FortiBleed: Defending Against Industrial-Scale Fortinet VPN Credential Spraying

Security researcher Volodymyr “Bob” Diachenko recently disclosed "FortiBleed," a startling exposure of a cybercriminal infrastructure designed for industrial-scale credential spraying. This was not a zero-day exploit in the traditional sense, but rather a massive, multi-operator campaign targeting Fortinet FortiGate SSL VPN devices globally. The attackers leveraged billions of login attempts to compromise organizations, and their operation was revealed only through a critical OpSec failure: an unprotected Elasticsearch database hosting their command-and-control (C2) and telemetry data.

For defenders, this is a wake-up call. The threat is not just the vulnerability of the VPN software, but the sheer volume and persistence of automated authentication attacks. If your Fortinet perimeter is exposed, it is statistically likely you are being targeted.

Technical Analysis

Affected Products & Platforms:

• Product: Fortinet FortiGate Firewalls
• Service: FortiGate SSL VPN (HTTPS/SSL VPN)
• Scope: Global exposure, targeting organizations across all sectors.

Attack Mechanics:

• Vector: Credential Spraying / Password Stuffing. The attackers utilize vast repositories of leaked credentials (username/password pairs) obtained from previous data breaches.
• Infrastructure: The operation utilized a complex network of scripts and bots managed via a central database. The "FortiBleed" exposure refers to the discovery of this unsecured database, which contained logs of billions of authentication attempts.
• Goal: Initial Access. By validating credentials against the SSL VPN portal, attackers gain a foothold into the internal network, bypassing perimeter firewalls.

Exploitation Status:

• Active: Confirmed active exploitation worldwide. The "FortiBleed" data leak confirms that the campaign is currently operational and has been running for an indeterminate period.
• CVE: No specific CVE is associated with this campaign (as it is an abuse of the authentication mechanism rather than a software flaw), though defenders should ensure all recent FortiOS advisories are reviewed.

Detection & Response

The primary indicators of this campaign are high-frequency authentication failures and successful logins from anomalous geolocations or IPs. The following rules hunt for the behavioral patterns associated with industrial-scale credential spraying.

SIGMA Rules

---
title: FortiBleed - High Volume SSL VPN Login Failures
id: 88f3a1b2-3c4d-4e5f-8a9b-0c1d2e3f4a5b
status: experimental
description: Detects potential FortiBleed credential spraying activity by identifying high volume of failed SSL VPN logins from a single source IP.
references:
  - https://securityaffairs.com/193931/hacking/fortibleed-exposes-global-credential-spraying-operation.html
author: Security Arsenal
date: 2026/04/06
tags:
  - attack.initial_access
  - attack.t1110.003
logsource:
  product: fortinet
  service: sslvpn
detection:
  selection:
    action: 'login'
    status: 'failure'
  condition: selection | count(src_ip) by dst_ip > 50
timeframe: 5m
falsepositives:
  - Misconfigured VPN clients
  - Internal red team exercises
level: high
---
title: FortiBleed - Successful VPN Login from Suspicious ASN
id: 99a4b2c3-4d5e-5f6a-9b0c-1d2e3f4a5b6c
status: experimental
description: Detects successful Fortinet SSL VPN logins from IP addresses associated with hosting providers or TOR exit nodes, often indicative of credential spraying success.
references:
  - https://securityaffairs.com/193931/hacking/fortibleed-exposes-global-credential-spraying-operation.html
author: Security Arsenal
date: 2026/04/06
tags:
  - attack.initial_access
  - attack.t1078
logsource:
  product: fortinet
  service: sslvpn
detection:
  selection:
    action: 'login'
    status: 'success'
  filter:
    asn_type:
      - 'isp'
      - 'business'
  condition: selection and not filter
falsepositives:
  - Legitimate remote work from cloud infrastructure
  - Traveling employees
level: medium

KQL (Microsoft Sentinel / Defender)

// Hunt for FortiBleed credential spraying patterns
// Adjust thresholds based on your baseline traffic volume
let StartTime = ago(24h);
let Threshold = 100;
CommonSecurityLog
| where DeviceVendor == "Fortinet"
| where DeviceProduct == "FortiGate"
| where Activity contains "login" 
| extend Status = coalesce(column_ifexists("Reason", ""), column_ifexists("Status", ""))
| where Status =~ "denied" or Status =~ "failure" 
| summarize FailedLogins = count() by SourceIP, DestinationUserName, bin(TimeGenerated, 5m)
| where FailedLogins > Threshold
| project TimeGenerated, SourceIP, DestinationUserName, FailedLogins
| sort by FailedLogins desc

Velociraptor VQL

-- Hunt for established VPN connections on endpoints
-- Identifying active VPN sessions that may be compromised or rogue
SELECT Pid, Name, CommandLine, LocalAddress, RemoteAddress, State
FROM netstat()
WHERE RemoteAddress =~ ':443' OR RemoteAddress =~ ':10443'
  AND (Name =~ 'FortiClient' OR Name =~ 'FortiSSLVPNdaemon' OR CommandLine =~ 'FortiClient')
  AND State =~ 'ESTABLISHED'

Remediation Script (Bash)

This Bash script is intended to be run on a Linux-based log aggregator (SIEM) or a central server that ingests FortiGate Syslogs. It parses logs to identify IPs exhibiting spraying behavior, allowing for automated blocking or alerting.

#!/bin/bash

# fortibleed_audit.sh
# Analyzes Fortinet syslog for potential FortiBleed credential spraying
# Usage: ./fortibleed_audit.sh /path/to/fortinet.log

LOG_FILE="$1"
THRESHOLD=50  # Alert if > 50 failed logins from one IP in the scan window

if [[ -z "$LOG_FILE" ]]; then
  echo "Usage: $0 <path_to_log_file>"
  exit 1
fi

echo "[+] Scanning $LOG_FILE for high-volume failed SSL VPN logins..."

# Grep for failed login attempts (common Fortinet log format) 
# Assumes log format contains 'action=login' and 'status=success/failure' or 'msg="denied"'
# Adjust regex based on your specific FortiOS logging format
grep "action=login" "$LOG_FILE" | grep "status=failure" | awk '{print $1}' | sort | uniq -c | sort -nr | awk -v thresh=$THRESHOLD '$1 > thresh {print "Suspicious IP: " $2 " - Attempts: " $1}'

echo "[+] Audit complete. Review output for IP addresses exceeding threshold of $THRESHOLD."
echo "[+] Recommended Action: Block identified IPs at the firewall perimeter immediately."

Remediation

To mitigate the risk of falling victim to the FortiBleed operation or similar credential spraying campaigns, implement the following defensive measures immediately:

1. Enforce MFA: Enable Multi-Factor Authentication (MFA) on all SSL VPN profiles. This is the single most effective control against credential stuffing.
2. Geo-Blocking: Restrict VPN access to specific countries where your users operate. Block traffic from known high-risk regions and hosting providers (ASNs) unless business-critical.
3. IP Reputation Filtering: Configure FortiGate local-in policies or integrate with threat intelligence feeds to automatically block IP addresses with poor reputations.
4. Account Lockout Policies: Implement aggressive account lockout policies (e.g., lock after 5 failed attempts) to slow down spraying operations.
5. Upgrade Firmware: Ensure FortiOS is on the latest supported release. While this specific attack is an auth bypass attempt, running the latest firmware ensures you have the latest security hardening.
6. Audit Admin Access: Ensure local admin accounts on the FortiGate are not exposed to the internet and use strong, unique passwords.

Official Advisory: Refer to the Fortinet Security Advisories for the latest patches and configuration guides.

Related Resources

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