CVE-2026-39901: monetr Auth Bypass Vulnerability

CVE-2026-39901 Overview

CVE-2026-39901 is an authorization bypass vulnerability in monetr, an open-source budgeting application focused on planning for recurring expenses. Prior to version 1.12.3, a transaction integrity flaw allows an authenticated tenant user to soft-delete synced non-manual transactions through the transaction update endpoint, despite the application explicitly blocking deletion of those transactions via the normal DELETE path. This bypass undermines the intended protection for imported transaction records and allows protected transactions to be hidden from normal views.

Critical Impact

Authenticated users can bypass transaction deletion restrictions, potentially hiding imported financial records and compromising data integrity within the budgeting application.

Affected Products

• monetr versions prior to 1.12.3

Discovery Timeline

• 2026-04-08 - CVE CVE-2026-39901 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-39901

Vulnerability Analysis

This vulnerability is classified under CWE-285 (Improper Authorization), which occurs when an application fails to properly verify that a user is authorized to perform a specific action. In the case of monetr, the application correctly implements access controls on the DELETE endpoint for synced transactions, preventing direct deletion of imported financial records. However, this protection is inconsistently applied across the application's API surface.

The transaction update endpoint lacks equivalent authorization checks, creating an alternative pathway that attackers can exploit. An authenticated tenant user can leverage this endpoint to set a soft-delete flag on transactions that should be protected from deletion. This inconsistency in authorization enforcement represents a classic authorization bypass pattern where different code paths lead to the same logical outcome without uniform security controls.

Root Cause

The root cause of this vulnerability is an inconsistent application of authorization controls across different API endpoints that affect the same protected resource. While the DELETE endpoint correctly blocks removal of synced non-manual transactions, the transaction update endpoint does not enforce the same restrictions when modifying the deletion status of these records. This allows the soft-delete mechanism to be abused as a backdoor for hiding protected transactions.

Attack Vector

The attack is conducted over the network by an authenticated user with tenant access to the monetr application. The attacker targets the transaction update endpoint rather than the protected DELETE endpoint. By crafting a request to the update endpoint that sets the soft-delete flag on a synced transaction, the attacker can effectively hide the transaction from normal application views.

The attack requires low complexity to execute—an authenticated user simply needs to identify the transaction update endpoint and send a modified request. User interaction is required in the form of authentication, but once authenticated, the attacker can target any synced transaction within their tenant scope. The integrity impact is high as protected financial records can be hidden, though confidentiality and availability are not directly affected.

Detection Methods for CVE-2026-39901

Indicators of Compromise

• Unexpected soft-delete flags appearing on synced or imported transactions
• Transaction update API calls that modify deletion status on non-manual transactions
• Audit log entries showing transaction updates that bypass normal deletion workflows
• User reports of missing imported transactions that were not manually deleted

Detection Strategies

• Monitor API access logs for transaction update requests that modify soft-delete fields
• Implement audit logging specifically for state changes on synced transaction records
• Create alerts for any modifications to protected transaction attributes via update endpoints
• Review application logs for patterns indicating endpoint abuse or authorization bypass attempts

Monitoring Recommendations

• Enable detailed logging on all transaction-related API endpoints
• Implement integrity checks comparing expected versus actual transaction states
• Set up automated alerts for unusual patterns in transaction soft-deletions
• Periodically audit synced transaction records for unexpected state changes

How to Mitigate CVE-2026-39901

Immediate Actions Required

• Upgrade monetr to version 1.12.3 or later immediately
• Review transaction audit logs for signs of exploitation prior to patching
• Restore any improperly soft-deleted synced transactions from backups if needed
• Verify the integrity of all imported financial records

Patch Information

The vulnerability is fixed in monetr version 1.12.3. Users should update to this version or later to address the authorization bypass. The patch ensures consistent authorization enforcement across both the DELETE endpoint and the transaction update endpoint, preventing authenticated users from soft-deleting protected synced transactions through alternative code paths.

For detailed information about the fix, refer to the GitHub Security Advisory.

Workarounds

• Restrict access to the transaction update endpoint at the network or application layer until patching is possible
• Implement additional server-side validation to block soft-delete operations on synced transactions
• Enable comprehensive audit logging to detect and investigate any exploitation attempts
• Consider temporarily disabling transaction sync functionality if the update cannot be applied immediately