By 
Cisco’s Talos security researchers have found that Alpine Linux Docker images have been sent with a NULL password for the root user, for the last three years.
As part of a reversion introduced in December 2015, the hard-coded credentials were included in the Official Alpine Linux Docker images since v3.3.
Containing a CVSS score of 9.8, this serious flaw was found to affect Alpine Docker versions 3.3 to 3.9, as well as Alpine Docker Edge.
The problem is that, in the impacted builds of the Alpine Docker Image, the /etc/shadow file comprises a blank field rather than 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 flaw was first spotted in 2015. Nevertheless, about a week after it was patched, a commit was pushed to shorten the reversion tests. The commit removed the ‘disable root by default’ flag from the ‘edge’ build properties file, thus reinstating the very same fault to following 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 susceptibility to the vendor in February. Several days later, it was revealed that the issue was made public on the Alpine Linux’s Github before, although it was not highlighted as a security issue and remained unsettled.
The researchers also highlighted that the root account should be openly restricted 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.
