Failing to properly encode information from external sources, language pack handling in the install tool is vulnerable to cross-site scripting.

References

• https://github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/2019-01-22-8.yaml
• https://typo3.org/security/advisory/typo3-core-sa-2019-004