CVE-2021-36741
CVE-2021-36741 is a security flaw in Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security, business antivirus software. Attackers can upload a file to Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security and make it run. 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]
Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security 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. Trend Micro has published a fix or advice.
Description
CISA calls it a multiple products improper input validation in Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security. In its words: “Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security contain an improper input validation vulnerability that allows a remote attacker to upload files.”[1]
The CVE record adds: “An improper input validation vulnerability in Trend Micro Apex One, Apex One as a Service, OfficeScan XG, and Worry-Free Business Security 10.0 SP1 allows a remote attached to upload arbitrary files on affected installations. Please note: an attacker must first obtain the ability to logon to the product�s management console in order to exploit this vulnerability.”[2]
The underlying weakness is path traversal (CWE-22), in MITRE’s list of common software weaknesses.
It scores 8.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; Trend Micro’s advisory has them[3]. CISA names the product as Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security.
Most families won’t have Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 97 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 17 Nov 2021 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 5%. That’s in the top 8% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Trend Micro has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security 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 17 Nov 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 8.8.[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 5%.[4] |
See also
Other Trend Micro Apex One, Apex One as a Service, and Worry-Free Business Security flaws
- CVE-2021-36742: multiple products improper input validation ·
Other Trend Micro flaws
- CVE-2026-34926, Trend Micro Apex One: (on-premise) directory traversal ·
- CVE-2025-54948, Trend Micro Apex One: OS command injection ·
- CVE-2023-41179, Trend Micro Apex One and Worry-Free Business Security: remote code execution ·
- CVE-2022-40139, Trend Micro Apex One and Apex One as a Service: improper validation ·
- CVE-2022-26871, Trend Micro Apex Central: arbitrary file upload ·
Other path traversal flaws
- CVE-2025-61624, Fortinet products: path traversal ·
- CVE-2021-3199, ONLYOFFICE Docs: server path traversal ·
- CVE-2026-104286, Fortinet FortiMail: path traversal ·
- CVE-2026-93616, Check Point products: path traversal ·
- CVE-2026-66384, JFrog Artifactory: improper limitation of a pathname to a restricted directory ·
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 Trend Micro flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Trend Micro Multiple Products Improper Input Validation Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-36741 Detail”. National Vulnerability Database, published 29 Jul 2021.
- ^ Trend Micro. Advisories and fixes: success.trendmicro.com, success.trendmicro.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.050, percentile 92.2, 31 Oct 2026.
- CVE Program. “CVE-2021-36741”. 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.