A security issue has been found in curl before version 7.78.0. When curl is instructed to download content using the metalink feature, the contents is verified against a hash provided in the metalink XML file. The metalink XML file points out to the client how to get the same content from a set of different URLs, potentially hosted by different servers and the client can then download the file from one or several of them in a serial or parallel manner.
If one of the servers hosting the contents has been breached and the contents of the specific file on that server is replaced with a modified payload, curl should detect this when the hash of the file mismatches after a completed download. It should remove the contents and instead try getting the contents from another URL. This is not done, and instead such a hash mismatch is only mentioned in text and the potentially malicious content is kept in the file on disk. There's a risk the user doesn't notice the message and instead assumes the file is fine.
This flaw exists only in the curl tool. libcurl is not affected.