Use-after-free in Linux kernel netfilter ctnetlink_dump_exp_ct()
CVE-2026-23458 is a use-after-free vulnerability in the Linux kernel netfilter ctnetlink component, specifically in ctnetlink_dump_exp_ct(). The function stores a conntrack pointer in cb->data for use by the netlink dump callback ctnetlink_exp_ct_dump_table(), but drops the conntrack reference immediately after netlink_dump_start(). If the netlink dump spans multiple rounds, a subsequent recvmsg() can invoke the dump callback after the conntrack object has already been freed, and nfct_help(ct) then dereferences freed memory via ct->ext. The root cause is missing netlink_dump_control .start and .done callbacks to retain and release the conntrack reference across the full lifetime of the dump operation. The fix adds those callbacks and also moves the nfct_help() call after the cb->args[0] early-return check to avoid unnecessary dereferencing.
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