Possible resource exhaustation of `expandvars` in `Tools/freeze/checkextensions.py`

Bug Description:
The code will re-scan the newly generated content after replacing the variable. If a variable is defined to contain itself (i.e., a recursive definition), this process will fall into infinite recursion.

This recursion can lead to two forms of attacks: one is to cause an infinite loop and deplete CPU resources, and the other is to cause the string to grow infinitely and deplete memory resources. The final result will all lead to the application crashing or being unresponsive.

Vulnerability Locations :
1.https://github.com/python/cpython/blob/e64395e8eb8d3a9e35e3e534e87d427ff27ab0a5/Tools/freeze/checkextensions.py#L72

Repair Status:
None

Common Information:

• CPython Version: main branch
• Operating System: Linux
• Credits: Finder is kexinoh (Xiangfan Wu) from QI-ANXIN Technology Research Institute.

Linked PRs

• gh-136093

从 Tools/freeze/checkextensions.py 约第 72 行开始，这里 expandvars 会在变量替换后重新扫描生成的内容。复现一个自引用变量定义，并检查链接的 gh-136093 工作；当递归展开不再耗尽 CPU 或内存，且现有行为保持不变时，即表示完成。