CVE-2016-4437
CVE-2016-4437 is a security flaw in Apache Shiro, open-source software from the Apache Software Foundation. Attackers can run their own code on Apache Shiro. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 3 Nov 2021.[1]
Apache Shiro 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.
Description
CISA calls it a code execution in Apache Shiro. In its words: “Apache Shiro contains a vulnerability which may allow remote attackers to execute code or bypass intended access restrictions via an unspecified request parameter when a cipher key has not been configured for the "remember me" feature.”[1]
The underlying weakness is improper access control (CWE-284), in MITRE’s list of common software weaknesses.[2]
It scores 9.8 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 doesn’t list version ranges for it. CISA names the product as Apache Shiro.
Most families won’t have Apache Shiro; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 1,975 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 3 May 2022 to fix it.[1]
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 93%. That’s in the top 1% of the flaws it scores.[3]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Apache Shiro 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, or not in use. CISA gave US government agencies until 3 May 2022 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 9.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[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 93%.[3] |
See also
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 access control flaws
- CVE-2015-3306, ProFTPD: improper access control ·
- CVE-2026-21962, Oracle HTTP Server and Oracle Weblogic Server Proxy Plug-in: improper access control ·
- CVE-2026-56290, Joomlack Page Builder: improper access control ·
- CVE-2026-34908, Ubiquiti UniFi OS: improper access control ·
- CVE-2026-48907, Widget Factory Joomla Content Editor: improper access control ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
Every Apache flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Apache Shiro Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2016-4437 Detail”. National Vulnerability Database, published 7 Jun 2016.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.930, percentile 99.8, 31 Oct 2026.
- CVE Program. “CVE-2016-4437”. 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.