The libcurl API function called curl_maprintf() can be tricked into doing a double-free due to an unsafe size_t multiplication, on systems using 32 bit size_t variables. The function is also used internally in numerous situations.
The function doubles an allocated memory area with realloc() and allows the size to wrap and become zero and when doing so realloc() returns NULL and frees the memory - in contrary to normal realloc() fails where it only returns NULL - causing libcurl to free the memory again in the error path.
This behavior is triggerable using the publicly exposed function. Systems with 64 bit versions of the size_t type are not affected by this issue.