Security readout for executives and security teams
Plain-English summary
CVE-2021-46933 is a Linux kernel flaw in USB gadget FunctionFS cleanup. A local user who can exercise this subsystem could trigger a reference-count underflow, leading to kernel use-after-free behavior and likely denial of service. The business risk is mainly availability on systems that expose USB gadget functionality.
Executive priority
Treat as a targeted availability risk, not an internet-wide emergency. Prioritize patching systems where Linux USB gadget functionality is part of the product, appliance, lab, or embedded fleet. Standard servers without this subsystem are lower urgency.
Technical view
The f_fs FunctionFS path can call ffs_data_clear multiple times after userland closes ep0 and unmounts f_fs. When an eventfd was supplied with USB descriptors, repeated eventfd_ctx_put calls can underflow the reference count. The fix clears ffs_eventfd after release, preventing duplicate puts.
Likely exposure
Exposure is most likely on Linux systems using USB gadget FunctionFS, commonly embedded, appliance, mobile, or device-mode USB deployments. General-purpose servers with no USB gadget FunctionFS use are less likely exposed. The CVSS vector indicates local access with low privileges is required.
Exploitation context
The bundle does not show CISA KEV listing or active exploitation evidence. The reported impact is availability-focused, with no confidentiality or integrity impact in CVSS. Evidence describes a reproducible kernel warning and use-after-free condition, but not remote exploitation.
Researcher notes
Focus validation on f_fs lifecycle handling around ep0 close, FunctionFS unmount, and eventfd cleanup. The source evidence supports a double or triple cleanup path causing eventfd_ctx_put over-release. Avoid assuming exploitability beyond local denial of service without further vendor evidence.
Mitigation direction
- Update affected Linux kernels to versions containing the stable FunctionFS fix.
- Prioritize embedded or device-mode USB systems using FunctionFS.
- Disable unused USB gadget FunctionFS functionality where operationally safe.
- Restrict local users able to configure or exercise USB gadget functions.
- Check Linux distribution advisories for backported fixed kernel packages.
Validation and detection
- Inventory Linux kernel versions against affected and fixed stable branches.
- Identify systems with USB gadget FunctionFS enabled or loaded.
- Confirm vendor kernel packages include the referenced f_fs cleanup fix.
- Review kernel logs for refcount underflow or use-after-free warnings.
- Validate mitigations on representative embedded or USB gadget devices.
Public sources used
- CVE Program
- CVE List V5
- Linux stable commit f976dd7011150244a7ba820f2c331e9fb253befa
- Linux stable commit cc8c8028c21b2a3842a1e98e99e55028df275919
- Linux stable commit 52500239e3f2d6fc77b6f58632a9fb98fe74ac09
- Linux stable commit 33f6a0cbb7772146e1c11f38028fffbfed14728b
- Linux stable commit 240fc586e83d645912accce081a48aa63a45f6ee
- Linux stable commit 1c4ace3e6b8575745c50dca9e76e0021e697d645
- Linux stable commit ebef2aa29f370b5096c16020c104e393192ef684
- Linux stable commit b1e0887379422975f237d43d8839b751a6bcf154
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-476: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-46933 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/f976dd7011150244a7ba820f2c331e9fb253befaCVE reference, Linux
- https://git.kernel.org/stable/c/cc8c8028c21b2a3842a1e98e99e55028df275919CVE reference, Linux
- https://git.kernel.org/stable/c/52500239e3f2d6fc77b6f58632a9fb98fe74ac09CVE reference, Linux
- https://git.kernel.org/stable/c/33f6a0cbb7772146e1c11f38028fffbfed14728bCVE reference, Linux
- https://git.kernel.org/stable/c/240fc586e83d645912accce081a48aa63a45f6eeCVE reference, Linux
- https://git.kernel.org/stable/c/1c4ace3e6b8575745c50dca9e76e0021e697d645CVE reference, Linux
- https://git.kernel.org/stable/c/ebef2aa29f370b5096c16020c104e393192ef684CVE reference, Linux
- https://git.kernel.org/stable/c/b1e0887379422975f237d43d8839b751a6bcf154CVE 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.
NULL Pointer Dereference
NULL Pointer Dereference represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.