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CVE-2023-27997

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in Fortinet FortiOS and FortiProxy SSL-VPN. For every flaw being used in attacks, see Exploited flaws; for Fortinet’s, see Fortinet flaws being used in attacks.

CVE-2023-27997 is a security flaw in Fortinet FortiOS and FortiProxy SSL-VPN, the operating system on Fortinet’s FortiGate firewalls. Attackers can run their own code on Fortinet FortiOS and FortiProxy SSL-VPN 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 13 Jun 2023, and says ransomware gangs have used it.[1]

Fortinet FortiOS and FortiProxy SSL-VPN 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 heap-based buffer overflow in Fortinet FortiOS and FortiProxy SSL-VPN. In its words: “Fortinet FortiOS and FortiProxy SSL-VPN contain a heap-based buffer overflow vulnerability which can allow an unauthenticated, remote attacker to execute code or commands via specifically crafted requests.”[1]

The CVE record adds: “A heap-based buffer overflow vulnerability [CWE-122] in FortiOS version 7.2.4 and below, version 7.0.11 and below, version 6.4.12 and below, version 6.0.16 and below and FortiProxy version 7.2.3 and below, version 7.0.9 and below, version 2.0.12 and below, version 1.2 all versions, version 1.1 all versions SSL-VPN may allow a remote attacker to execute arbitrary code or commands via specifically crafted requests.”[2]

The underlying weakness is heap buffer overflow (CWE-122), in MITRE’s list of common software weaknesses.

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; Fortinet’s advisory has them[3]. CISA names the product as Fortinet FortiOS and FortiProxy SSL-VPN.

Most families won’t have Fortinet FortiOS and FortiProxy SSL-VPN; it’s for whoever looks after IT at work.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 13 Jun 2023, 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 4 Jul 2023 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 86%. 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

Fortinet has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]

  1. Check whether you use it.Fortinet FortiOS and FortiProxy SSL-VPN 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.
  2. Send this page to your IT person.Ask them to confirm it’s fixed, or not in use. CISA gave US government agencies until 4 Jul 2023 to fix it.
  3. 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

DateEvent
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, 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 86%.[4]

See also

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-2025-68686, Fortinet FortiOS: exposure of sensitive information to an unauthorized actor ·
  • CVE-2026-39808, Fortinet FortiSandbox: OS command injection ·

Other heap buffer overflow flaws

  • CVE-2026-94127, F5 BIG-IP APM: heap-based buffer overflow ·
  • CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
  • CVE-2025-32706, Microsoft Windows: common log file system (CLFS) driver heap-based buffer overflow ·
  • CVE-2025-24993, Microsoft Windows: NTFS heap-based buffer overflow ·
  • CVE-2025-21418, Microsoft Windows: ancillary function driver for winSock heap-based buffer overflow ·

Every Fortinet flaw on CISA’s list · Everything added in June 2023

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Fortinet FortiOS and FortiProxy SSL-VPN Heap-Based Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 13 Jun 2023.
  2. ^ National Institute of Standards and Technology. “CVE-2023-27997 Detail”. National Vulnerability Database, published 13 Jun 2023.
  3. ^ Fortinet. Advisories and fixes: fortiguard.com, fortiguard.com.
  4. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.857, percentile 99.7, 31 Oct 2026.
  5. CVE Program. “CVE-2023-27997”. 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.