REDHAT-BUG-2509761: Low severity sssd vulnerability

A flaw was found in SSSD. The sssnssprotocolfillinitgr() function in the NSS responder (src/responder/nss/nssprotocolgrent.c) pre-allocates the reply packet for all group entries using ssspacketgrow() but does not shrink the packet when groups are skipped (non-POSIX, incomplete, or filtered groups). ssspacketgrow() uses tallocreallocsize(), which does not zero-fill newly allocated memory. The trailing unwritten bytes therefore contain uninitialized heap data from the sssdnss process and are transmitted to the client at the grown packet length. A local attacker can exploit this by sending SSSNSSINITGR (0x0026) requests to the world-writable NSS responder socket (/var/lib/sss/pipes/nss), receiving uninitialized heap content in the reply tail. Through heap grooming (for example, a preceding getpwnam query), the leak can disclose other users' cached directory records and process heap pointers. The leaked data is limited to the sssdnss heap (directory-level information); credentials reside in separate sssdpam and sssdbe processes. Reported via PSIRTSUPT-20553 by BreachX Zero Day Labs.