[MEDIUM] CVE-2020-26138 FormField: with square brackets in field name skips validation

PKSA-pq7g-1pwh-dw3n CVE-2020-26138 GHSA-7mv4-4xpg-xq44

Affected package: silverstripe/framework

Affected version: >=3.0.0,<4.0.0|>=4.0.0,<4.7.4

Reported by:
GitHub, FriendsOfPHP/security-advisories