This indicates an attack attempt to exploit a Buffer Overflow vulnerability in cURL/libcurl.
The vulnerability is caused by an error when the vulnerable software handles a crafted SASL DIGEST-MD5 authentication. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application.
cURL Project cURL 7.26.0 to 7.28.1
System Compromise: Remote attackers can execute arbitrary code in the context of the affected system.
Apply the most recent upgrade or patch from the vendor.