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Linux KernelOperating system · Linux

CVE-2026-53080

MEDIUM · 5.5 CVSS v3.1 Published 2026-06-24
Fix available
A fix is available. Upgrade to 5.10.259 / 5.15.210 or later.
See remediation →
57/100
Remediation priority · Elevated
Zero-click 8 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_fw: fix NULL dereference of "old" filters before change() Like pointed out by Sashiko [1], since commit ed76f5edccc9 ("net: sched: protect filter_chain list with filter_chain_lock mutex") TC filters are added to a shared block and published to datapath before their ->change() function is called. This is a problem for cls_fw: an invalid filter created with the "old" method can still classify some packets before it is destroyed by the validation logic added by Xiang. Therefore, insisting with repeated runs of the following script: # ip link add dev crash0 type dummy # ip link set dev crash0 up # mausezahn crash0 -c 100000 -P 10 \ > -A 4.3.2.1 -B 1.2.3.4 -t udp "dp=1234" -q & # sleep 1 # tc qdisc add dev crash0 egress_block 1 clsact # tc filter add block 1 protocol ip prio 1 matchall \ > action skbedit mark 65536 continue # tc filter add block 1 protocol ip prio 2 fw # ip link del dev crash0 can still make fw_classify() hit the WARN_ON() in [2]: WARNING: ./include/net/pkt_cls.h:88 at fw_classify+0x244/0x250 [cls_fw], CPU#18: mausezahn/1399 Modules linked in: cls_fw(E) act_skbedit(E) CPU: 18 UID: 0 PID: 1399 Comm: mausezahn Tainted: G E 7.0.0-rc6-virtme #17 PREEMPT(full) Tainted: [E]=UNSIGNED_MODULE Hardware name: Red Hat KVM, BIOS 1.16.3-2.el9 04/01/2014 RIP: 0010:fw_classify+0x244/0x250 [cls_fw] Code: 5c 49 c7 45 00 00 00 00 00 41 5d 41 5e 41 5f 5d c3 cc cc cc cc 5b b8 ff ff ff ff 41 5c 41 5d 41 5e 41 5f 5d c3 cc cc cc cc 90 <0f> 0b 90 eb a0 0f 1f 80 00 00 00 00 90 90 90 90 90 90 90 90 90 90 RSP: 0018:ffffd1b7026bf8a8 EFLAGS: 00010202 RAX: ffff8c5ac9c60800 RBX: ffff8c5ac99322c0 RCX: 0000000000000004 RDX: 0000000000000001 RSI: ffff8c5b74d7a000 RDI: ffff8c5ac8284f40 RBP: ffffd1b7026bf8d0 R08: 0000000000000000 R09: ffffd1b7026bf9b0 R10: 00000000ffffffff R11: 0000000000000000 R12: 0000000000010000 R13: ffffd1b7026bf930 R14: ffff8c5ac8284f40 R15: 0000000000000000 FS: 00007fca40c37740(0000) GS:ffff8c5b74d7a000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fca40e822a0 CR3: 0000000005ca0001 CR4: 0000000000172ef0 Call Trace: <TASK> tcf_classify+0x17d/0x5c0 tc_run+0x9d/0x150 __dev_queue_xmit+0x2ab/0x14d0 ip_finish_output2+0x340/0x8f0 ip_output+0xa4/0x250 raw_sendmsg+0x147d/0x14b0 __sys_sendto+0x1cc/0x1f0 __x64_sys_sendto+0x24/0x30 do_syscall_64+0x126/0xf80 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7fca40e822ba Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb b8 0f 1f 00 f3 0f 1e fa 41 89 ca 64 8b 04 25 18 00 00 00 85 c0 75 15 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 7e c3 0f 1f 44 00 00 41 54 48 83 ec 30 44 89 RSP: 002b:00007ffc248a42c8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c RAX: ffffffffffffffda RBX: 000055ef233289d0 RCX: 00007fca40e822ba RDX: 000000000000001e RSI: 000055ef23328c30 RDI: 0000000000000003 RBP: 000055ef233289d0 R08: 00007ffc248a42d0 R09: 0000000000000010 R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000001e R13: 00000000000186a0 R14: 0000000000000000 R15: 00007fca41043000 </TASK> irq event stamp: 1045778 hardirqs last enabled at (1045784): [<ffffffff864ec042>] __up_console_sem+0x52/0x60 hardirqs last disabled at (1045789): [<ffffffff864ec027>] __up_console_sem+0x37/0x60 softirqs last enabled at (1045426): [<ffffffff874d48c7>] __alloc_skb+0x207/0x260 softirqs last disabled at (1045434): [<ffffffff874fe8f8>] __dev_queue_xmit+0x78/0x14d0 Then, because of the value in the packet's mark, dereference on 'q->handle' with NULL 'q' occurs: BUG: kernel NULL pointer dereference, address: 0000000000000038 [...] RIP: 0010:fw_classify+0x1fe/0x250 [cls_fw] [...] Skip "old-style" classification on shared blocks, so that the NULL dereference is fixed and WARN_ON() is not hit anymore in the short lifetime of invalid cls_fw "old-style" filters. [1] https://sashiko.dev/#/patchset/2 ---truncated---

