CVE-2026-39901: Authorization Bypass and Transaction Integrity Flaw in monetr

monetr is a budgeting application designed to facilitate planning for recurring expenses, heavily relying on the integrity of imported transaction data. A core security requirement of the application is the immutability of "synced" transactions, which represent verifiable financial events imported from external banking sources. Prior to version 1.12.3, the application failed to enforce this immutability across all API endpoints, exposing a critical authorization bypass vulnerability tracked as CVE-2026-39901.

This vulnerability manifests as an Improper Authorization flaw (CWE-285) within the transaction management API. Authenticated tenant users possess the ability to interact with both manual and synced transactions. While the application explicitly prohibits the deletion of synced transactions via the dedicated DELETE HTTP method, it fails to apply the same authorization logic to the transaction update routine.

By leveraging the PUT HTTP method against the transaction update endpoint, an authenticated user can supply arbitrary values for server-managed metadata fields. Specifically, injecting a timestamp into the deletedAt field triggers a soft-delete operation within the backend database. This action completely bypasses the intended business logic constraints, allowing users to arbitrarily manipulate their financial history and undermine the fundamental integrity of the application's audit mechanisms.

The root cause of CVE-2026-39901 lies in insecure data binding and insufficient input validation within the application's Go-based backend. The application implements an update routine that accepts a JSON payload representing a Transaction object. During processing, the framework unmarshals this payload directly into the internal domain model structure.

The application logic restricts a small, hardcoded set of fields during this update process. However, it fails to implement an exhaustive allowlist approach for mutable fields or explicitly strip sensitive system-level fields from the incoming payload. Consequently, the deletedAt property, which dictates the soft-delete state of a record in the database, is accepted and mapped directly from user input.

This architectural oversight creates an exploitable disparity between the DELETE and PUT handlers. The DELETE route correctly inspects the metadata of the target transaction, verifying if it is marked as a "synced" or non-manual record. If the condition is met, the application rejects the request with an explicit authorization error. In contrast, the PUT route implicitly trusts the data binding process and persists the object without repeating these authorization checks.

The vulnerability stems from the implementation of the update handler, which directly processes incoming JSON payloads without adequately filtering sensitive struct tags. In the vulnerable versions (prior to 1.12.3), the backend receives the PUT request and maps it to the Transaction entity.

The application attempts to manage state by processing the unmarshaled data, but it neglects to zero out the deletedAt field. The ORM (Object-Relational Mapping) layer subsequently interprets the presence of a non-null deletedAt timestamp as a valid soft-delete operation and executes an UPDATE query against the database, persisting the hidden state.

// Vulnerable Update Handler Implementation
func UpdateTransaction(c *gin.Context) {
    var req Transaction
    if err := c.ShouldBindJSON(&req); err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
        return
    }
 
    // Flaw: The application fails to sanitize req.DeletedAt
    // Only specific business fields are checked, ignoring system metadata
 
    if err := db.Save(&req).Error; err != nil {
        c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to update"})
        return
    }
    c.JSON(http.StatusOK, req)
}

The remediation introduced in version 1.12.3 resolves this flaw by enforcing strict control over the fields permitted during an update operation. The patch modifies the update logic to selectively map only user-modifiable attributes from the request payload to the persistent model, effectively discarding any user-supplied values for internal tracking fields.

// Patched Update Handler Implementation
func UpdateTransaction(c *gin.Context) {
    var req UpdateTransactionRequest
    if err := c.ShouldBindJSON(&req); err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
        return
    }
 
    var tx Transaction
    if err := db.First(&tx, id).Error; err != nil {
        c.JSON(http.StatusNotFound, gin.H{"error": "Not found"})
        return
    }
 
    // Fix: Explicitly map only allowed fields.
    // The deletedAt field is strictly controlled by the DELETE handler.
    tx.Amount = req.Amount
    tx.Category = req.Category
    tx.Notes = req.Notes
 
    if err := db.Save(&tx).Error; err != nil {
        c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to update"})
        return
    }
    c.JSON(http.StatusOK, tx)
}

Exploitation of CVE-2026-39901 is highly deterministic and requires minimal prerequisites. An attacker must possess valid credentials for a tenant account within the monetr application. No elevated administrative privileges are necessary. The attacker must first identify the unique identifier of a synced transaction they intend to hide.

The attacker observes that submitting a standard DELETE /transactions/{id} request results in a deterministic rejection by the application's authorization middleware. The server responds with an error indicating that synced transactions cannot be deleted manually. This confirms the presence of the intended business logic constraint.

To bypass this restriction, the attacker crafts a malicious PUT request directed at the /transactions/{id} endpoint. The JSON payload includes standard transaction fields alongside the injected deletedAt property. The value supplied must be a valid ISO 8601 timestamp to ensure the ORM processes it correctly without throwing a validation exception.

PUT /api/v1/transactions/550e8400-e29b-41d4-a716-446655440000 HTTP/1.1
Host: app.monetr.local
Authorization: Bearer <valid_tenant_token>
Content-Type: application/json
 
{
  "amount": 150.00,
  "notes": "Legitimate update attempt",
  "deletedAt": "2026-04-08T22:00:00Z"
}

Upon receiving this payload, the backend application binds the data and executes the update. The database records the transaction as soft-deleted. Consequently, the transaction disappears from the user's interface and all subsequent financial calculations, achieving the objective of the bypass.

The primary impact of this vulnerability is a severe degradation of data integrity within the budgeting application. The CVSS v3.1 score is calculated at 5.7 (Medium), with the vector CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N. This score reflects the necessity of low-level privileges and user interaction, juxtaposed against the high integrity impact of the exploit.

While the vulnerability does not expose sensitive data to unauthorized parties or facilitate arbitrary code execution, it subverts the core purpose of the monetr application. By allowing users to selectively hide imported bank transactions, the application can no longer provide an accurate or reliable financial overview. This fundamentally undermines user trust in the platform's accounting mechanisms.

> [!NOTE] > Although the records are only "soft-deleted" and remain in the underlying database schema, they are completely excluded from application-level views and calculations. This functional removal is equivalent to a permanent deletion from the perspective of the application's business logic.

Furthermore, the flaw highlights a systemic issue in how API endpoints enforce authorization. The discrepancy in security controls between different HTTP methods operating on the same resource is a classic manifestation of incomplete access control modeling.

The official remediation for CVE-2026-39901 is to upgrade the monetr application to version 1.12.3 or later. The patch implements a strict allowlist approach for data binding on the transaction update endpoint, ensuring that system-managed fields like deletedAt cannot be modified via client-supplied JSON payloads.

There are no practical configuration-based workarounds available for vulnerable deployments. Administrators operating instances prior to 1.12.3 must apply the software update immediately to restore the integrity of the transaction handling logic. Attempting to mitigate the issue at the reverse proxy level is complex and error-prone, as it requires deep inspection and modification of JSON request bodies.

For detection purposes, security teams should implement monitoring on application request logs. Specifically, alerts should be configured to trigger when PUT requests to the /transactions/ path contain the deletedAt key within the payload. Correlating these requests with the target transaction's synchronization status can definitively identify exploitation attempts.

> [!TIP] > In environments where immediate patching is impossible, configuring a Web Application Firewall (WAF) to block PUT requests containing the exact string "deletedAt" in the body directed at /transactions/* can serve as a temporary compensating control.