CVE-2016-10033
CVE-2016-10033 is a security flaw in PHPMailer, the programming language behind many websites. Attackers can run their own code on PHPMailer. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 7 Jul 2025.[1]
PHPMailer 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 command injection in PHP PHPMailer. In its words: “PHPMailer contains a command injection vulnerability because it fails to sanitize user-supplied input. Specifically, this issue affects the 'mail()' function of 'class.phpmailer.php' script. An attacker can exploit this issue to execute arbitrary code within the context of the application. Failed exploit attempts will result in a denial-of-service condition.”[1]
The underlying weakness is command injection (CWE-77), CWE-88, 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 PHP PHPMailer.
Most families won’t have PHPMailer; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 7 Jul 2025, 3,111 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 28 Jul 2025 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 over 99%. 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 mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.”[1]
- Check whether you use it.PHPMailer 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 28 Jul 2025 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 over 99%.[3] |
See also
Other PHP flaws
- CVE-2019-11043, PHP FastCGI Process Manager (FPM): buffer overflow ·
- CVE-2012-1823, PHP: PHP-CGI query string parameter ·
Other command injection flaws
- CVE-2026-105134, AhsayCBS: command injection ·
- CVE-2026-96951, Microsoft Windows: command injection ·
- CVE-2016-3081, Apache Struts: command injection ·
- CVE-2026-8037, Progress LoadMaster: command injection ·
- CVE-2025-29635, D-Link DIR-823X: command injection ·
Added the same day
- CVE-2019-9621, Synacor Zimbra Collaboration Suite (ZCS): server-side request forgery (SSRF) ·
- CVE-2019-5418, Rails Ruby on Rails: path traversal ·
- CVE-2014-3931, Looking Glass Multi-Router Looking Glass (MRLG): buffer overflow ·
Every PHP flaw on CISA’s list · Everything added in July 2025
References
- ^ Cybersecurity and Infrastructure Security Agency. “PHPMailer Command Injection Vulnerability”. Known Exploited Vulnerabilities Catalog, added 7 Jul 2025.
- ^ National Institute of Standards and Technology. “CVE-2016-10033 Detail”. National Vulnerability Database, published 30 Dec 2016.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.997, percentile 100.0, 31 Oct 2026.
- CVE Program. “CVE-2016-10033”. 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.