On architectures that do not have a vsnprintf() in their standard
library Python attempts to emulate it. When doing so, the implementation
ambitiously allocates more memory than requested without verifying the
sanity of the summed value. As a result it becomes possible (although
unlikely) for an integer overflow to occur misallocating memory and
causing a buffer overflow.

 53 int
 54 PyOS_vsnprintf(char *str, size_t size, const char  *format, va_list va)
 55 {
 56         int len;  /* # bytes written, excluding \0 */
[...]
 60         assert(str != NULL);
 61         assert(size > 0);
 62         assert(format != NULL);
 63 
[...]
 67         /* Emulate it. */
 68         buffer = PyMem_MALLOC(size + 512);
 69         if (buffer == NULL) {
 70                 len = -666;
 71                 goto Done;
 72         }
 73 
 74         len = vsprintf(buffer, format, va);
 75         if (len < 0)
 76                 /* ignore the error */;
 77 
 78         else if ((size_t)len >= size + 512)
 79                 Py_FatalError("Buffer overflow in
PyOS_snprintf/PyOS_vsnprintf");
 80 
 81         else {
 82                 const size_t to_copy = (size_t)len < size ?
 83                                         (size_t)len : size - 1;
 84                 assert(to_copy < size);
 85                 memcpy(str, buffer, to_copy);
 86                 str[to_copy] = '\0';
 87         }
 88         PyMem_FREE(buffer);
 89 Done:
[...]
 91         str[size-1] = '\0';
 92         return len;
 93 }