The remote Debian host is missing a security-related update.
Robert Swiecki reported that mod_auth_digest does not properly initialize or reset the value placeholder in [Proxy-]Authorization headers of type 'Digest' between successive key=value assignments, leading to information disclosure or denial of service.
Upgrade the apache2 packages. For the oldstable distribution (jessie), this problem has been fixed in version 2.4.10-10+deb8u10. For the stable distribution (stretch), this problem has been fixed in version 2.4.25-3+deb9u2.