Pillow before 8.1.1 allows attackers to cause a denial of service (memory consumption) because the reported size of a contained image is not properly checked for a BLP container, and thus an attempted memory allocation can be very large.
https://security.gentoo.org/glsa/202107-33
https://pillow.readthedocs.io/en/stable/releasenotes/8.1.1.html