Exfiltration of hashed SMB credentials on Windows via file:// redirect

Impact

When following a redirect, Electron delays a check for redirecting to file:// URLs from other schemes. The contents of the file is not available to the renderer following the redirect, but if the redirect target is a SMB URL such as file://some.website.com/, then in some cases, Windows will connect to that server and attempt NTLM authentication, which can include sending hashed credentials.

Patches

This issue has been fixed in all current stable versions of Electron. Specifically, these versions contain the fixes:

• 21.0.0-beta.1
• 20.0.1
• 19.0.11
• 18.3.7

We recommend all apps upgrade to the latest stable version of Electron.

Workarounds

If upgrading isn’t possible, this issue can be addressed without upgrading by preventing redirects to file:// URLs in the WebContents.on('will-redirect') event, for all WebContents:

app.on('web-contents-created', (e, webContents) => {
  webContents.on('will-redirect', (e, url) => {
    if (/^file:/.test(url)) e.preventDefault()
  })
})

For more information

If you have any questions or comments about this advisory, email us at [email protected].

Credit

Thanks to user @coolcoolnoworries for reporting this issue.