This flaw allows an attacker to insert cookies at will into a running program using libcurl, if the specific series of conditions are met.
libcurl performs transfers. In its API, an application creates "easy handles" that are the individual handles for single transfers.
libcurl provides a function call that duplicates en easy handle called curl_easy_duphandle.
If a transfer has cookies enabled when the handle is duplicated, the
cookie-enable state is also cloned - but without cloning the actual
cookies. If the source handle did not read any cookies from a specific file on
disk, the cloned version of the handle would instead store the file name as
none (using the four ASCII letters, no quotes).
Subsequent use of the cloned handle that does not explicitly set a source to
load cookies from would then inadvertently load cookies from a file named
none - if such a file exists and is readable in the current directory of the
program using libcurl. And if using the correct file format of course.