Local Administrator and User Password Hash Invalidation on Config Downgrade
Rolling back firmware or restoring configuration archives from FortiOS 7.4 to FortiOS 7.2 corrupts local administrator and user password hashes due to backward-incompatible PBKDF2 salt formatting, resulting in immediate administrative GUI and SSH lockout upon reboot.
Technical Root Cause Analysis
FortiOS 7.4 introduced enhanced security hardening for stored credentials, upgrading the password derivation function from legacy SHA-256 with fixed 16-byte salts to an advanced PBKDF2-HMAC-SHA512 scheme. In the 7.4 configuration file, encrypted strings under config system admin and config user local are serialized with modern version prefixes (e.g. $6$ or $s$) and extended salt vectors.
When an administrator downgrades a firewall from FortiOS 7.4.x to 7.2.x, or imports a 7.4 backup configuration onto a unit running 7.2 firmware, the older 7.2 configuration compiler (fcnfg) parses the text file during early kernel boot.
Because the 7.2 parser does not recognize the 7.4 PBKDF2 schema, it either truncates the hash string during input validation or flags the entire password token as malformed. As a defensive failure behavior, FortiOS zeroes the stored password hash in the user database (user_db) while keeping the username. Upon subsequent boot, all local administrator passwords fail authentication checks, producing immediate lockout across Web GUI, SSH, and API interfaces.
Affected Firmware & Blast Radius Matrix
| Downgrade Vector | Vulnerable Builds | Target Firmware | Status & Workaround |
|---|---|---|---|
| FortiOS 7.4 → 7.2 Rollback | 7.4.0 – 7.4.4 | 7.2.0 – 7.2.8 | Maintainer reset or clean native config restore |
| FortiOS 7.4 → 7.0 Rollback | 7.4.0 – 7.4.4 | 7.0.0 – 7.0.14 | TFTP BIOS format & clean installation required |
| FortiOS 7.2.8+ Target | Supported | 7.2.9+ Clean | Parser patch recognizes 7.4 hashes on rollback |
Platform Impact: All physical and virtual FortiGate models undergoing firmware downgrade or configuration restoration from FortiOS 7.4 to FortiOS 7.2.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands via direct RJ45 / USB serial console connection to verify administrator presence and system checksum consistency:
Diagnostic Commands
# 1. View configured system administrators and password hash status
show system admin
# 2. Check configuration database integrity and parser checksum errors
diagnose sys checkum fgt
# 3. Verify whether administrator lockout thresholds have been triggered
diagnose sys admin-lockout list
# 4. Check system configuration status and active firmware image version
get system statusRemediation & Workaround Steps (Teaser Preview)
Step-by-step remediation procedures available in the interactive Ferrite platform:
Frequently Asked Questions
FortiOS 7.4 introduced a new PBKDF2 hash scheme with extended salts. Older FortiOS 7.2 parsers fail to decode the new format and invalidate the credentials in RAM during boot, locking all local accounts.
No. When fncfg fails to parse the hash, it marks the account credentials as invalid or unauthenticated rather than resetting it to blank, preventing unauthenticated access.
Format and re-flash the FortiOS 7.2 firmware image via BIOS TFTP or GUI firmware install, followed by restoring the configuration backup that was specifically created under FortiOS 7.2 before the upgrade.