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CVE-2026-74656
ipv4: fix use-after-free in fib_nhc_update_mtu()
Published 2026-08-22 (2 months ago) by the Linux kernel CNA. Subsystem include/net. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74656 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
fib_nhc_update_mtu() walks the nexthop exception table under RTNL, but RTNL does not serialize this walk with PMTU exception updates. The walk uses rcu_dereference_protected() with a constant true condition without holding fnhe_lock. The following interleaving can therefore occur: CPU 0 CPU 1 fib_nhc_update_mtu() update_or_create_fnhe() load fnhe spin_lock_bh(&fnhe_lock) fnhe_remove_oldest() unlink fnhe kfree_rcu(fnhe, rcu) <quiescent state> access fnhe after grace period KASAN reported: BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410 Read of size 8 at addr ffff888107d49000 by task poc/90 Call Trace: fib_nhc_update_mtu+0x3df/0x410 fib_sync_mtu+0x7a/0xd0 fib_netdev_event+0x229/0x3f0 netif_set_mtu_ext+0x33a/0x570 dev_set_mtu+0x88/0x120 The same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a pair and other writers serialize them with fnhe_lock. RCU alone prevents reclamation, but would still allow concurrent writers to leave a mixed pair. Walk the table under RCU and acquire fnhe_lock only while updating each exception. RCU keeps the current entry alive while the short critical section serializes its paired PMTU fields. This avoids holding the global lock while scanning all 2048 buckets for every nexthop.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
include/net/route.h
net/ipv4/fib_semantics.c
net/ipv4/route.c
- What this is
- NETWORKING [IPv4/IPv6]MAINTAINERS section matched on include/net/route.h · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_INET /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sid, bookworm · linux-6.12y
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_INET
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-22 (2 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.13%, higher than 2.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- include/net
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 4.19 | 5.10.265 | fd39e7118664 |
| 4.19 | 5.15.216 | e1e602d6b22d |
| 4.19 | 6.1.183 | e00f7d2b5f25 |
| 4.19 | 6.6.152 | dfe388da13aa |
| 4.19 | 6.12.104 | 5a28a4b22dde |
| 4.19 | 6.18.45 | 63996ffc594d |
| 4.19 | 7.1.9 | ed503eaad62f |
| 4.19 | 7.2 | bc5bde9ce3cc |
| 3.16.62 | 0 | 0 |
| 4.4.162 | 0 | 0 |
| 4.9.134 | 0 | 0 |
| 4.14.77 | 0 | 0 |
| 4.18.15 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The UAF is hit in fib_nhc_update_mtu(), entered only via NETDEV_CHANGEMTU from dev_set_mtu()/netif_set_mtu_ext() (SIOCSIFMTU or RTM_NEWLINK IFLA_MTU); per kernel CNA guidance this administrative netdev path is Local even though the racing update_or_create_fnhe() leg is driven by received ICMP PMTU packets.
- AC:LAttack complexity, low
- KASAN reproduced this from task poc/90 by racing dev_set_mtu() against concurrent PMTU exception creation/eviction; an attacker controls both CPUs by running an MTU-change thread alongside ICMP or socket traffic that floods update_or_create_fnhe() and fnhe_remove_oldest(), without depending on uncontrollable victim state.
- PR:LPrivileges required, low
- Changing interface MTU requires CAP_NET_ADMIN checked via ns_capable(net->user_ns, CAP_NET_ADMIN) on SIOCSIFMTU and RTM_SETLINK; unprivileged local users obtain this in a user+network namespace (unshare -Urn) and can then race MTU changes against PMTU exception updates on the same host.
- UI:NUser interaction, none
- Exploitation requires only the attacker's own MTU ioctl/netlink operations and concurrent network or socket traffic to populate/evict fib_nh_exception entries; no victim mount, click, or other voluntary action is needed beyond normal host networking.
- S:UScope, unchanged
- The slab use-after-free corrupts host kernel IPv4 routing state within the same kernel security authority; it is not a VM escape, IOMMU bypass, or other cross-boundary scope change.
- C:HConfidentiality impact, high
- KASAN reported a slab use-after-free read of 8 bytes in fib_nhc_update_mtu() on a fib_nh_exception freed by kfree_rcu() from fnhe_remove_oldest(); UAF on this kmalloc object enables heap grooming for arbitrary kernel memory disclosure.
- I:HIntegrity impact, high
- The buggy walk both reads and writes fnhe_pmtu and fnhe_mtu_locked on potentially freed fib_nh_exception objects without fnhe_lock, providing standard slab reuse primitives for attacker-controlled kernel writes and potential control-flow hijack.
- A:HAvailability impact, high
- The confirmed KASAN slab-use-after-free in fib_nhc_update_mtu() can immediately BUG/oops the kernel; the attacker can retrigger the race by repeating MTU toggles concurrent with PMTU exception flooding for persistent denial of service.