An attacker who uses this vulnerability can craft a PDF which leads to an infinite loop. This requires accessing the children of a TreeObject, for example as part of outlines.
This has been fixed in pypdf==6.7.1.
If you cannot upgrade yet, consider applying the changes from PR #3645.
6.7.1Exploitability
AV:LAC:LAT:NPR:NUI:NVulnerable System
VC:NVI:NVA:HSubsequent System
SC:NSI:NSA:N6.9/CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N