### Impact
An attacker who uses this vulnerability can craft a PDF which leads to long runtimes. This requires loading relevant parts of a file (like their pages) where the same, but non-existent indirect object is referenced multiple times.
### Patches
This has been fixed in [pypdf==6.21.2](https://github.com/py-pdf/pypdf/releases/tag/6.21.2).
### Workarounds
If you cannot upgrade yet, consider applying the changes from PR [#4240](https://github.com/py-pdf/pypdf/pull/4240).