It has been discovered that request handling in Extbase can be vulnerable to insecure deserialization. User submitted payload has to be signed with a corresponding HMAC-SHA1 using the sensitive TYPO3 encryptionKey as secret - invalid or unsigned payload is not deserialized.
However, since sensitive information could have been leaked by accident (e.g. in repositories or in commonly known and unprotected backup files), there is the possibility that attackers know the private encryptionKey and are able to calculate the required HMAC-SHA1 to allow a malicious payload to be deserialized.
Requirements for successfully exploiting this vulnerability (all of the following):
github.com/advisories/GHSA-hh95-5xm5-v8v7
github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/2019-12-17-7.yaml
github.com/TYPO3/typo3/commit/57e4ed35a6e58521a931855e702b2688b3bc3d62
github.com/TYPO3/typo3/commit/b1626ad8fd4aebedc15e424a76f86094d78b2564
typo3.org/security/advisory/typo3-psa-2019-011