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An out-of-bounds write flaw was found in the Linux kernel’s seq_file in the Filesystem layer. This flaw allows a local attacker with a user privilege to gain access to out-of-bound memory, leading to a system crash or a leak of internal kernel information. The issue results from not validating the size_t-to-int conversion prior to performing operations.

From PCI DSS point of views, primary concerns are operating system user account security. Verification on the necessities of allowing access given to System, restrict only the mandatory rights to login with logging, ePBF etc. Patch management, especially critical, should be complete in 30 days.

• PCI DSS Requirements 2.1 – Always change vendor-supplied defaults and remove or disable unnecessary default accounts before installing a system on the network. Verify /etc/password have proper settings, delete or set to “nologin”, preventing non mandatory users can login using vulnerability to compromise the system.
• PCI DSS Requirements 6.2 – Ensure that all system components and software are protected from known vulnerabilities by installing applicable vendor supplied security patches. Install critical security patches within one month of release.

Verify Operating System vendors have releasing relate patch and complete patch update within 1 month. If there are no updates from the vendors, necessary mitigation process should be in place.

Patch updates resolving this vulnerability (CVE-2021-33909) noted by Qualys Security Research Team, see following form for Patch listing:

SOURCE	RISK LEVEL
NESSUS https://www.tenable.com/cve/CVE-2021-33909	CVSS (v2) 7.2
NIST NVD https://nvd.nist.gov/vuln/detail/CVE-2021-33909	CVSS (v3) 7.8
Redhat https://access.redhat.com/security/cve/cve-2021-33909	CVSS (v3) 7.0
CVE https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-33909	Source: MITRE
* Update on 2021/09/10

Qualys Security Research Team has proven vulnerability by accessing root rights in vulnerable OS of : Ubuntu 20.04、Ubuntu 20.10、Ubuntu 21.04、Debian 11 and Fedora 34 Workstation. Other Linux OS may result in I.O.C. generate from this vulnerability. Linux Servers patch fix as follow:

OPERATING SYSTEM	SECURITY PATCH LINK
Redhat	https://access.redhat.com/security/cve/cve-2021-33909
CentOS	https://centosfaq.org/centos/its-been-six-days-since-cvd-2021-33909-was-patched-in-rhel-whats-the-holdup-for-stream-8/
CentOS	https://centos.pkgs.org/8-stream/centos-baseos-x86_64/kernel-4.18.0-326.el8.x86_64.rpm.html
SUSE	https://www.suse.com/security/cve/CVE-2021-33909.html
ubuntu	https://ubuntu.com/security/CVE-2021-33909
* Update on 2021/09/10

If there are no updates from the vendors, necessary mitigation process should be in place.

• sysctl kernel.unprivileged_userns_clone=1
  # unprivileged_userns_clone set as 0
• sysctl kernel.unprivileged_bpf_disabled=1
  # unprivileged_bpf_disabled set as 1
• For technical details, please refer to below link:
  https://www.qualys.com/2021/07/20/cve-2021-33909/sequoia-local-privilege-escalation-linux.txt

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