CVE-2021-44228
CVE-2021-44228 is a security flaw in Apache Log4j2, open-source software from the Apache Software Foundation. Attackers can run their own code on Apache Log4j2. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 10 Dec 2021, and says ransomware gangs have used it.[1]
Apache Log4j2 is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version log4j-core* of Apache Log4j2.
Description
CISA calls it a remote code execution in Apache Log4j2. In its words: “Apache Log4j2 contains a vulnerability where JNDI features do not protect against attacker-controlled JNDI-related endpoints, allowing for remote code execution.”[1]
The CVE record adds: “Apache Log4j2 2.0-beta9 through 2.15.0 (excluding security releases 2.12.2, 2.12.3, and 2.3.1) JNDI features used in configuration, log messages, and parameters do not protect against attacker controlled LDAP and other JNDI related endpoints. An attacker who can control log messages or log message parameters can execute arbitrary code loaded from LDAP servers when message lookup substitution is enabled. From log4j 2.15.0, this behavior has been disabled by default. From version 2.16.0 (along with 2.12.2, 2.12.3, and 2.3.1), this functionality has been completely removed. Note that this vulnerability is specific to log4j-core and does not affect log4net, log4cxx, or other Apache Logging Services projects.”[2]
The underlying weakness is improper input validation (CWE-20), resource exhaustion (CWE-400), deserialization of untrusted data (CWE-502), in MITRE’s list of common software weaknesses.
It scores 10.0 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists one fixed version: everything before it is affected.[2]
| Product | Fixed in |
|---|---|
| Apache Log4j2 | log4j-core* |
Most families won’t have Apache Log4j2; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 10 Dec 2021, within a day of it being made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 24 Dec 2021 to fix it.[1] CISA says ransomware gangs have used it.
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at over 99%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
Apache has published a fix or advice.[3] CISA’s required action: “For all affected software assets for which updates exist, the only acceptable remediation actions are: 1) Apply updates.”[1]
- Check whether you use it.Apache Log4j2 is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version log4j-core* of Apache Log4j2), or not in use. CISA gave US government agencies until 24 Dec 2021 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 10.0.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks, noting that ransomware gangs have used it.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at over 99%.[4] |
See also
Other Apache Log4j2 flaws
- CVE-2021-45046: deserialization of untrusted data ·
Other Apache flaws
- CVE-2016-3081, Apache Struts: command injection ·
- CVE-2026-34486, Apache Tomcat: missing encryption of sensitive data ·
- CVE-2026-34197, Apache ActiveMQ: improper input validation ·
- CVE-2024-38475, Apache HTTP Server: improper escaping of output ·
- CVE-2025-24813, Apache Tomcat: path equivalence ·
Other improper input validation flaws
- CVE-2026-93952, Arista VeloCloud Orchestrator: improper input validation ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
- CVE-2026-12569, PTC Windchill and FlexPLM: improper input validation ·
- CVE-2026-34910, Ubiquiti UniFi OS: improper input validation ·
- CVE-2026-6973, Ivanti Endpoint Manager Mobile (EPMM): improper input validation ·
Added the same day
- CVE-2021-44515, Zoho Desktop Central: authentication bypass ·
- CVE-2021-44168, Fortinet FortiOS: arbitrary file download ·
- CVE-2021-35394, Realtek Jungle Software Development Kit (SDK): jungle SDK remote code execution ·
- CVE-2020-8816, Pi-hole AdminLTE: remote code execution ·
- CVE-2020-17463, Fuel CMS: SQL injection ·
Every Apache flaw on CISA’s list · Everything added in December 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Apache Log4j2 Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 10 Dec 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-44228 Detail”. National Vulnerability Database, published 10 Dec 2021.
- ^ Apache. Advisories and fixes: lists.fedoraproject.org, lists.fedoraproject.org, logging.apache.org, msrc-blog.microsoft.com, oracle.com, and 1 more on NVD’s record.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 1.000, percentile 100.0, 31 Oct 2026.
- CVE Program. “CVE-2021-44228”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.