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CVE Record

CVE-2025-21696: mm: clear uffd-wp PTE/PMD state on mremap()

In the Linux kernel, the following vulnerability has been resolved: mm: clear uffd-wp PTE/PMD state on mremap() When mremap()ing a memory region previously registered with userfaultfd as write-protected but without UFFD_FEATURE_EVENT_REMAP, an inconsistency in flag clearing leads to a mismatch between the vma flags (which have uffd-wp cleared) and the pte/pmd flags (which do not have uffd-wp cleared). This mismatch causes a subsequent mprotect(PROT_WRITE) to trigger a warning in page_table_check_pte_flags() due to setting the pte to writable while uffd-wp is still set. Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any such mremap() so that the values are consistent with the existing clearing of VM_UFFD_WP. Be careful to clear the logical flag regardless of its physical form; a PTE bit, a swap PTE bit, or a PTE marker. Cover PTE, huge PMD and hugetlb paths.

MediumCVSS 5.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This Linux kernel flaw can let a local, low-privileged user trigger a kernel warning that may affect system availability. It is not described as remote, and the source bundle does not show data theft or integrity impact.

Executive priority

Treat as a routine but real availability-risk kernel update. Prioritize shared Linux infrastructure, container hosts, and systems with untrusted local users, but do not escalate as remotely exploitable based on these sources.

Technical view

During mremap() of a userfaultfd write-protected memory region without UFFD_FEATURE_EVENT_REMAP, VMA uffd-wp state is cleared but PTE/PMD state can remain. A later writable mprotect() can hit page_table_check_pte_flags(), producing an availability-impacting kernel warning. The fix clears uffd-wp state across PTE, huge PMD, swap PTE, and marker paths.

Likely exposure

Linux systems running affected kernel versions are relevant, especially where untrusted local users or workloads can exercise memory-management features. The bundle identifies Linux as the affected product and does not name distributions or appliances.

Exploitation context

CVE data marks this as local, low-complexity, low-privilege, no-user-interaction, availability-only. KEV is false, and the provided sources do not support active exploitation claims.

Researcher notes

Focus review on userfaultfd write-protect, mremap(), mprotect(PROT_WRITE), and consistency between VMA and PTE/PMD uffd-wp state. Evidence is limited to CVE metadata and kernel stable references; no exploit details or distribution matrix are provided.

Mitigation direction

  • Update to a Linux kernel containing the cited stable fixes.
  • Use distribution vendor kernel advisories for packaged backports.
  • Prioritize multi-user, shared, and container-host systems.
  • Restrict untrusted local access where patching is delayed.

Validation and detection

  • Inventory Linux kernel versions across servers and container hosts.
  • Confirm vendor packages include the referenced stable commits.
  • Check whether affected hosts allow untrusted local workloads.
  • Review kernel logs for relevant page_table_check warnings.
Prepared
Confidence
high
Sources
4

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-2025-21696 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
5.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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
1ADP providers
3Source links

SSVC decision data

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

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
5.5CVSS 3.1MediumCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H1.83.6CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

5.5Medium
CVSS 3.1 vector shape for CVE-2025-21696Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux63b2d4174c4ad1f40b48d7138e71bcb564c1fe03, 63b2d4174c4ad1f40b48d7138e71bcb564c1fe03unaffected
LinuxLinux5.7, 0, 6.12.11, 6.13affected
Weakness

CWE details

No CWE listed

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