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Linux KernelOperating system · Linux

CVE-2021-34981

MEDIUM · 6.7 CVSS v3.1 Published 2024-05-07
Fix available
A fix is available. Upgrade to 5.10.42 or later.
See remediation →
69/100
Remediation priority · Elevated
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Linux Kernel Bluetooth CMTP Module Double Free Privilege Escalation Vulnerability. This vulnerability allows local attackers to escalate privileges on affected installations of Linux Kernel. An attacker must first obtain the ability to execute high-privileged code on the target system in order to exploit this vulnerability. The specific flaw exists within the CMTP module. The issue results from the lack of validating the existence of an object prior to performing further free operations on the object. An attacker can leverage this vulnerability to escalate privileges and execute code in the context of the kernel. Was ZDI-CAN-11977.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A double-free vulnerability in the Linux Kernel Bluetooth CMTP module allows local attackers with existing high-privileged code execution to escalate to kernel context. The flaw stems from missing validation of object existence before performing free operations on it.

MitigationApply the Linux kernel security update that patches the CMTP module double-free vulnerability. This typically involves updating the kernel package and rebooting the system.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:< 5.10.42

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
High
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check running kernel version
    Run 'uname -r' or 'cat /proc/version' to obtain the kernel version number
    Affected if The kernel version is earlier than 5.10.42 (for example, 5.10.41, 5.9.x, or older)
  2. Verify CMTP module is loaded
    Run 'lsmod | grep cmtp' to list the CMTP module if it is currently loaded into memory
    Affected if The cmtp module appears in the lsmod output, indicating it is actively loaded
  3. Check if CMTP module exists
    Run 'modinfo cmtp' or check for the existence of /lib/modules/$(uname -r)/kernel/net/bluetooth/cmtp.ko*'
    Affected if The CMTP module file exists on the system and matches the kernel version from step 1
  4. Check Bluetooth subsystem status
    Run 'rfkill list' or 'hciconfig -a' to see if Bluetooth is enabled and which Bluetooth modules are in use
    Affected if Bluetooth is enabled and the CMTP module would be invoked as part of Bluetooth CMTP protocol handling

The system is affected if the running kernel version is earlier than 5.10.42 AND the CMTP module is loaded or loadable on the system.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.10.42 or later
Fixed in 5.10.42
Interim mitigation

Apply the Linux kernel security update that patches the CMTP module double-free vulnerability. This typically involves updating the kernel package and rebooting the system.

Recommended fix High confidence

Linux Kernel 5.10.42 or later stable release

  1. 1. Check the current running kernel version using: uname -r
  2. 2. Update the system package repository to ensure you have access to the latest kernel packages
  3. 3. Upgrade the Linux kernel to version 5.10.42 or later using your distribution's package manager (e.g., apt-get update && apt-get install linux-image-<version> for Debian/Ubuntu, or yum/dnf for RHEL-based systems)
  4. 4. Reboot the system to load the new kernel
  5. 5. Verify the new kernel version is running using: uname -r
  6. 6. Confirm the version is >= 5.10.42
Caveat Kernel upgrades may require matching kernel headers and modules; ensure dependent packages are compatible. Some custom drivers or modules may need recompilation.

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Linux Kernel Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing12.0 h
  • Review / QA4.0 h
25.0 hours of engineering $4,200
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Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-34981 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data