Pillow is a Python imaging library. From 5.1.0 until 13.0.0, Pillow can enter an infinite loop while rewriting PDF pages. PdfParser.rewrite pages() follows Parent relationships without detecting a cycle in the page tree. An attacker can supply a cyclic PDF to an application that appends an image to that PDF. This can hold the worker at full CPU and deny service. This issue is fixed in version 13.0.0.