A flaw was found in SSSD. In src/responder/pam/pamsrv_cmd.c, pam_forwarder_parse_data() dereferences prctx->cli_protocol_version->version without first checking whether the client negotiated a protocol version with SSS_GET_VERSION. The pointer is initialized to NULL at connection setup and is only populated during version negotiation. A local user able to connect to the PAM responder socket can send a PAM command before SSS_GET_VERSION, causing a NULL pointer dereference and terminating sssd_pam. Repeated requests can sustain denial of SSSD-backed authentication. The review describes a header-only 16-byte request reaching the vulnerable switch. The maintainer confirmed that recent versions do not initialize a non-NULL default and that rejecting requests when cli_protocol_version is NULL is a valid fix. A possible similar issue in sudosrv_cmd() was noted by the maintainer but is not established as part of this flaw. This is distinct from CVE-2026-90995 (missing service item in get_domain_request_type()) and CVE-2026-104042 (zero-length authentication token out-of-bounds read). Source: PSIRTSUPT-25890, finding H-09; security review dated 2026-10-21 and maintainer assessment.