Linux‑PAM versions up to 1.7.2 contain an observable timing discrepancy (CWE‑208) in the pam_userdb module’s plaintext‑password comparison routine, which allows a local or network‑adjacent attacker to recover a target account’s plaintext password by measuring response‑timing differences. The comparison uses a length equality check followed by a byte‑by‑byte comparison with strncmp() (or strncasecmp() when case‑insensitive mode is enabled); the time to reject a candidate depends on the index of the first differing byte and on whether the candidate’s length matches the stored password, thereby leaking the password length and successive prefix bytes.

The flaw is present in Linux‑PAM versions through 1.7.2 whenever the pam_userdb module is configured with crypt=none, an unrecognized crypt method, or with no crypt argument, causing the module to store and compare credentials in cleartext. Services that use pam_userdb for authentication – such as SSH, sudo, or other PAM‑enabled applications – may thus expose user passwords to an attacker who can repeatedly trigger authentication.

The CVSS score of 6.9 indicates moderate severity. No EPSS score is available, and the vulnerability is not listed in the CISA KEV catalog. A local or network‑adjacent attacker who can repeatedly drive authentication against the affected service can recover the full plaintext password by timing the comparison. The attack requires only that the service using pam_userdb be reachable, so restricting network exposure can reduce the likelihood of exploitation.