The specific version of cURL that the client is running is vulnerable to a man-in-the-middle spoofing attack.
cURL and libcurl with mbedTLS and PolarSSL contain a flaw as TLS certificates are not properly validated. By spoofing the TLS/SSL server via a certificate that appears valid, an attacker with the ability to intercept network traffic (e.g. MitM, DNS cache poisoning) can disclose and optionally manipulate transmitted data. Technical Information: This flaw only exists when libcurl is built to use mbedTLS or PolarSSL as TLS backend.
It has been reported that this has been fixed. Please refer to the product listing for upgraded versions that address this vulnerability. In addition, the vendor has released a patch for some older versions.