CVE-2026-74656
ipv4: fix use-after-free in fib_nhc_update_mtu()
Description
In the Linux kernel, the following vulnerability has been resolved: ipv4: fix use-after-free in fib_nhc_update_mtu() 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.
INFO
Published Date :
Aug. 22, 2026, 4:16 p.m.
Last Modified :
Aug. 22, 2026, 4:16 p.m.
Remotely Exploit :
No
Source :
416baaa9-dc9f-4396-8d5f-8c081fb06d67
Affected Products
The following products are affected by CVE-2026-74656
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
Solution
- Apply the provided patch to the Linux kernel.
- Update the kernel to a patched version.
- Walk table under RCU and lock updates.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-74656.
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-74656 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-74656
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-74656 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-74656 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
New CVE Received by 416baaa9-dc9f-4396-8d5f-8c081fb06d67
Aug. 22, 2026
Action Type Old Value New Value Added Affected [{'repo': 'https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git', 'vendor': 'Linux', 'product': 'Linux', 'versions': [{'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': 'fd39e711866498ae94fcf9acf6f422a4f045b681', 'versionType': 'git'}, {'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': 'e1e602d6b22d5cb1641c4459c487eb18bf569e0a', 'versionType': 'git'}, {'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': 'e00f7d2b5f2540a3415a229c982af7a25ff6362e', 'versionType': 'git'}, {'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': 'dfe388da13aa784851e5ebbea90afbb099075761', 'versionType': 'git'}, {'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': '5a28a4b22dde92f9d293b94236314b8d6181dc4a', 'versionType': 'git'}, {'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': '63996ffc594d128ccec8fc0983f91effd2d3adc4', 'versionType': 'git'}, {'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': 'ed503eaad62f20cdd5122d7c3078a648a99c8f16', 'versionType': 'git'}, {'status': 'affected', 'version': 'af7d6cce53694a88d6a1bb60c9a239a6a5144459', 'lessThan': 'bc5bde9ce3cc36502839dfe98e068f7303a50982', 'versionType': 'git'}, {'status': 'affected', 'version': 'b427832009b97a3ee412ef643a80b15372a7754d', 'versionType': 'git'}, {'status': 'affected', 'version': '2b7e4c735933be79882aba2bed9afa789e03c62f', 'versionType': 'git'}, {'status': 'affected', 'version': '8d59c3a6376bbc6dd3f7303968a719ffba75a4f1', 'versionType': 'git'}, {'status': 'affected', 'version': '9b4869cf385aa16f89c0f019eed4ec4e36aa441c', 'versionType': 'git'}, {'status': 'affected', 'version': 'ff34695ced21e2dfa04d0fa1c5f6c35011fa8117', 'versionType': 'git'}, {'status': 'affected', 'version': '3.16.62', 'lessThan': '3.17', 'versionType': 'semver'}, {'status': 'affected', 'version': '4.4.162', 'lessThan': '4.5', 'versionType': 'semver'}, {'status': 'affected', 'version': '4.9.134', 'lessThan': '4.10', 'versionType': 'semver'}, {'status': 'affected', 'version': '4.14.77', 'lessThan': '4.15', 'versionType': 'semver'}, {'status': 'affected', 'version': '4.18.15', 'lessThan': '4.19', 'versionType': 'semver'}], 'programFiles': ['include/net/route.h', 'net/ipv4/fib_semantics.c', 'net/ipv4/route.c'], 'defaultStatus': 'unaffected'}, {'repo': 'https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git', 'vendor': 'Linux', 'product': 'Linux', 'versions': [{'status': 'affected', 'version': '4.19'}, {'status': 'unaffected', 'version': '0', 'lessThan': '4.19', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '5.10.265', 'versionType': 'semver', 'lessThanOrEqual': '5.10.*'}, {'status': 'unaffected', 'version': '5.15.216', 'versionType': 'semver', 'lessThanOrEqual': '5.15.*'}, {'status': 'unaffected', 'version': '6.1.183', 'versionType': 'semver', 'lessThanOrEqual': '6.1.*'}, {'status': 'unaffected', 'version': '6.6.152', 'versionType': 'semver', 'lessThanOrEqual': '6.6.*'}, {'status': 'unaffected', 'version': '6.12.104', 'versionType': 'semver', 'lessThanOrEqual': '6.12.*'}, {'status': 'unaffected', 'version': '6.18.45', 'versionType': 'semver', 'lessThanOrEqual': '6.18.*'}, {'status': 'unaffected', 'version': '7.1.9', 'versionType': 'semver', 'lessThanOrEqual': '7.1.*'}, {'status': 'unaffected', 'version': '7.2', 'versionType': 'original_commit_for_fix', 'lessThanOrEqual': '*'}], 'programFiles': ['include/net/route.h', 'net/ipv4/fib_semantics.c', 'net/ipv4/route.c'], 'defaultStatus': 'affected'}] Added Description In the Linux kernel, the following vulnerability has been resolved: ipv4: fix use-after-free in fib_nhc_update_mtu() 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. Added Reference https://git.kernel.org/stable/c/5a28a4b22dde92f9d293b94236314b8d6181dc4a Added Reference https://git.kernel.org/stable/c/63996ffc594d128ccec8fc0983f91effd2d3adc4 Added Reference https://git.kernel.org/stable/c/bc5bde9ce3cc36502839dfe98e068f7303a50982 Added Reference https://git.kernel.org/stable/c/dfe388da13aa784851e5ebbea90afbb099075761 Added Reference https://git.kernel.org/stable/c/e00f7d2b5f2540a3415a229c982af7a25ff6362e Added Reference https://git.kernel.org/stable/c/e1e602d6b22d5cb1641c4459c487eb18bf569e0a Added Reference https://git.kernel.org/stable/c/ed503eaad62f20cdd5122d7c3078a648a99c8f16 Added Reference https://git.kernel.org/stable/c/fd39e711866498ae94fcf9acf6f422a4f045b681