Tampered Fluent Forms Pro 6.2.7 installs a persistent backdoor
A tampered Fluent Forms Pro 6.2.7 build distributed through the vendor's former update server added libs/class-license-sync.php and loaded it from fluentformpro.php. The rogue code exposed wp-update/v1, contacted apii.observer, planted PHP in mu-plugins and uploads, created passwordless administrator access and scheduled persistence that survives plugin removal. This is a supply-chain compromise rather than an ordinary request-validation flaw.
- Component
- Fluent Forms Pro
- Plugin slug
fluentformpro- Affected
- 6.2.7
- Safe version
6.2.10- Published
- Aug 13, 2026
This CVE was published Aug 13, 2026 and is one of 4 known issues for this plugin.
Update, patch or deactivate.
Replace the plugin with a vendor-verified clean 6.2.10-or-later build and follow the vendor's incident-response checks. Updating or deleting the plugin alone does not remove planted persistence; inspect mu-plugins, uploads, scheduled tasks and administrator accounts, rotate credentials, and preserve evidence.
A safe version is available, so updating to that version or later is the preferred remediation. If an immediate update is not practical, consider a targeted application patch or temporarily restricting the affected functionality.
Deactivate only when warranted by your risk profile, or when advised by your hosting provider in the limited circumstances where the vulnerability cannot otherwise be mitigated. If you’re unsure which action is appropriate, contact Fused or your hosting provider for guidance.
Technical description
Fluent Forms Pro 6.2.7 contains an embedded malicious code vulnerability introduced via a tampered plugin build served through a decommissioned update server. The tampered build introduced a rogue PHP file (libs/class-license-sync.php), loaded via a require_once directive added to fluentformpro.php, that established a backdoor REST API endpoint, dropped persistent PHP files in mu-plugins and uploads directories, installed a passwordless administrator account, and registered scheduled tasks that survived plugin removal.
CVE / CNA vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X