CyberSecBulletin
CVE-2021-3493Exploited in the wildNVD status: Analyzed

Linux Kernel Privilege Escalation

Published Updated By the CyberSecBulletin desk

CVE-2021-3493 is a high-severity incorrect authorization vulnerability in linux kernel from Ubuntu, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists versions 5.8.0-50.56, 5.4.0-72.80, 4.15.0-142.146 as the first without the flaw, so updating linux kernel to one of those releases or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on October 20, 2022, which means attacks have been observed in the wild.

Patch now

Attackers are already exploiting this vulnerability, according to CISA.

What to do

  1. Update linux kernel to 5.8.0-50.56 or 5.4.0-72.80 or 4.15.0-142.146 or 4.4.0-209.241, whichever matches your release line, or a later version.
  2. Update Ubuntu Linux to 18.04 or 20.04 or 20.10, whichever matches your release line, or a later version.
  3. CISA's required action: Apply updates per vendor instructions.
  4. Read the advisory on ubuntu.com for workarounds and exact fixed builds.

Affected products and versions

VendorProductAffected versionsFixed in
Ubuntulinux kernel5.8 kernel up to, but not including, 5.8.0-50.56; 5.4 kernel up to, but not including, 5.4.0-72.80; 4.15 kernel up to, but not including, 4.15.0-142.146; 4.4 kernel up to, but not including, 4.4.0-209.2415.8.0-50.56, 5.4.0-72.80, 4.15.0-142.146, 4.4.0-209.241
CanonicalUbuntu Linuxbefore 18.04; 18.04.1 up to, but not including, 20.04; before 20.1018.04, 20.04, 20.10

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
LocalThe attacker needs local access or must trick a user into running something.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
LowA basic user account is enough.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
HighAll data handled by the component can be read.
Integrity
HighData or code can be modified at will.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.

Scoring systemScoreSeverityScored by
CVSS 3.18.8HighCNA (ubuntu.com)
CVSS 3.17.8HighNVD
CVSS 2.07.2HighNVD

Official description

The overlayfs implementation in the linux kernel did not properly validate with respect to user namespaces the setting of file capabilities on files in an underlying file system. Due to the combination of unprivileged user namespaces along with a patch carried in the Ubuntu kernel to allow unprivileged overlay mounts, an attacker could use this to gain elevated privileges.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Incorrect Authorization

The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check.

How developers prevent this class of flaw

  • Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) to enforce the roles at the appropriate boundaries. Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
  • Ensure that access control checks are performed related to the business logic. These checks may be different than the access control checks that are applied to more generic resources such as files, connections, processes, memory, and database records. For example, a database may restrict access for medical records to a specific database user, but each record might only be intended to be accessible to the patient and the patient's doctor.
  • Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, consider using authorization frameworks such as the JAAS Authorization Framework and the OWASP ESAPI Access Control feature.

More Incorrect Authorization vulnerabilities (CWE-863)

Timeline

  1. CVE record published.
  2. Added to CISA's Known Exploited Vulnerabilities catalogue.
  3. Deadline for US federal agencies to remediate, set by CISA.
  4. CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
  5. Record last changed materially (score, affected products, status or references).

References

Third-party and government advisories

Technical write-ups (may include proof of concept)

Recent Incorrect Authorization vulnerabilities

Spotted an error? Tell us.