CVE-2026-27584: Actualbudget Information Disclosure Flaw

CVE-2026-27584 Overview

CVE-2026-27584 is a critical authentication bypass vulnerability in ActualBudget, a local-first personal finance tool. Prior to version 26.2.1, missing authentication middleware in the ActualBudget server component allows any unauthenticated user to query the SimpleFIN and Pluggy.ai integration endpoints and read sensitive bank account balance and transaction information.

This vulnerability represents a severe security flaw classified as CWE-306 (Missing Authentication for Critical Function), where endpoints handling sensitive financial data lack proper authentication controls. An attacker with network access to an affected ActualBudget Server instance can retrieve users' bank account balances and complete transaction histories without any credentials.

Critical Impact

Unauthenticated attackers can remotely access and exfiltrate sensitive bank account balances and transaction histories from any network-accessible ActualBudget Server instance with SimpleFIN or Pluggy.ai integrations configured.

Affected Products

• ActualBudget Actual versions prior to 26.2.1
• ActualBudget Server instances with SimpleFIN integration configured
• ActualBudget Server instances with Pluggy.ai integration configured

Discovery Timeline

• 2026-02-24 - CVE-2026-27584 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-27584

Vulnerability Analysis

The vulnerability exists in the ActualBudget sync-server component, specifically affecting the SimpleFIN and Pluggy.ai integration endpoints. These endpoints are designed to facilitate bank account synchronization but were deployed without the required validateSessionMiddleware authentication check. This oversight allows any user with network access to the server to directly query these endpoints and retrieve sensitive financial information.

The attack is straightforward to execute as it requires no prior authentication or special privileges. An attacker simply needs network connectivity to the vulnerable ActualBudget Server instance. The impact is significant given the nature of the exposed data—complete bank account balances and transaction histories represent highly sensitive personal financial information.

Root Cause

The root cause is the omission of the validateSessionMiddleware authentication middleware from the Express.js application handlers for both the SimpleFIN (app-simplefin.js) and Pluggy.ai (app-pluggyai.js) integration modules. While other parts of the application properly enforce session validation, these integration endpoints were left unprotected, creating an authentication bypass condition.

Attack Vector

The attack vector is network-based and requires no user interaction or prior authentication. An attacker can exploit this vulnerability by:

1. Identifying a network-accessible ActualBudget Server instance
2. Sending unauthenticated HTTP POST requests to the SimpleFIN or Pluggy.ai endpoints (e.g., /status)
3. Receiving sensitive bank account data including balances and transaction history in the response

The following patches were applied to remediate the vulnerability:

SimpleFIN Integration Fix (packages/sync-server/src/app-simplefin/app-simplefin.js):

 
 import { handleError } from '../app-gocardless/util/handle-error';
 import { SecretName, secretsService } from '../services/secrets-service';
-import { requestLoggerMiddleware } from '../util/middlewares';
+import {
+  requestLoggerMiddleware,
+  validateSessionMiddleware,
+} from '../util/middlewares';
 
 const app = express();
 export { app as handlers };
-app.use(express.json());
 app.use(requestLoggerMiddleware);
+app.use(express.json());
+app.use(validateSessionMiddleware);
 
 app.post(
   '/status',

Source: GitHub Commit Details

Pluggy.ai Integration Fix (packages/sync-server/src/app-pluggyai/app-pluggyai.js):

 
 import { handleError } from '../app-gocardless/util/handle-error';
 import { SecretName, secretsService } from '../services/secrets-service';
-import { requestLoggerMiddleware } from '../util/middlewares';
+import {
+  requestLoggerMiddleware,
+  validateSessionMiddleware,
+} from '../util/middlewares';
 
 import { pluggyaiService } from './pluggyai-service';
 
 const app = express();
 export { app as handlers };
-app.use(express.json());
 app.use(requestLoggerMiddleware);
+app.use(express.json());
+app.use(validateSessionMiddleware);
 
 app.post(
   '/status',

Source: GitHub Commit Details

Detection Methods for CVE-2026-27584

Indicators of Compromise

• Unusual HTTP POST requests to SimpleFIN or Pluggy.ai endpoints (/status, /accounts, /transactions) from external or untrusted IP addresses
• Access logs showing requests to /simplefin/* or /pluggyai/* paths without corresponding authenticated sessions
• High volume of API calls to financial integration endpoints from single sources
• Network traffic patterns indicating bulk data exfiltration of financial information

Detection Strategies

• Review web server access logs for unauthenticated requests to /app-simplefin/ and /app-pluggyai/ endpoints
• Implement network monitoring to detect anomalous traffic patterns to the ActualBudget Server
• Audit session logs to identify requests to financial integration endpoints that lack valid session tokens
• Deploy web application firewall (WAF) rules to alert on requests to sensitive endpoints without proper authentication headers

Monitoring Recommendations

• Enable detailed access logging for all ActualBudget Server endpoints
• Configure alerting for requests to financial integration endpoints from non-whitelisted IP addresses
• Monitor for bulk data retrieval patterns that may indicate active exploitation
• Implement rate limiting on sensitive endpoints to reduce impact of automated attacks

How to Mitigate CVE-2026-27584

Immediate Actions Required

• Upgrade ActualBudget to version 26.2.1 or later immediately
• Restrict network access to ActualBudget Server instances using firewall rules until patching is complete
• Review access logs for evidence of prior exploitation
• Consider temporarily disabling SimpleFIN and Pluggy.ai integrations if immediate upgrade is not possible

Patch Information

ActualBudget has released version 26.2.1 which addresses this vulnerability by adding the validateSessionMiddleware authentication check to both the SimpleFIN and Pluggy.ai integration endpoints. The fix ensures all requests to these sensitive financial data endpoints require a valid authenticated session.

For detailed information about the security fix, refer to:

• GitHub Security Advisory GHSA-m2cq-xjgm-f668
• GitHub Commit Details

Workarounds

• Restrict network access to ActualBudget Server to trusted networks only using firewall rules
• Place ActualBudget Server behind a reverse proxy with additional authentication requirements
• Disable SimpleFIN and Pluggy.ai integrations until the patch can be applied
• Implement VPN requirements for accessing the ActualBudget Server

# Example: Restrict access to ActualBudget Server using iptables
# Allow only trusted network (e.g., 192.168.1.0/24)
iptables -A INPUT -p tcp --dport 5006 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 5006 -j DROP