pypdf is a pure-python PDF library capable of splitting, merging, cropping, and
transforming the pages of PDF files. In version 2.10.5 an attacker who uses
this
vulnerability can craft a PDF which leads to an infinite loop. This infinite
loop blocks the current process and can utilize a single core of the CPU by
100%. It does not affect memory usage. That is, for example, the case if the
user extracted metadata from such a malformed PDF. Versions prior to 2.10.5
throw an error, but do not hang forever. This issue was fixed with
https://github.com/py-pdf/pypdf/pull/1331 which has been included in release
2.10.6. Users are advised to upgrade. Users unable to upgrade should modify
`PyPDF2/generic/_data_structures.py::read_object` to an an error throwing case.
See GHSA-hm9v-vj3r-r55m for details. 

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2023-36807