For the past three years, Alpine Linux Docker images have been shipped with a NULL password for the root user, Cisco’s Talos security researchers have discovered.
The hard-coded credentials were included in the Official Alpine Linux Docker images since v3.3, as part of a regression introduced in December 2015.
Featuring a CVSS score of 9.8, this Critical vulnerability was found to impact Alpine Docker versions 3.3 to 3.9, as well as Alpine Docker Edge.
The issue is that, in the affected builds of the Alpine Docker Image, the /etc/shadow file contains a blank field instead of the encrypted password. This means that the system treats the root user as having no password.
“Due to the nature of this issue, systems deployed using affected versions of the Alpine Linux container that utilize Linux PAM, or some other mechanism that uses the system shadow file as an authentication database, may accept a NULL password for the root user,” Talos explains.
The vulnerability was first spotted in 2015. However, about a week after it was patched, a commit was pushed to simplify the regression tests. The commit removed the ‘disable root by default’ flag from the ‘edge’ build properties file, thus reintroducing the very same flaw to subsequent builds.
With the default build options appearing to have been copied from this properties file, the flaw has been missing from all tagged builds since December 2015.
Talos reported the vulnerability to the vendor in February. Several days later, it was discovered that the issue was made public on the Alpine Linux’s Github before, although it was not flagged as a security issue and remained unresolved.
The researchers also point out that the root account should be explicitly disabled in Docker images that are based on the affected versions.
“The likelihood of exploitation of this vulnerability is environment-dependent, as successful exploitation requires that an exposed service or application utilise Linux PAM, or some other mechanism which uses the system shadow file as an authentication database,” Talos notes.