The four libcurl functions curl_escape(), curl_easy_escape(), curl_unescape and curl_easy_unescape perform string URL percent escaping and unescaping. They accept custom string length inputs in signed integer arguments. (The functions having names without "easy" being the deprecated versions of the others.)
The provided string length arguments were not properly checked and due to arithmetic in the functions, passing in the length 0xffffffff (2^32-1 or UINT_MAX or even just -1) would end up causing an allocation of zero bytes of heap memory that curl would attempt to write gigabytes of data into.
The use of 'int' for this input type in the API is of course unwise but has remained so in order to maintain the API over the years.