Siemens SIMATIC S7-1500 TM MFP Improper Update of Reference Count (CVE-2026-46099)

#%NASL_MIN_LEVEL 80900
##
# (C) Tenable, Inc.
##

include('compat.inc');

if (description)
{
  script_id(506470);
  script_version("1.2");
  script_set_attribute(attribute:"plugin_modification_date", value:"2026/10/21");

  script_cve_id("CVE-2026-46099");

  script_name(english:"Siemens SIMATIC S7-1500 TM MFP Improper Update of Reference Count (CVE-2026-46099)");

  script_set_attribute(attribute:"synopsis", value:
"The remote OT asset is affected by a vulnerability.");
  script_set_attribute(attribute:"description", value:
"In the Linux kernel, the following vulnerability has been resolved:

net: ipv6: fix NOREF dst use in seg6 and rpl lwtunnels

seg6_input_core() and rpl_input() call ip6_route_input() which sets a
NOREF dst on the skb, then pass it to dst_cache_set_ip6() invoking
dst_hold() unconditionally.
On PREEMPT_RT, ksoftirqd is preemptible and a higher-priority task can
release the underlying pcpu_rt between the lookup and the caching
through a concurrent FIB lookup on a shared nexthop.
Simplified race sequence:

  ksoftirqd/X                       higher-prio task (same CPU X)
  -----------                       --------------------------------
  seg6_input_core(,skb)/rpl_input(skb)
    dst_cache_get()
      -> miss
    ip6_route_input(skb)
      -> ip6_pol_route(,skb,flags)
         [RT6_LOOKUP_F_DST_NOREF in flags]
        -> FIB lookup resolves fib6_nh
           [nhid=N route]
        -> rt6_make_pcpu_route()
           [creates pcpu_rt, refcount=1]
             pcpu_rt->sernum = fib6_sernum
             [fib6_sernum=W]
           -> cmpxchg(fib6_nh.rt6i_pcpu,
                      NULL, pcpu_rt)
              [slot was empty, store succeeds]
      -> skb_dst_set_noref(skb, dst)
         [dst is pcpu_rt, refcount still 1]

                                    rt_genid_bump_ipv6()
                                      -> bumps fib6_sernum
                                         [fib6_sernum from W to Z]
                                    ip6_route_output()
                                      -> ip6_pol_route()
                                        -> FIB lookup resolves fib6_nh
                                           [nhid=N]
                                        -> rt6_get_pcpu_route()
                                             pcpu_rt->sernum != fib6_sernum
                                             [W <> Z, stale]
                                          -> prev = xchg(rt6i_pcpu, NULL)
                                          -> dst_release(prev)
                                             [prev is pcpu_rt,
                                              refcount 1->0, dead]

    dst = skb_dst(skb)
    [dst is the dead pcpu_rt]
    dst_cache_set_ip6(dst)
      -> dst_hold() on dead dst
      -> WARN / use-after-free

For the race to occur, ksoftirqd must be preemptible (PREEMPT_RT without
PREEMPT_RT_NEEDS_BH_LOCK) and a concurrent task must be able to release
the pcpu_rt. Shared nexthop objects provide such a path, as two routes
pointing to the same nhid share the same fib6_nh and its rt6i_pcpu
entry.

Fix seg6_input_core() and rpl_input() by calling skb_dst_force() after
ip6_route_input() to force the NOREF dst into a refcounted one before
caching.
The output path is not affected as ip6_route_output() already returns a
refcounted dst.

This plugin only works with Tenable.ot.
Please visit https://www.tenable.com/products/tenable-ot for more information.");
  script_set_attribute(attribute:"see_also", value:"https://cert-portal.siemens.com/productcert/html/ssa-784472.html");
  script_set_attribute(attribute:"solution", value:
"Refer to the vendor advisory.");
  script_set_cvss_base_vector("CVSS2#AV:N/AC:H/Au:N/C:C/I:C/A:C");
  script_set_cvss_temporal_vector("CVSS2#E:U/RL:OF/RC:C");
  script_set_cvss3_base_vector("CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H");
  script_set_cvss3_temporal_vector("CVSS:3.0/E:U/RL:O/RC:C");
  script_set_attribute(attribute:"cvss_score_source", value:"CVE-2026-46099");

  script_set_attribute(attribute:"exploitability_ease", value:"No known exploits are available");
  script_set_attribute(attribute:"exploit_available", value:"false");
  script_cwe_id(911);

  script_set_attribute(attribute:"vuln_publication_date", value:"2026/10/13");
  script_set_attribute(attribute:"patch_publication_date", value:"2026/10/13");
  script_set_attribute(attribute:"plugin_publication_date", value:"2026/10/20");

  script_set_attribute(attribute:"plugin_type", value:"remote");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:siemens:simatic_s7-1500_tm_mfp");
  script_set_attribute(attribute:"generated_plugin", value:"former");
  script_end_attributes();

  script_category(ACT_GATHER_INFO);
  script_family(english:"Tenable.ot");

  script_copyright(english:"This script is Copyright (C) 2026 and is owned by Tenable, Inc. or an Affiliate thereof.");

  script_dependencies("tenable_ot_api_integration.nasl");
  script_require_keys("Tenable.ot/Siemens");

  exit(0);
}


include('tenable_ot_cve_funcs.inc');

get_kb_item_or_exit('Tenable.ot/Siemens');

var asset = tenable_ot::assets::get(vendor:'Siemens');

var vuln_cpes = {
    "cpe:/o:siemens:simatic_s7-1500_tm_mfp" :
        {"versionEndIncluding" : "1.1.1", "versionStartIncluding" : "1.1.1", "family" : "S71500", "orderNumbers": ["6ES7558-1AA00-0AB0"]}
};

tenable_ot::cve::compare_and_report(asset:asset, cpes:vuln_cpes, severity:SECURITY_HOLE);