Clipboard-based DOM-XSS

Impact

A self Cross-Site Scripting vulnerability exists in the @github/paste-markdown library. If the clipboard data contains the string <table>, a div is dynamically created, and the clipboard content is copied into its innerHTML property without any sanitization, resulting in improper execution of JavaScript in the browser of the victim (the user who pasted the code). Users directed to copy text from a malicious website and paste it into pages that utilize this library are affected.

The following @github/paste-markdown code snippet is triggered when the user pastes something and the browser's clipboard data contains an entry whose content-type is text/HTML.

function generateText(transfer: DataTransfer): string | undefined {
  if (Array.from(transfer.types).indexOf('text/html') === -1) return

  let html = transfer.getData('text/html')
  if (!/<table/i.test(html)) return

  html = html.replace(/<meta.*?>/, '')

  const el = document.createElement('div')
  el.innerHTML = html
  const tables = el.querySelectorAll('table')

  for (const table of tables) {
    if (table.closest('[data-paste-markdown-skip]')) {
      table.replaceWith(new Text(table.textContent || ''))
    }
    const formattedTable = tableMarkdown(table)
    table.replaceWith(new Text(formattedTable))
  }

  return el.innerHTML
}

Patches

A security patch was released in version 0.3.4.

Workarounds

A Content Security Policy that prevents unsafe-inline helps reduce the likelihood of this vulnerability being exploited in modern browsers.

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