CVE-2026-27933: Manyfold Auth Bypass Vulnerability

CVE-2026-27933 Overview

CVE-2026-27933 is a session hijacking vulnerability affecting Manyfold, an open source, self-hosted web application designed for managing collections of 3D models with a focus on 3D printing workflows. The vulnerability allows attackers to hijack user sessions through cookie leakage in proxy caches, potentially leading to unauthorized access to user accounts and sensitive data.

Critical Impact

Attackers can intercept and reuse session cookies cached by proxy servers, enabling complete account takeover without requiring authentication credentials.

Affected Products

• Manyfold versions prior to 0.133.0
• Self-hosted Manyfold instances deployed behind proxy caches
• Manyfold deployments using reverse proxies without proper cache-control headers

Discovery Timeline

• 2026-02-26 - CVE CVE-2026-27933 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-27933

Vulnerability Analysis

This vulnerability stems from insufficient session expiration controls (CWE-613), where session cookies are improperly handled when requests pass through proxy caches. When a user authenticates to a Manyfold instance that sits behind a caching proxy, the session cookie may be stored in the proxy's cache. Subsequent requests from other users passing through the same proxy could potentially receive the cached response containing another user's session cookie, effectively allowing session hijacking.

The attack requires specific conditions to be successful: the Manyfold instance must be deployed behind a proxy cache, and the application must not be setting appropriate cache-control headers to prevent sensitive authentication data from being cached.

Root Cause

The root cause of CVE-2026-27933 lies in missing or insufficient Cache-Control headers on responses containing session cookies. Without explicit directives such as no-store or private, intermediate proxy caches may store and serve responses containing sensitive session data to unintended recipients. This is classified as CWE-613 (Insufficient Session Expiration), as the session data persists in an uncontrolled location (the proxy cache) beyond its intended scope.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker must position themselves to receive cached responses from a proxy server that has stored another user's session cookie. The attack flow typically involves:

1. A legitimate user authenticates to the Manyfold application
2. The authentication response, including the session cookie, is cached by an intermediary proxy
3. An attacker makes a request through the same proxy server
4. The proxy serves the cached response containing the victim's session cookie
5. The attacker uses the stolen session cookie to impersonate the victim

The vulnerability mechanism involves improper cache-control header configuration. When responses containing session cookies lack appropriate Cache-Control: no-store or Cache-Control: private directives, proxy caches may store these responses. For detailed technical information, refer to the GitHub Security Advisory GHSA-g949-hmvj-2r76.

Detection Methods for CVE-2026-27933

Indicators of Compromise

• Multiple sessions originating from different IP addresses using identical session tokens
• Unusual access patterns where a session token appears in requests from geographically disparate locations
• Proxy cache logs showing cached responses containing Set-Cookie headers
• User reports of unauthorized access to their accounts without credential compromise

Detection Strategies

• Monitor authentication logs for sessions being used from multiple distinct source IPs
• Implement session binding to client fingerprints (IP address, User-Agent) and alert on mismatches
• Review proxy cache configurations to identify cached responses containing authentication headers
• Deploy web application firewall rules to detect and block requests with suspicious session reuse patterns

Monitoring Recommendations

• Enable detailed logging on proxy servers to track which responses are being cached
• Implement real-time alerting for session anomalies such as concurrent use from different networks
• Regularly audit HTTP response headers to ensure proper cache-control directives are present
• Monitor for unusual spikes in session creation or authentication events

How to Mitigate CVE-2026-27933

Immediate Actions Required

• Upgrade Manyfold to version 0.133.0 or later immediately
• Invalidate all existing user sessions and require re-authentication
• Review and configure proxy cache settings to exclude responses with Set-Cookie headers
• Implement proper Cache-Control headers on your reverse proxy as an additional layer of defense

Patch Information

The vulnerability has been fixed in Manyfold version 0.133.0. The patch implements proper cache-control headers to prevent session cookies from being stored in proxy caches. Users should upgrade to this version or later to remediate the vulnerability. For complete release notes and download information, see the GitHub Release v0.133.0.

Workarounds

• Configure proxy servers to never cache responses containing Set-Cookie headers
• Add explicit Cache-Control: no-store, private headers at the proxy level for all authentication-related endpoints
• Implement additional session validation such as binding sessions to client IP addresses
• Consider deploying Manyfold without intermediate caching proxies until the upgrade can be completed

# Nginx proxy configuration to prevent caching of sensitive responses
location / {
    proxy_pass http://manyfold_backend;
    
    # Prevent caching of responses with Set-Cookie headers
    proxy_no_cache $http_set_cookie;
    proxy_cache_bypass $http_set_cookie;
    
    # Add cache-control headers for authentication endpoints
    add_header Cache-Control "no-store, no-cache, must-revalidate, private" always;
}