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CVE Record

CVE-2025-21765: ipv6: use RCU protection in ip6_default_advmss()

In the Linux kernel, the following vulnerability has been resolved: ipv6: use RCU protection in ip6_default_advmss() ip6_default_advmss() needs rcu protection to make sure the net structure it reads does not disappear.

HighCVSS 8.1Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A Linux kernel concurrency flaw in IPv6 code can allow a network-related kernel object to disappear while still being read. The supplied CVSS 8.1 assessment indicates potentially severe confidentiality, integrity, and availability impact, although exploitation requires difficult timing conditions. Kernel updates containing the upstream fix are the appropriate remediation direction.

Executive priority

Treat this as a high-priority kernel maintenance issue, especially for exposed or critical Linux infrastructure. Schedule vendor-approved updates and reboots promptly, while avoiding emergency claims unsupported by exploitation evidence. Escalate systems whose package status cannot be mapped confidently to a fixed vendor build.

Technical view

ip6_default_advmss() reads a network namespace structure without the required read-copy-update protection, creating a race with object removal. The stable-kernel changes add RCU protection to prevent the structure from disappearing during access. The supplied vector is network-accessible, unauthenticated, high-complexity, and requires no user interaction.

Likely exposure

Potential exposure includes systems running affected Linux kernel versions that can reach the relevant IPv6 path. The supplied version data is ambiguous and should not be treated as complete package-level guidance. Map the running kernel and distribution package to Linux stable commits or the applicable vendor advisory.

Exploitation context

The supplied record is not in CISA KEV, and no provided source reports active exploitation. The CVSS vector describes network reachability without privileges, but high attack complexity suggests a timing-dependent race. Public exploit availability and practical reliability are not established by this source bundle.

Researcher notes

The core issue is missing RCU protection around a network-structure read in ip6_default_advmss(). Eight stable-kernel commits are referenced, indicating fixes across maintained branches. The supplied affected-version representation lacks clear range semantics, so researchers should inspect the CVE record, commit ancestry, and downstream backport status before declaring a build vulnerable.

Mitigation direction

  • Install a vendor-supported kernel containing the applicable Linux stable fix.
  • Reboot into the updated kernel; installing a package alone does not replace the running kernel.
  • If immediate updating is impossible, obtain temporary mitigation guidance from the operating-system vendor.
  • Prioritize externally exposed and high-value Linux systems using affected kernel packages.

Validation and detection

  • Inventory running kernel releases, distribution package revisions, and IPv6 usage across Linux assets.
  • Compare each package against its distribution advisory and the referenced stable-kernel fix.
  • After updating and rebooting, confirm the running kernel is the remediated build.
  • Verify vulnerability scanners recognize vendor backports rather than relying only on upstream version numbers.
Prepared
Confidence
medium
Sources
12

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

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CVE-2025-21765 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
8.1 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:H/PR:N/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.

1CVSS vectors
3Timeline events
2ADP providers
14Source links

CVSS vector scores

1 official score

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
8.1CVSS 3.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H2.25.9Linux

Vulnerability scoring details

Base CVSS 3.1 score

8.1High
CVSS 3.1 vector shape for CVE-2025-21765Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:H/PR:N/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

CVECVE Program Container
siemens-SADPADP container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux5578689a4e3c04f2d43ea39736fd3fa396d80c6e, 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, 5578689a4e3c04f2d43ea39736fd3fa396d80c6eunaffected
LinuxLinux2.6.26, 0, 5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.79, 6.12.16, 6.13.4, 6.14affected
Weakness

CWE details

No CWE listed

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