CVE-2026-27898: Vaultwarden Information Disclosure Flaw

CVE-2026-27898 Overview

CVE-2026-27898 is an Insecure Direct Object Reference (IDOR) vulnerability in Vaultwarden, an unofficial Bitwarden-compatible server written in Rust (formerly known as bitwarden_rs). This vulnerability allows an authenticated regular user to access sensitive cipher data belonging to other users by exploiting a flaw in the partial update endpoint's authorization mechanism.

Prior to version 1.35.4, an authenticated user can specify another user's cipher_id and call the PUT /api/ciphers/{id}/partial endpoint. Although the standard retrieval API correctly denies access to unauthorized ciphers, the partial update endpoint returns a 200 OK response and exposes sensitive cipher details including name, notes, data, and secureNote fields.

Critical Impact

Authenticated attackers can access other users' sensitive password vault data including credentials, secure notes, and other stored secrets, undermining the core security guarantees of the password management system.

Affected Products

• Vaultwarden versions prior to 1.35.4
• Self-hosted Bitwarden-compatible deployments using Vaultwarden

Discovery Timeline

• 2026-03-04 - CVE CVE-2026-27898 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-27898

Vulnerability Analysis

This vulnerability is classified as CWE-639 (Authorization Bypass Through User-Controlled Key), commonly known as an Insecure Direct Object Reference (IDOR). The flaw exists in the authorization logic of the partial update endpoint for cipher management.

The core issue stems from inconsistent authorization checks between different API endpoints handling cipher data. While the standard cipher retrieval endpoints properly validate that the requesting user owns the cipher they're attempting to access, the partial update endpoint at /api/ciphers/{id}/partial fails to perform this ownership verification before processing the request and returning cipher details in the response.

An authenticated attacker can enumerate or guess cipher IDs belonging to other users and submit PUT requests to the vulnerable endpoint. Even though the attacker cannot modify the target cipher (as the operation may still fail authorization at the write level), the API response itself contains the full cipher details, resulting in unauthorized data disclosure.

Root Cause

The root cause is insufficient authorization validation in the partial update endpoint's request handling logic. The endpoint processes the request and retrieves cipher details before validating whether the authenticated user has permission to access that specific cipher. This creates a scenario where the authorization check occurs too late in the request lifecycle, after sensitive data has already been included in the response body.

Attack Vector

The attack is network-based and requires the attacker to have valid authentication credentials for the Vaultwarden instance. The attack flow involves:

1. An attacker authenticates to the Vaultwarden instance with valid credentials
2. The attacker identifies or enumerates cipher IDs belonging to other users
3. The attacker crafts PUT requests to /api/ciphers/{id}/partial using victim cipher IDs
4. The vulnerable endpoint returns 200 OK responses containing the victim's cipher details
5. The attacker extracts sensitive information including passwords, notes, and other stored credentials

The vulnerability requires low attack complexity with no user interaction needed. An attacker only needs a valid account on the same Vaultwarden instance to exploit this flaw and access other users' vault contents.

Detection Methods for CVE-2026-27898

Indicators of Compromise

• Unusual patterns of PUT requests to /api/ciphers/{id}/partial endpoints from single user sessions
• API requests targeting cipher IDs that don't belong to the requesting user
• Sequential or bulk requests to the partial update endpoint with incrementing cipher IDs
• High volume of 200 OK responses from the partial endpoint without corresponding successful updates

Detection Strategies

• Implement API request logging and monitor for anomalous access patterns to cipher endpoints
• Enable audit logging in Vaultwarden to track cipher access attempts
• Configure alerts for users accessing cipher endpoints with IDs outside their ownership scope
• Deploy web application firewalls (WAF) with rules to detect enumeration attacks against REST APIs

Monitoring Recommendations

• Review Vaultwarden access logs for bulk requests to the /api/ciphers/*/partial endpoint pattern
• Monitor for authenticated sessions making excessive requests to cipher-related endpoints
• Implement rate limiting on cipher endpoints to slow down potential enumeration attempts
• Enable network-level monitoring to detect unusual API traffic patterns to Vaultwarden instances

How to Mitigate CVE-2026-27898

Immediate Actions Required

• Upgrade Vaultwarden to version 1.35.4 or later immediately
• Audit access logs for signs of exploitation prior to patching
• Consider rotating stored credentials if unauthorized access is suspected
• Review and restrict network access to Vaultwarden instances to trusted networks only

Patch Information

This vulnerability has been patched in Vaultwarden version 1.35.4. The fix implements proper authorization checks in the partial update endpoint to ensure users can only access cipher data they own. For detailed patch information and security advisory, refer to the GitHub Security Advisory.

Workarounds

• If immediate patching is not possible, consider temporarily disabling external access to the Vaultwarden instance
• Implement network-level access controls (VPN, IP allowlisting) to limit who can reach the Vaultwarden API
• Deploy a reverse proxy with request filtering to block suspicious patterns to the vulnerable endpoint
• Monitor API logs closely for exploitation attempts while planning the upgrade

# Upgrade Vaultwarden to patched version
# Using Docker (most common deployment):
docker pull vaultwarden/server:1.35.4
docker stop vaultwarden
docker rm vaultwarden
docker run -d --name vaultwarden -v /vw-data/:/data/ -p 80:80 vaultwarden/server:1.35.4

# Verify the version after upgrade
docker exec vaultwarden vaultwarden --version