The URL percent-encoding decode function in libcurl is called
curl_easy_unescape
. Internally, even if this function would be made to
allocate a destination buffer larger than 2GB, it would return that new length
in a signed 32 bit integer variable, thus the length would get either just
truncated or both truncated and turned negative. That could then lead to
libcurl writing outside of its heap based buffer.
This can be triggered by a user on a 64bit system if the user can send in a custom (very large) URL to a libcurl using program.