In Eclipse Jetty, the Digest authentication server-side component uses ISO-8859-1 to encode the password as bytes.
This was done because the initial specification for HTTP did not specify explicitly a charset, and it was assumed to be ISO-8859-1 for historical reasons.
If the password contains characters that cannot be represented in ISO-8859-1, they are silently replaced by ?. This happens with passwords that contain Chinese, Cyrillic or Greek characters, for example: αβ123 converts to ??123.
An attacker can send a request with a digest Authorization header crafted with a password made of only ? characters; the server would match any password of the same length that contains non-ISO-8859-1 characters.
Recent HTTP Digest RFC-7616 supports a charset parameters that defaults to UTF-8 that allows for correct encoding/decoding of passwords.
Exploitability
AV:NAC:LAT:NPR:NUI:NVulnerable System
VC:HVI:NVA:NSubsequent System
SC:NSI:NSA:N8.7/CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N