{"id":"CVE-2026-23274","state":"PUBLISHED","title":"netfilter: xt_IDLETIMER: reject rev0 reuse of ALARM timer labels","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnetfilter: xt_IDLETIMER: reject rev0 reuse of ALARM timer labels\n\nIDLETIMER revision 0 rules reuse existing timers by label and always call\nmod_timer() on timer->timer.\n\nIf the label was created first by revision 1 with XT_IDLETIMER_ALARM,\nthe object uses alarm timer semantics and timer->timer is never initialized.\nReusing that object from revision 0 causes mod_timer() on an uninitialized\ntimer_list, triggering debugobjects warnings and possible panic when\npanic_on_warn=1.\n\nFix this by rejecting revision 0 rule insertion when an existing timer with\nthe same label is of ALARM 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xt_IDLETIMER: reject rev0 reuse of ALARM timer labels","descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nnetfilter: xt_IDLETIMER: reject rev0 reuse of ALARM timer labels\n\nIDLETIMER revision 0 rules reuse existing timers by label and always call\nmod_timer() on timer->timer.\n\nIf the label was created first by revision 1 with XT_IDLETIMER_ALARM,\nthe object uses alarm timer semantics and timer->timer is never initialized.\nReusing that object from revision 0 causes mod_timer() on an uninitialized\ntimer_list, triggering debugobjects warnings and possible panic when\npanic_on_warn=1.\n\nFix this by rejecting revision 0 rule insertion when an existing timer with\nthe same label is of ALARM 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- This is accessed through the iptables/netfilter sockopt interface (IPT_SO_SET_REPLACE), which requires local system access to create iptables rules. This is a local attack vector via setsockopt syscall.\nAC:L - The attacker can reliably trigger this by first creating a rev1 IDLETIMER rule with XT_IDLETIMER_ALARM flag and a chosen label, then creating a rev0 rule with the same label. No race condition or special conditions needed — this is a deterministic two-step operation.\nPR:L - The iptables interface requires CAP_NET_ADMIN, but ip_tables.c uses ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN), meaning an unprivileged user can obtain this capability via user namespaces (unshare -Urn), making this reachable from a low-privilege user.\nUI:N - No user interaction is required. The attacker creates both iptables rules themselves without any victim action.\nS:U - The vulnerability stays within the kernel's security scope — there is no crossing of a security boundary like VM escape or sandbox escape. It's a standard kernel privilege escalation scenario.\nC:H - The bug calls mod_timer() on an uninitialized timer_list from kmalloc'd memory. Since kmalloc does not zero memory, the timer_list contains stale heap content which an attacker can influence via heap spraying, potentially achieving arbitrary read through controlled function pointers and timer wheel manipulation.\nI:H - Operating mod_timer() on a timer_list with attacker-influenced stale heap data (uninitialized function pointer, hlist_node entries) can corrupt timer wheel data structures and potentially achieve arbitrary code execution if the function pointer is controlled through heap spraying.\nA:H - At minimum, mod_timer() on uninitialized memory triggers debug_assert_init() WARN which causes panic with panic_on_warn=1. 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