The vulnerability resides in the DigestAuth implementation of http4s. The nonce‑count tracking stores lastNc + 1 instead of the highest nonce‑count value it has accepted when a legitimate client sends noncontiguous nonce‑count values. As a result, the stored counter can be lower than the maximum value seen by the server. A can replay the request multiple times, with the server accepting each replay as a fresh authenticated request. Successful replays allow the attacker to trigger any state‑changing operations on the server as the captured user. This flaw attacks (CWE‑294).

The issue affects applications that use the http4s HTTP client or server libraries before version 0.23.35 and before the milestone 1.0.0‑M47. Any deployment that includes those earlier releases of http4s is vulnerable.

The CVSS score of 5.9 indicates moderate severity. The EPSS score is < 1%, and the vulnerability is not listed in the CISA KEV catalog, suggesting very low current exploitation activity. The likely attack vector is a passive network eavesdropper who can capture an Authorization header during a legitimate request. After capturing the header, the attacker can replay the request to the target server. Exploitation requires no special host access beyond the ability to intercept traffic but relies on legitimate authenticated session information. The flaw does not require arbitrary code execution or privilege escalation beyond performing actions as the captured user.