Thousands of Jenkins servers will let anonymous users become admins

Two vulnerabilities discovered and patched over the summer expose Jenkins servers to mass exploitation.

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Thousands, if not more, Jenkins servers are vulnerable to data theft, takeover, and cryptocurrency mining attacks. This is because hackers can exploit two vulnerabilities to gain admin rights or log in using invalid credentials on these servers.

Both vulnerabilities were discovered by security researchers from CyberArk, were privately reported to the Jenkins team, and received fixes over the summer. But despite patches for both issues, there are still thousands of Jenkins servers available online.

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Jenkins is a web application for continuous integration built in Java that allows development teams to run automated tests and commands on code repositories based on test results, and even automate the process of deploying new code to production servers.

Jenkins is a popular component in many companies' IT infrastructure and these servers are very popular with both freelancers and enterprises alike.

Two very dangerous flaws

Over the summer, CyberArk researchers discovered a vulnerability (tracked as CVE-2018-1999001) that allows an attacker to provide malformed login credentials that cause Jenkins servers to move their config.xml file from the Jenkins home directory to another location.

If an attacker can cause the Jenkins server to crash and restart, or if he waits for the server to restart on its own, the Jenkins server then boots in a default configuration that features no security.

In this weakened setup, anyone can register on the Jenkins server and gain administrator access. With an administrator role in hand, an attacker can access private corporate source code, or even make code modifications to plant backdoors in a company's apps.

This lone issue would have been quite bad on its own, but CyberArk researchers also discovered a second Jenkins vulnerability --CVE-2018-1999043.

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This second bug, they said, allowed an attacker to create ephemeral user records in the server's memory, allowing an attacker a short period when they could authenticate using ghost usernames and credentials.

Both vulnerabilities were fixed, the first in July and the second in August, but as we've gotten accustomed to in the past few years of covering security flaws, not all server owners have bothered to install these security updates.

Thousands of servers exposed to hackers

"Using this link, we can see there are close to 78,000 total online Jenkins installations," Nimrod Stoler, a security researcher with CyberArk, told ZDNet in an email. "Since our attack example doesn't require the attacker to be logged in, any of these could have been attacked."

"On top of the roughly 78,000 installation number, there are also installations within closed networks that can't be accessed online (and thus not visible in Shodan), so the roughly 78,000 number is just a piece of the larger number," Stoler told us. "Again, anyone with network access can pull off this attack."

ZDNet has used the same Shodan engine to fine tune the search query for ten Jenkins server versions known to be vulnerable to the above vulnerabilities.


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Within a few minutes, ZDNet was able to discover over 2,000 vulnerable Jenkins servers, but we believe the total number of Internet-accessible vulnerable servers might even go over 10,000.

Earlier this year, a cyber-criminal group abused a multitude of older vulnerabilities to take over Jenkins instances and abuse them to mine cryptocurrency at their behest, earning an astounding $3.4 million worth of Monero (at the time) in the span of a few months.

Rarely do you see a more perfect set of vulnerabilities that can be exploited en masse with extensive damages. Jenkins server owners are advised to patch as soon as possible and avoid having hackers roam free through their servers.

CyberArk researchers have also published a technical report detailing the inner workings of these two flaws this week.

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