Hackers Planted Backdoor in Webmin, Popular Utility for Linux/Unix Servers

Aug 20, 2019

Following the public disclosure of a critical zero-day
vulnerability in Webmin last week, the project’s maintainers today
revealed that the flaw was not actually the result of a coding
mistake made by the programmers.

Instead, it was secretly planted by an unknown hacker who
successfully managed to inject a backdoor at some point in its
build infrastructure—that surprisingly persisted into various
releases of Webmin (1.882 through 1.921) and eventually remained
hidden for over a year.

With over 3 million downloads per year, Webmin is one of
the world’s most popular open-source web-based applications for
managing Unix-based systems, such as Linux, FreeBSD, or OpenBSD
servers.

Webmin offers a simple user interface (UI) to manage users and
groups, databases, BIND, Apache, Postfix, Sendmail, QMail, backups,
firewalls, monitoring and alerts, and much more.

The story started when Turkish researcher Özkan Mustafa Akkuş
publicly presented a zero-day remote code execution vulnerability
in the Webmin at DefCon on August 10, without giving any advance
notice to the affected project maintainers.

“We received no advance notification of it, which is unusual and
unethical on the part of the researcher who discovered it. But, in
such cases there’s nothing we can do but fix it ASAP,” said Joe
Cooper, one of the project’s developers.

Besides revealing the flaw to the public, Akkuş also released a
Metasploit
module for this vulnerability that aims to automate the
exploitation using the Metasploit framework.

The vulnerability, tracked as CVE-2019-15107, was introduced in a
security feature that has been designed to let Webmin administrator
enforce a password expiration policy for other users’ accounts.

According to the researcher, the security flaw resides in the
password reset page and allows a remote, unauthenticated attacker
to execute arbitrary commands with root privileges on affected
servers just by adding a simple pipe command (“|”) in the old
password field through POST requests.

In a blog post published today, Cooper said
that the team is still investigating how and when the backdoor was
introduced, but confirmed that the official Webmin downloads were
replaced by the backdoored packages only on the project’s
SourceForge repository, and not on the Webmin’s GitHub repositories.
^[2]^[3]

Cooper also stressed that the affected password expiration feature
doesn’t come enabled by default for Webmin accounts, which means
that most versions are not vulnerable in their default
configuration, and the flaw only affects Webmin admins who have
manually enabled this feature.

“To exploit the malicious code, your Webmin installation must have
Webmin → Webmin Configuration → Authentication → Password expiry
policy set to Prompt users with expired passwords to enter a new
one. This option is not set by default, but if it is set, it allows
remote code execution,” Cooper said.

However, another security researcher on Twitter later revealed that Webmin
version 1.890 is affected in the default configuration, as the
hackers appear to have modified the source code to enable password
expiration feature by default for all Webmin users.

These unusual changes in the Webmin source code were red-flagged ^[5]
by an administrator late last year, but surprisingly, Webmin
developers never suspected that it was not their mistake, but the
code was actually modified by someone else intentionally.

According to a Shodan search, Webmin has more than 218,000
Internet-exposed instances available at the time of writing, mostly
located in the United States, France, and Germany—of which over
13,000 instances are running vulnerable Webmin version
1.890.

Webmin developers have now removed the malicious backdoor in
its software to address the vulnerability and released the clean
versions, Webmin 1.930 and Usermin version 1.780.

The latest Webmin and Usermin releases also address a handful of
cross-site scripting (XSS) vulnerabilities that were responsibly
disclosed by a different security researcher who has been rewarded
with a bounty.

So, Webmin administrators are strongly recommended to update
their packages as soon as possible.

^[1]^[4]

References

^{^}
Metasploit module
(pentest.com.tr)
^{^}
blog post
(www.virtualmin.com)
^{^}
GitHub repositories
(github.com)
^{^}
revealed
(twitter.com)
^{^}
red-flagged
(github.com)

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OWASP TOP !0

Injection. Injection flaws, such as SQL, NoSQL, OS, and LDAP injection, occur when untrusted data is sent to an interpreter as part of a command or query. The attacker’s hostile data can trick the interpreter into executing unintended commands or accessing data without proper authorization. Broken Authentication. Application functions related to authentication and session management are often implemented incorrectly, allowing attackers to compromise passwords, keys, or session tokens, or to exploit other implementation flaws to assume other users’ identities temporarily or permanently. Sensitive Data Exposure. Many web applications and APIs do not properly protect sensitive data, such as financial, healthcare, and PII. Attackers may steal or modify such weakly protected data to conduct credit card fraud, identity theft, or other crimes. Sensitive data may be compromised without extra protection, such as encryption at rest or in transit, and requires special precautions when exchanged with the browser. XML External Entities (XXE). Many older or poorly configured XML processors evaluate external entity references within XML documents. External entities can be used to disclose internal files using the file URI handler, internal file shares, internal port scanning, remote code execution, and denial of service attacks. Broken Access Control. Restrictions on what authenticated users are allowed to do are often not properly enforced. Attackers can exploit these flaws to access unauthorized functionality and/or data, such as access other users’ accounts, view sensitive files, modify other users’ data, change access rights, etc. Security Misconfiguration. Security misconfiguration is the most commonly seen issue. This is commonly a result of insecure default configurations, incomplete or ad hoc configurations, open cloud storage, misconfigured HTTP headers, and verbose error messages containing sensitive information. Not only must all operating systems, frameworks, libraries, and applications be securely configured, but they must be patched/upgraded in a timely fashion. Cross-Site Scripting XSS. XSS flaws occur whenever an application includes untrusted data in a new web page without proper validation or escaping, or updates an existing web page with user-supplied data using a browser API that can create HTML or JavaScript. XSS allows attackers to execute scripts in the victim’s browser which can hijack user sessions, deface web sites, or redirect the user to malicious sites. Insecure Deserialization. Insecure deserialization often leads to remote code execution. Even if deserialization flaws do not result in remote code execution, they can be used to perform attacks, including replay attacks, injection attacks, and privilege escalation attacks. Using Components with Known Vulnerabilities. Components, such as libraries, frameworks, and other software modules, run with the same privileges as the application. If a vulnerable component is exploited, such an attack can facilitate serious data loss or server takeover. Applications and APIs using components with known vulnerabilities may undermine application defenses and enable various attacks and impacts. Insufficient Logging & Monitoring. Insufficient logging and monitoring, coupled with missing or ineffective integration with incident response, allows attackers to further attack systems, maintain persistence, pivot to more systems, and tamper, extract, or destroy data. Most breach studies show time to detect a breach is over 200 days, typically detected by external parties rather than internal processes or monitoring.

Hackers Planted Backdoor in Webmin, Popular Utility for Linux/Unix Servers

References

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