CURL-CVE-2016-5421

libcurl is vulnerable to a use after free flaw.

libcurl works with easy handles using the type 'CURL *' that are objects the application creates using curl_easy_init(). They are the handles that are all each associated with a single transfer at a time. libcurl also has an internal struct that represents and holds most state that is related to a single connection. An easy handle can hold references to one or many such connection structs depending on the requested operations.

When using libcurl's multi interface, an application performs transfers by adding one or more easy handles to the multi handle and then it can drive all those transfers in parallel.

Due to a flaw, libcurl could leave a pointer to a freed connection struct dangling in an easy handle that was previously added to a multi handle when curl_multi_cleanup() is called with an easy handle still added to it. This does not seem to cause any notable harm if the handle is then closed properly.

However, if the easy handle would instead get used again with the easy interface and curl_easy_perform() to do another transfer, it would blindly use the connection struct pointer now pointing to freed memory.

An application could be made to allocate its own fake version of the connect struct, fill in some data and then have the curl_easy_perform() call do something that clearly was not intended by the original code.

For example, this could be an application using a component or library that uses libcurl to do something against fixed URLs or fixed hostnames or with a set of fixed options, but using this flaw the application can then make the component to do something completely different and unintended.

Pseudo code for a bad application

easy = curl_easy_init();
curl_easy_setopt(easy, CURLOPT_URL, "http://example.com/");

// --- start of code to confuse libcurl ---
multi = curl_multi_init();
curl_multi_add_handle(multi, easy);
curl_multi_perform(multi, &still_running);
curl_multi_cleanup(multi);

// --- attack code
allocate_fake_connection_struct()
fill_in_fake_connection_struct()

// ---- end of confusion code

// now this is called, it will not use example.com at all even if the
// option above asks for it...

curl_easy_perform(easy);

This flaw can also be exploited using libcurl bindings in other languages.