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CVE-2018-18955

In the Linux kernel 4.15.x through 4.19.x before 4.19.2, map_write() in kernel/user_namespace.c allows privilege escalation because it mishandles nested user namespaces with more than 5 UID or GID ranges. A user who has CAP_SYS_ADMIN in an affected user namespace can bypass access controls on resources outside the namespace, as demonstrated by reading /etc/shadow. This occurs because an ID transformation takes place properly for the namespaced-to-kernel direction but not for the kernel-to-namespaced direction.

References

• https://github.com/bcoles/kernel-exploits
• https://people.canonical.com/~ubuntu-security/cve/2018/CVE-2018-18955.html
• https://access.redhat.com/security/cve/cve-2018-18955