
+  token.content = ((firstLine && firstLine.trim() ? firstLine + '\n' : '') +
   state.getLines(start + 1, next, state.tShift[start], true) +
-  (lastLine && lastLine.trim() ? lastLine : '')
+  (lastLine && lastLine.trim() ? lastLine : ''))
+    // Replace curly braces with temporary placeholders to prevent
+    // markdown-it-attrs from interpreting them as attribute delimiters.
+    .replaceAll('{', BRACE_OPEN_PLACEHOLDER)
+    .replaceAll('}', BRACE_CLOSE_PLACEHOLDER)
+    // Replace pipe with temporary placeholder to prevent markdown
+    // table parser from interpreting it as a cell delimiter.
+    .replaceAll('|', PIPE_PLACEHOLDER)
   token.map = [ start, state.line ]
   token.markup = '$$'
   return true