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CVE-2025-21702 high CVSS 7.8

Vulnerability in Linux kernel (CVE-2025-21702)

Summary

Vulnerability in Linux kernel (CVE-2025-21702). Successful exploitation can lead to full system takeover. Exploitable via NET_XMIT_CN.

Original description

The description as published by the source.

In 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() c…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2025-21702 has been found in Linux kernel.
Attackers can target a specific entry point like NET_XMIT_CN over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 7.8/10.
What to do: apply the vendor's official patched release.
If unsure, ask your IT team or search "Linux kernel CVE-2025-21702" on the vendor's site.

CVE-2025-21702 (Linux kernel) — CVSS v3 7.8
Attack vector: local / no user interaction
Attack surface: NET_XMIT_CN / hfsc / hfsc_enqueue / pfifo_tail_enqueue
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (unclassified) exists in Linux kernel. Attackers reach the vulnerable code path via NET_XMIT_CN.

📍 Affected scope

Check the vendor advisory for the affected versions of Linux kernel. Attack surface: NET_XMIT_CN / hfsc / hfsc_enqueue / pfifo_tail_enqueue.

🔥 Severity

Severity: High (CVSS 7.8/10). Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to the patched release per vendor advisory.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    Audit SBOM/dependencies for affected components.

    資産台帳・SBOM・依存マニフェストで Linux kernel を使っているサービスとバージョンを洗い出す。

  2. 2
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

Linux Kernel
[{"type":"ECOSYSTEM","events":[{"introduced":"2.6.34"},{"fixed":"5.4.291"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.5.0"},{"fixed":"5.10.235"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.11.0"},{"fixed":"5.15.179"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.130"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.83"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.14"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.13.3"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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