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CVE Record

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.

HighCVSS 7.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2025-21703 is a Linux kernel use-after-free flaw in traffic control queuing logic. It is rated high because a local low-privileged attacker could potentially affect confidentiality, integrity, and availability. The provided sources do not show known active exploitation, but kernel flaws with local privilege impact should be prioritized where multi-user or containerized workloads exist.

Executive priority

Patch in the normal high-priority kernel update cycle, faster for shared or container-hosting environments. There is no source-backed evidence of active exploitation, but the potential impact spans data exposure, tampering, and system availability if locally exploited.

Technical view

The resolved Linux kernel bug is in netem backlog accounting. sch->q.qlen was updated too late before qdisc_tree_reduce_backlog(), so parent qdiscs could miss qlen_notify(). With DRR, that missed notification could leave stale active-list state and produce a use-after-free condition, mapped to CWE-416.

Likely exposure

Exposure is most relevant to Linux systems running affected kernel builds where local users or workloads can reach traffic-control queuing paths. Systems with delegated network administration, container hosts, shared servers, or appliances using netem/DRR-like queuing deserve closer review. The bundle does not prove remote exposure.

Exploitation context

The CVSS vector is local, low complexity, low privileges, and no user interaction. CISA KEV status in the bundle is false, and no cited source states active exploitation. Treat this as a serious local kernel vulnerability, not a confirmed internet-scale exploitation event.

Researcher notes

Focus validation on kernel traffic-control state transitions around netem backlog reduction and DRR qlen_notify handling. The evidence identifies the bug class and fix direction but does not provide enough public source detail to assert exploit reliability, affected distributions beyond listed references, or compensating controls.

Mitigation direction

  • Apply vendor or distribution kernel updates containing the referenced stable fixes.
  • Check Debian LTS guidance if operating affected Debian LTS kernels.
  • Prioritize shared, multi-tenant, container-host, and developer systems first.
  • Review and limit unnecessary local traffic-control administration privileges.
  • Monitor kernel vendor advisories for backport status and replacement packages.

Validation and detection

  • Inventory deployed Linux kernel versions and distribution backport levels.
  • Compare systems against the CVE record and referenced stable commits.
  • Identify hosts using netem, DRR, or delegated traffic-control capabilities.
  • Confirm updated kernels are booted, not merely installed.
  • Document exceptions where vendor guidance is pending or incomplete.
Prepared
Confidence
medium
Sources
11

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-416: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cve · low confidence lookup

CVE-2025-21703 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.8 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
3Timeline events
2ADP providers
10Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: total

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9Linux
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

7.8High
CVSS 3.1 vector shape for CVE-2025-21703Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
cvssV3_1other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux83c6ab12f08dcc09d4c5ac86fdb89736b28f1d31, 216509dda290f6db92c816dd54b83c1df9da9e76, c2047b0e216c8edce227d7c42f99ac2877dad0e4, 10df49cfca73dfbbdb6c4150d859f7e8926ae427, 3824c5fad18eeb7abe0c4fc966f29959552dca3e, 356078a5c55ec8d2061fcc009fb8599f5b0527f9, f8d4bc455047cf3903cd6f85f49978987dbb3027, f8d4bc455047cf3903cd6f85f49978987dbb3027, 5.4.288, 5.10.232, 5.15.175, 6.1.121, 6.6.67, 6.12.6unaffected
LinuxLinux6.13, 0, 5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.78, 6.12.14, 6.13.3, 6.14affected
Weakness

CWE details

CWE-416

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-416 · source CWE mapping

Use After Free

Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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