Linux KernelOperating system · Linux
CVE-2025-21702
A fix is available.
Upgrade to 5.4.291 / 5.10.235 or later.
See remediation →
72/100
Remediation priority · Elevated
Zero-click
Patch available
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 · uneditedIn the Linux kernel, the following vulnerability has been resolved: pfifo_tail_enqueue: Drop new packet when sch->limit == 0 Expected behaviour: In case we reach scheduler's limit, pfifo_tail_enqueue() will drop a packet in scheduler's queue and decrease scheduler's qlen by one. Then, pfifo_tail_enqueue() enqueue new packet and increase scheduler's qlen by one. Finally, pfifo_tail_enqueue() return `NET_XMIT_CN` status code. Weird behaviour: In case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a scheduler that has no packet, the 'drop a packet' step will do nothing. This means the scheduler's qlen still has value equal 0. Then, we continue to enqueue new packet and increase scheduler's qlen by one. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by one and return `NET_XMIT_CN` status code. The problem is: Let's say we have two qdiscs: Qdisc_A and Qdisc_B. - Qdisc_A's type must have '->graft()' function to create parent/child relationship. Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`. - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`. - Qdisc_B is configured to have `sch->limit == 0`. - Qdisc_A is configured to route the enqueued's packet to Qdisc_B. Enqueue packet through Qdisc_A will lead to: - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B) - Qdisc_B->q.qlen += 1 - pfifo_tail_enqueue() return `NET_XMIT_CN` - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A. The whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1. Replace 'hfsc' with other type (for example: 'drr') still lead to the same problem. This violate the design where parent's qlen should equal to the sum of its childrens'qlen. Bug impact: This issue can be used for user->kernel privilege escalation when it is reachable.
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 analysisA detailed technical summary for this CVE is being prepared.
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:
>= 2.6.34, < 5.4.291>= 5.5, < 5.10.235>= 5.11, < 5.15.179>= 5.16, < 6.1.130>= 6.2, < 6.6.83>= 6.7, < 6.12.14>= 6.13, < 6.13.3= 6.14CVSS 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
- High
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
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 5.4.291 / 5.10.235 / 5.15.179 or later
Fixed in
5.4.2915.10.2355.15.179
Vendor patch
git.kernel.org →
Recommended fix
High confidence
Upgrade to Linux kernel >= 5.4.291, >= 5.10.235, >= 5.15.179, or >= 6.1.130 (recommended: 6.1.x LTS such as 6.1.130 or later)
- Obtain the appropriate kernel upgrade package from your Linux distribution's package repository or kernel.org (e.g., for Debian/Ubuntu: apt-get update && apt-get install linux-image-<version>, for RHEL/CentOS: yum update kernel)
- Ensure the new kernel version is at or above: 5.4.291, 5.10.235, 5.15.179, or 6.1.130
- Install the new kernel package using your distribution's package manager
- Update bootloader configuration (GRUB) if necessary to ensure the new kernel boots by default
- Reboot the system to load the patched kernel
- Verify the running kernel version using 'uname -r' and confirm it matches a fixed release
- Verify the fix is present by checking that pfifo_tail_enqueue drops packets when sch->limit == 0 (the vulnerable code path should no longer exist)
Caveat Kernel upgrades may require updated kernel modules or drivers; ensure compatibility with user-space tools and proprietary modules before rebooting
Generated from the published advisory — verify against the referenced sources before acting.
We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,950.
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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 sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-21702 in production — separate from our analysis above.
Know something about CVE-2025-21702?
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- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
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