Description
Linux-PAM through 1.7.2 contains an observable timing discrepancy (CWE-208) in the pam_userdb module's plaintext-password comparison path in modules/pam_userdb/pam_userdb.c that allows a local or network-adjacent attacker able to repeatedly drive authentication through a calling service to recover the plaintext password of a target account by measuring response-timing differences. The comparison uses strncmp() (or strncasecmp() when PAM_ICASE_ARG is set) preceded by a length-equality check, so 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, leaking the password length and individual prefix bytes. The vulnerable path is reached when the administrator configures pam_userdb with crypt=none, with an unrecognized crypt method, or without a crypt= argument, causing the module to store and compare credentials in plaintext.
An attacker repeatedly authenticates through a service that calls pam_userdb (plaintext mode) and measures the time the service takes to reject each candidate password to learn the password length and recover the password one byte at a time.
Problem types
CWE-208 Observable Timing Discrepancy
Product status
Any version
Credits
Xurshidbek Sobirjonov
References
github.com/linux-pam/linux-pam (Linux-PAM - upstream repository)
github.com/...am/blob/master/modules/pam_userdb/pam_userdb.c (Vulnerable plaintext-password comparison in pam_userdb.c (master))
github.com/...ux-pam/blob/master/libpam/include/pam_inline.h (pam_consttime_streq helper available for the remediation)
cwe.mitre.org/data/definitions/208.html (CWE-208: Observable Timing Discrepancy)