CVE-2025-21703 — netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()
In the Linux kernel, the following vulnerability has been resolved: netem: Update sch->q.qlen before qdisc_tree_reduce_backlog() qdisc_tree_reduce_backlog() notifies parent qdisc only if child qdisc becomes empty, therefore we need to reduce the backlog of the child qdisc before calling it. Otherwise it would miss the opportunity to call cops->qlen_notify(), in the case of DRR, it resulted in UAF since DRR uses ->qlen_notify() to maintain its active list.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.3% (30-day exploitation probability)+3
- Technical severity (high)+6
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
- Documented patch — git.kernel.org (NIST National Vulnerability Database)
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Linux Linux, Linux Linux Kernel).
- Apply the fixed version or the patch documented under Remediation.
- Search available logs for exploit probes matching the vulnerability class (memory corruption), errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Linux | Linux |
≥ 10df49cfca73dfbbdb6c4150d859f7e8926ae427 and < 7b79ca9a1de6a428d486ff52fb3d602321c08f55version 6.13≥ 6.12.6 and < 6.12.14≥ 6.6.67 and < 6.6.78≥ 6.1.121 and < 6.1.129≥ 5.15.175 and < 5.15.179≥ 5.10.232 and < 5.10.235≥ 5.4.288 and < 5.4.291 and 7 more≥ f8d4bc455047cf3903cd6f85f49978987dbb3027 and < 638ba5089324796c2ee49af10427459c2de35f71≥ f8d4bc455047cf3903cd6f85f49978987dbb3027 and < 839ecc583fa00fab785fde1c85a326743657fd32≥ 356078a5c55ec8d2061fcc009fb8599f5b0527f9 and < 6312555249082d6d8cc5321ff725df05482d8b83≥ 3824c5fad18eeb7abe0c4fc966f29959552dca3e and < 1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5≥ c2047b0e216c8edce227d7c42f99ac2877dad0e4 and < 98a2c685293aae122f688cde11d9334dddc5d207≥ 216509dda290f6db92c816dd54b83c1df9da9e76 and < 7f31d74fcc556a9166b1bb20515542de7bb939d1≥ 83c6ab12f08dcc09d4c5ac86fdb89736b28f1d31 and < e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c |
Vulnerable | CVE Program (cve.org) e CISA ADP |
| Linux | Linux Kernel | ≥ 5.4.288 and < 5.4.291version 6.14-rc1≥ 6.13 and < 6.13.3≥ 6.12.6 and < 6.12.14≥ 6.6.67 and < 6.6.78≥ 6.1.121 and < 6.1.129≥ 5.15.175 and < 5.15.179≥ 5.10.232 and < 5.10.235 | Vulnerable | NIST National Vulnerability Database |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 7.8 | CVSS 3.1 | High | 1.8 | 5.9 | 416baaa9-dc9f-4396-8d5f-8c081fb06d67 via NIST National Vulnerability Database |
| 7.8 | CVSS 3.1 | High | 1.8 | 5.9 | 134c704f-9b21-4f2e-91b3-4a467353bcc0 via NIST National Vulnerability Database |
| 7.8 | CVSS 3.1 | High | — | — | GitHub, Inc. via GitHub Security Advisories |
| 7.8 | CVSS 3.1 | High | — | — | Linux via CVE Program (cve.org) e CISA ADP |
| 7.8 | CVSS 3.1 | High | — | — | CISA-ADP via CVE Program (cve.org) e CISA ADP |
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (score shown at the top: 416baaa9-dc9f-4396-8d5f-8c081fb06d67) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
EPSS over time FIRST
Oct 14, 2026 → Oct 16, 2026 · 3 reading(s) · max 0.30%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Local/physical vector: focus on endpoint telemetry (EDR) from affected hosts.
- Correlate service crashes/restarts with inbound traffic; repeated crashes may indicate attempts.
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · linux/linux
- SSVC assessment updated
- Record published
- Fix documented
- Affected products added · linux/linux kernel
- CVSS changed · 7.8
- Source status changed · → Modified
- Record created in this database
- 2025
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 2× References added · Oct 14, 2026 – Oct 14, 2026
- 1× Official description updated · Oct 14, 2026 – Oct 14, 2026
References
13 on the record
- git.kernel.org/stable/c/7b79ca9a1de6a428d486ff52fb3d602321c08f55
- git.kernel.org/stable/c/638ba5089324796c2ee49af10427459c2de35f71
- git.kernel.org/stable/c/98a2c685293aae122f688cde11d9334dddc5d207
- git.kernel.org/stable/c/e395fec75ac2dbffc99b4bce57b7f1f3c5449f2c
- git.kernel.org/stable/c/7f31d74fcc556a9166b1bb20515542de7bb939d1
- git.kernel.org/stable/c/839ecc583fa00fab785fde1c85a326743657fd32
- git.kernel.org/stable/c/1f8e3f4a4b8b90ad274dfbc66fc7d55cb582f4d5
- git.kernel.org/stable/c/6312555249082d6d8cc5321ff725df05482d8b83
- github.com/advisories/GHSA-cg86-m5xc-jqrm
- nvd.nist.gov/vuln/detail/CVE-2025-21703
- cert-portal.siemens.com/productcert/html/ssa-784472.html
- cert-portal.siemens.com/productcert/html/ssa-503939.html
- lists.debian.org/debian-lts-announce/2025/03/msg00028.html
Elsewhere on this site
Related CVEs (same products)
- Critical CVE-2026-99999 ipv4: validate checksum_start before completing checksum 2d
- Unknown CVE-2026-98432 ipv4: free inet_opt and ireq_opt after an RCU grace period 2d
- Unknown CVE-2026-98424 wifi: cfg80211: undo netns switch if renaming the wiphy fails 3d
- Unknown CVE-2026-98425 wifi: mac80211: don't warn when an IBSS has no channel to scan 3d
- Unknown CVE-2026-98426 wifi: mwifiex: fix IRQ leak using wrong index in MSI-X error path 3d
- Unknown CVE-2026-98427 net/sched: codel: bound the dropping loop per dequeue call 3d
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2025-21703 is "netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()", rated High. It affects Linux (Linux); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.8 describes technical severity. EPSS 0.30% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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