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Kernel NULL pointer dereference vulnerability in net/sched/cls_fw (fw_classify function). Since commit ed76f5edccc9, TC filters are published to datapath before their ->change() function validates them. This allows invalid 'old-style' cls_fw filters to still classify packets, causing NULL dereference on 'q->handle' when 'q' is NULL and hitting a WARN_ON in the short window before validation destroys the invalid filter.

MitigationApply the kernel patch to skip old-style classification on shared blocks; until patch is available, avoid using cls_fw filters on shared blocks with the 'old' method.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 5.1, < 5.10.259>= 5.11, < 5.15.210>= 5.16, < 6.1.176>= 6.2, < 6.6.143>= 6.7, < 6.12.93>= 6.13, < 6.18.35>= 6.19, < 7.0.10

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check kernel version against affected ranges
    Run `uname -r` or `cat /proc/version` and compare the kernel version to the affected ranges: >= 5.1 to < 5.10.259, >= 5.11 to < 5.15.210, >= 5.16 to < 6.1.176, >= 6.2 to < 6.6.143, >= 6.7 to < 6.12.93, >= 6.13 to < 6.18.35, or >= 6.19 to < 7.0.10
    Affected if The kernel version falls within any of the listed affected ranges
  2. Verify cls_fw classifier is in use
    Run `tc filter show` or `tc class show` to list any configured traffic control filters. Also check `lsmod | grep -i fw` or look for fw classifier references in `/proc/net/`
    Affected if Any fw (firewall) classifier filters are actively configured in the traffic control subsystem
  3. Check for fw filters on shared blocks
    Run `tc filter show dev <interface>` for each network interface to inspect filter configurations. Look for 'handle' values and block associations in the output. Check if any fw filters reference shared blocks using `tc qdisc show` with block ID information
    Affected if Fw filters are configured on shared blocks (block ID associated with the qdisc)
  4. Inspect packet mark usage with fw filters
    Review firewall rules (iptables, nftables) for MARK targets that set packet marks. Cross-reference these mark values with any fw classifier filters seen in step 2 and 3. Use `iptables -t mangle -L -n -v` or `nft list ruleset` to list mark actions
    Affected if Packet marks are set via firewall rules AND those marks could match fw filter rules on shared blocks

If the kernel version is affected AND cls_fw filters are configured on shared blocks with packet marking in use, the environment is vulnerable to kernel crashes via NULL pointer dereference.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.10.259 / 5.15.210 / 6.1.176 or later
Fixed in 5.10.2595.15.2106.1.176
Interim mitigation

Apply the kernel patch to skip old-style classification on shared blocks; until patch is available, avoid using cls_fw filters on shared blocks with the 'old' method.

Recommended fix High confidence

Upgrade to Linux kernel >= 5.10.259, >= 5.15.210, >= 6.1.176, or >= 6.6.143 (depending on your major version branch)

  1. Identify the currently running Linux kernel version using `uname -r`
  2. Check if the running kernel version falls within any of the affected ranges: >= 5.1 and < 5.10.259, >= 5.11 and < 5.15.210, >= 5.16 and < 6.1.176, or >= 6.2 and < 6.6.143
  3. If affected, update the system packages to install a newer kernel version
  4. For RHEL/CentOS: run `yum update kernel` or `dnf update kernel`
  5. For Debian/Ubuntu: run `apt-get update && apt-get upgrade` or `apt-get dist-upgrade`
  6. Reboot the system to load the new kernel using `systemctl reboot`
  7. Verify the new kernel version is running with `uname -r` and confirm it is a fixed version
Caveat Kernel upgrades may require system reboot and could introduce compatibility issues with custom kernel modules or older drivers; test in staging first

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Linux Kernel Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing8.0 h
  • Review / QA4.0 h
21.0 hours of engineering $3,600
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

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