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

Comienza en Tools/freeze/checkextensions.py, alrededor de la línea 72, donde expandvars vuelve a analizar el contenido generado después del reemplazo de variables. Reproduce una definición de variable autorreferencial e inspecciona el trabajo enlazado gh-136093; se considera terminado cuando la expansión recursiva ya no agota la CPU ni la memoria y el comportamiento existente permanece intacto.