CVE-2019-5591
CVE-2019-5591 is a security flaw in Fortinet FortiOS, the operating system on Fortinet’s FortiGate firewalls. Attackers can read information from Fortinet FortiOS that should be private from anywhere, without logging in. 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, and says ransomware gangs have used it.[1]
Fortinet FortiOS 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. Fortinet has published a fix or advice.
Description
CISA calls it a default configuration in Fortinet FortiOS. In its words: “Fortinet FortiOS contains a default configuration vulnerability that may allow an unauthenticated attacker on the same subnet to intercept sensitive information by impersonating the Lightweight Directory Access Protocol (LDAP) server.”[1]
The underlying weakness is missing authentication (CWE-306), in MITRE’s list of common software weaknesses.[2]
It scores 6.5 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; Fortinet’s advisory has them[3]. CISA names the product as Fortinet FortiOS.
Most families won’t have Fortinet FortiOS; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 446 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] 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 19%. That’s in the top 3% 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
Fortinet has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Fortinet FortiOS 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 6.5.[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 19%.[4] |
See also
Other Fortinet FortiOS flaws
- CVE-2025-68686: exposure of sensitive information to an unauthorized actor ·
- CVE-2019-6693: use of hard-coded credentials ·
- CVE-2024-21762: out-of-bound write ·
- CVE-2022-41328: path traversal ·
- CVE-2022-42475: heap-based buffer overflow ·
- CVE-2021-44168: arbitrary file download ·
Other Fortinet flaws
- CVE-2025-61624, Fortinet products: path traversal ·
- CVE-2026-104286, Fortinet FortiMail: path traversal ·
- CVE-2025-25249, Fortinet products: heap-based buffer overflow ·
- CVE-2026-39808, Fortinet FortiSandbox: OS command injection ·
- CVE-2026-25089, Fortinet FortiSandbox: OS command injection ·
Other missing authentication flaws
- CVE-2026-111357, Ivanti Connect Secure: missing authentication for critical function ·
- CVE-2026-67277, MikroTik RouterOS: missing authentication for critical function ·
- CVE-2026-81578, PaperCut NG/MF: missing authentication for critical function ·
- CVE-2026-72529, TrueConf Server: missing authentication for critical function ·
- CVE-2026-56164, Microsoft SharePoint Server: missing authentication for critical function ·
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 Fortinet flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Fortinet FortiOS Default Configuration Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2019-5591 Detail”. National Vulnerability Database, published 14 Aug 2020.
- ^ Fortinet. Advisories and fixes: fortiguard.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.186, percentile 97.3, 31 Oct 2026.
- CVE Program. “CVE-2019-5591”. 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.