Failing to properly encode user input, backend forms are vulnerable to Cross-Site Scripting. A valid backend user account is needed to exploit this vulnerability.

References

• https://github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/2017-09-05-1.yaml
• https://typo3.org/security/advisory/typo3-core-sa-2017-004
• https://typo3.org/teams/security/security-bulletins/typo3-core/typo3-core-sa-2017-004