EEF-CVE-2026-82759

## Summary

Use of a One-Way Hash with a Predictable Salt vulnerability in team-alembic AshAuthentication allows readers of the audit store to recover the client IP addresses that the audit log add-on's `:hash` privacy mode is meant to pseudonymise.

`AshAuthentication.AddOn.AuditLog.IpPrivacy.hash_ip/1` computes a single unkeyed `:crypto.hash(:sha256, salt <> ip)` and truncates the result to 16 hexadecimal characters. The salt is read from the `:audit_log_ip_salt` or `:secret` application config keys, and falls back to the constant `"default-salt-change-in-production"` published in the library source when neither is set, with nothing warning that the default is in use. The IPv4 space is only 2^32 values and SHA-256 is fast, so the whole hash table is precomputable and every stored value maps back to its source address. Truncating to 16 characters does not help, and even a configured salt leaves the hash cheap enough to enumerate once it leaks.

This issue affects ash_authentication: from 4.12.0 before 4.15.0 and from 5.0.0-rc.0 before 5.0.0-rc.14.

## Configurations

Only applications that enable the `audit_log` add-on and set its `ip_privacy` mode to `:hash` store the affected value. Other modes (`:none`, `:truncate`, `:exclude`) do not reach `hash_ip/1`.

The complete break additionally requires the default salt, that is neither `:audit_log_ip_salt` nor `:secret` configured under the `:ash_authentication` application. This is the out-of-the-box state: no warning or error is raised when the hardcoded salt is in use.