Security readout for executives and security teams
Plain-English summary
CVE-2025-21809 is a Linux kernel locking bug in rxrpc/AFS handling. Under certain local conditions, the kernel can deadlock while cleaning up peer references, causing availability impact. The published CVSS is 5.5 medium, with local access and low privileges required.
Executive priority
Treat as a moderate availability issue. Patch through normal kernel maintenance, with higher priority for systems using AFS/rxrpc or where local-user denial of service would affect critical services.
Technical view
AFS address-list cleanup runs under RCU in BH context and may release rxrpc_peer references. rxrpc_put_peer removes peers from a hash table using peer_hash_lock. The lock was taken inconsistently between normal and softirq contexts, creating a deadlock risk. The fix changes rxnet->peer_hash_lock back to BH-disabling locking.
Likely exposure
Exposure is limited to Linux systems running affected kernel versions with relevant rxrpc/AFS code present or reachable. The bundle lists Linux as affected, including versions 6.8, 6.12.13, 6.13.2, and 6.14, but organizations should confirm exact distro backports.
Exploitation context
The source bundle does not report active exploitation, and KEV is false. CVSS indicates local attack vector, low privileges, no user interaction, and high availability impact. Evidence supports denial-of-service risk, not data theft or integrity compromise.
Researcher notes
The evidence is a resolved Linux kernel concurrency flaw, not a published exploit. Analysis should focus on kernel version lineage, distro backports, and whether local users can exercise AFS/rxrpc paths. Avoid assuming remote reachability from the available sources.
Mitigation direction
- Apply vendor kernel updates incorporating the referenced stable fixes.
- For custom kernels, include the upstream stable peer_hash_lock BH-locking fix.
- Prioritize hosts using AFS or rxrpc-dependent workloads.
- Check Linux distribution advisories for backported fixed package versions.
- Schedule reboots where required to load the corrected kernel.
Validation and detection
- Inventory running Linux kernel versions across exposed fleets.
- Confirm whether AFS or rxrpc kernel components are enabled or loaded.
- Compare installed kernels with vendor-fixed versions or referenced stable commits.
- Review kernel logs for lockdep warnings or unexplained deadlocks.
- Verify rebooted systems are running the updated kernel.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
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-667: Exact CWE lookup
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Open ATT&CK lookupCVE-2025-21809 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- 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
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.
SSVC decision data
CVSS vector scores
1 official scoreWe 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.
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H1.83.6CISA-ADPVulnerability scoring details
Base CVSS 3.1 score
5.5MediumVector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- https://git.kernel.org/stable/c/10ba5a3d57af20e494e0d979d1894260989235ddCVE reference, Linux
- https://git.kernel.org/stable/c/0e77dd41689637ac4e1b8fe0f27541f373640855CVE reference, Linux
- https://git.kernel.org/stable/c/79d458c13056559d49b5e41fbc4b6890e68cf65bCVE reference, Linux
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Locking
Improper Locking represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.