A flaw was found in SSSD. The extractauthtokv1() function in the PAM responder (src/responder/pam/pamsrvcmd.c) reads a 32-bit authtokenlength from the client request but does not validate it against the remaining buffer size before passing it to sssauthtoksetpassword(). When sssauthtoksetstring() processes the token, it accesses str[len-1] using the unvalidated length, resulting in a heap-buffer-overflow read. A local attacker can trigger this by connecting to the world-writable PAM responder socket (/var/lib/sss/pipes/pam) and sending a crafted protocol v1 authentication request with an oversized authtokenlength value. The stock PAM client uses protocol v3, so this is only reachable via raw socket access. Protocol v2 includes an equivalent bounds check (c+datasize > blen), which is missing from v1. Successful exploitation crashes the sssdpam responder, causing a denial of service for SSSD-mediated authentication. Reported via PSIRTSUPT-20553 by BreachX Zero Day Labs.