External risk intelligence

Fider Tenant Isolation Bypass via Email Verification Key

CVE advisorySeverity: CRITICAL (CVSS 9.6)

CVE-2026-65840

Fider is a feedback portal typically deployed as a public-facing web application. While this specific vulnerability affects multi-tenant configurations, the nature of the software as a web-based portal intended for user suggestions means it is commonly exposed to the internet in real-world deployments.

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This vulnerability affects Fider, a feedback portal used for feature requests, specifically in its multi-tenant configuration. It allows an attacker to bypass tenant isolation by using an email verification key from one tenant to access and potentially take over accounts in another. This could lead to unauthorized access and data exposure across different customer instances.

Bypasses tenant security in Fider's multi-tenant setup.
Leaders should remember it if using Fider in multi-tenant mode.
Confirm relevance and exposure of multi-tenant Fider.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by obtaining an email verification key from one tenant and then using it to impersonate a user in a different tenant. This allows them to bypass tenant isolation controls and take over accounts on other tenants within the same Fider instance.

Attacker needs a verification key.
Attacker uses key on different tenant.
Complete bypass of tenant isolation.

Live Threat

Current exploitation, exposure, and threat context

When Fider is deployed in multi-tenant mode, an attacker could leverage an email verification key from one tenant to gain unauthorized access to another tenant's account, effectively bypassing tenant isolation. This could lead to account takeover across different tenants within the same Fider instance.

Tenant account data.
Email verification key misuse.
Unauthorized account takeover.

Operational Fix

Recommended remediation, mitigation, and detection steps

To address this vulnerability, application owners responsible for Fider instances in multi-tenant mode should be the primary responders. The immediate practical move is to identify all deployed Fider instances, determine if they are operating in multi-tenant mode, and assess their external reachability and business criticality. Once accountable owners are confirmed, remediation efforts can be planned based on the assessed risk.

Application owners should lead remediation.
Verify multi-tenant mode and exposure.
Plan and coordinate necessary updates.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Fider?

Fider is an open-source feedback portal designed to help organizations collect, manage, and prioritize feature requests and suggestions from their users in a centralized web interface.

What does CWE-639 mean for CVE-2026-65840?

CWE-639, or Authorization Bypass Through User-Controlled Key, describes a weakness where an application incorrectly validates a key or identifier. In this CVE, the system fails to verify that an email authentication key belongs to the specific tenant session, allowing a user to bypass security boundaries.

How can an attacker trigger this vulnerability?

An attacker triggers this by obtaining an email verification key from one Fider tenant and presenting that same key to a different tenant within the same instance to authenticate. Note that standard, single-host installations where this multi-tenant configuration is disabled are not affected by this specific issue.

Is my Fider instance at risk?

According to Halo Surface Signal, Fider is typically deployed as a public-facing web application. You should be concerned if your instance is configured in multi-tenant mode, as this makes the authorization bypass accessible to network-based attackers rather than being limited to internal users.

When should I update Fider?

If you are running a multi-tenant Fider environment, you should prioritize updating to version 0.36.0 immediately. First, confirm your current version and verify if the multi-tenant setting is enabled, then coordinate with your team to apply the patch provided by the developers.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.