CVE-2026-102248: Rebuild Authentication Bypass Vulnerability

CVE-2026-102248 Overview

CVE-2026-102248 is an improper authentication vulnerability [CWE-287] affecting Rebuild versions up to 4.4.7 and 4.5.0-beta5. The flaw resides in an unknown component of the /user/login file, which serves as the Login Endpoint. Attackers can manipulate the endpoint remotely to bypass authentication controls without user interaction or prior credentials. Public exploit code exists and may be leveraged by opportunistic attackers. The vendor was contacted about the disclosure but did not respond, leaving affected deployments without a coordinated fix at time of publication.

Critical Impact

Remote attackers can bypass authentication on the Rebuild login endpoint, gaining unauthorized access to application functions and data.

Affected Products

• Rebuild versions up to and including 4.4.7
• Rebuild 4.5.0-beta5
• Login Endpoint component (/user/login)

Discovery Timeline

• 2026-09-29 - CVE CVE-2026-102248 published to NVD
• 2026-09-29 - Last updated in NVD database

Technical Details for CVE-2026-102248

Vulnerability Analysis

The vulnerability is classified under [CWE-287: Improper Authentication]. It affects the /user/login handler in Rebuild, an open-source business application platform. The flaw allows a remote attacker to manipulate login processing so that authentication checks are bypassed or improperly enforced.

Because the login endpoint is the primary trust boundary for the application, a bypass here can grant an attacker access to authenticated application functionality. The confidentiality, integrity, and availability impacts are all rated as low individually, reflecting scoped rather than complete compromise. However, follow-on access to business data and workflows remains a concrete risk.

Exploit code is publicly available through the referenced GitHub issue and VulDB entry, which lowers the barrier for opportunistic attackers to weaponize the flaw.

Root Cause

The root cause is an improper authentication check within the login processing logic of /user/login. Public technical details are limited, but the classification under CWE-287 indicates that the endpoint fails to correctly verify the identity claimed by a client under specific manipulated request conditions. Consult the GitHub Issue CVE-25 for the researcher's technical writeup.

Attack Vector

The attack vector is Network. An unauthenticated attacker sends a crafted request to the /user/login endpoint over HTTP or HTTPS. No user interaction and no prior privileges are required. Because Rebuild is commonly deployed as a web-accessible application, exposed instances are directly reachable by remote adversaries. Refer to the public references for exploit specifics rather than fabricated proof-of-concept code.

Detection Methods for CVE-2026-102248

Indicators of Compromise

• Unexpected successful authentication events at /user/login from unfamiliar source IP addresses
• Session tokens issued without a corresponding credential validation entry in application logs
• Login requests containing anomalous or malformed parameters targeting /user/login
• Access to authenticated Rebuild functionality by accounts with no recent legitimate sign-in history

Detection Strategies

• Correlate web server access logs for POST /user/login requests against application authentication success logs to find mismatches
• Alert on brute-force patterns or repeated probing of /user/login from single sources
• Deploy a web application firewall rule to inspect and log requests to the login endpoint
• Baseline normal login request structures and flag deviations in parameters, headers, or content types

Monitoring Recommendations

• Enable verbose authentication logging in Rebuild and forward logs to a centralized SIEM
• Monitor for privilege changes, data exports, or administrative actions following login events
• Track new session creation rates for the Rebuild application against historical baselines
• Review outbound network activity from the Rebuild host for signs of data staging or exfiltration

How to Mitigate CVE-2026-102248

Immediate Actions Required

• Restrict network exposure of the Rebuild /user/login endpoint to trusted networks or VPN users only
• Place the application behind a reverse proxy or web application firewall with authentication controls
• Force password rotation and invalidate all existing sessions for Rebuild user accounts
• Audit recent authentication logs and administrative actions for indicators of unauthorized access

Patch Information

No vendor patch is available at time of publication. According to the disclosure, the vendor was contacted but did not respond. Monitor the VulDB advisory for CVE-2026-102248 and the Rebuild project repository for updates. Once a patched release is published, upgrade beyond versions 4.4.7 and 4.5.0-beta5.

Workarounds

• Enforce network-layer access controls to limit who can reach the login endpoint
• Require an authenticating reverse proxy (for example, mutual TLS or SSO) in front of Rebuild
• Enable multi-factor authentication on any upstream identity provider protecting the application
• Consider taking internet-exposed Rebuild instances offline until a vendor fix is confirmed

# Example: restrict access to /user/login using an Nginx allowlist
location /user/login {
    allow 10.0.0.0/8;      # internal network
    allow 192.168.0.0/16;  # management network
    deny all;
    proxy_pass http://rebuild_backend;
}