Marshmallow is a lightweight library for converting complex objects to and from simple Python datatypes. In versions from 3.0.0rc1 to before 3.26.2 and from 4.0.0 to before 4.1.2, Schema.load(data, many=True) is vulnerable to denial of service attacks. A moderately sized request can consume a disproportionate amount of CPU time. This issue has been patched in version 3.26.2 and 4.1.2.
https://github.com/marshmallow-code/marshmallow/security/advisories/GHSA-428g-f7cq-pgp5
https://github.com/marshmallow-code/marshmallow/commit/d24a0c9df061c4daa92f71cf85aca25b83eee508