CVE-2026-25892: Adminer DOS Vulnerability

CVE-2026-25892 Overview

CVE-2026-25892 is an improper input validation vulnerability affecting Adminer, the popular open-source database management software. The vulnerability exists in Adminer's version check mechanism where the ?script=version endpoint lacks proper origin validation and accepts POST data from any source. An attacker can exploit this by sending a version[] parameter which PHP converts to an array, causing openssl_verify() to receive an array instead of a string, resulting in a TypeError that returns HTTP 500 errors to all users.

Critical Impact

This denial of service vulnerability allows unauthenticated remote attackers to make Adminer completely unavailable to all legitimate users by triggering persistent HTTP 500 errors through a simple malformed POST request.

Affected Products

• Adminer v5.4.1 and earlier versions
• All Adminer installations using the default version check mechanism
• Self-hosted Adminer deployments with the ?script=version endpoint accessible

Discovery Timeline

• 2026-02-09 - CVE CVE-2026-25892 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-25892

Vulnerability Analysis

This vulnerability stems from a type confusion issue in PHP's handling of POST parameters combined with missing origin validation on a critical endpoint. Adminer implements a version check mechanism where adminer.org sends signed version information via JavaScript postMessage, which the browser then POSTs to the ?script=version endpoint. The fundamental security flaw is that this endpoint does not validate the origin of incoming POST requests, allowing any source to submit data.

When an attacker submits the version parameter as an array (using version[] syntax), PHP automatically converts this to an array data type. The openssl_verify() function, which expects a string argument for signature verification, receives this array instead. This type mismatch causes PHP to throw a TypeError exception. The unhandled exception results in an HTTP 500 Internal Server Error response that persists across subsequent page loads for all users, effectively creating a denial of service condition.

Root Cause

The root cause of CVE-2026-25892 is twofold: first, the ?script=version endpoint lacks proper origin validation, accepting POST data from any source rather than only from trusted adminer.org origins. Second, there is no input type validation to ensure the version parameter is a string before passing it to openssl_verify(). This combination of CWE-20 (Improper Input Validation) allows attackers to inject malformed data that triggers type confusion in cryptographic verification functions.

Attack Vector

The attack can be executed remotely over the network without any authentication or user interaction. An attacker simply needs to craft a POST request to the vulnerable Adminer instance's ?script=version endpoint with a malformed version[] parameter. This can be accomplished through a simple HTTP POST request from any location that can reach the target Adminer installation. The attack requires no privileges and has no complexity barriers, making it trivially exploitable once a vulnerable Adminer instance is identified.

The exploitation flow involves:

1. Attacker identifies an Adminer installation running version 5.4.1 or earlier
2. Attacker sends a POST request to target-adminer/?script=version with version[] as a parameter
3. PHP converts version[] to an array
4. On next page load, openssl_verify() receives an array instead of expected string
5. TypeError is thrown, resulting in HTTP 500 error for all users

Detection Methods for CVE-2026-25892

Indicators of Compromise

• HTTP 500 errors occurring persistently on Adminer pages after a POST request to ?script=version
• Unusual POST requests to the ?script=version endpoint from non-adminer.org sources
• PHP error logs showing TypeError exceptions in openssl_verify() calls
• Multiple requests with version[] array parameters in web server access logs

Detection Strategies

• Monitor web server access logs for POST requests to ?script=version containing array parameters like version[]
• Configure PHP error logging to capture TypeError exceptions and alert on openssl_verify() failures
• Implement web application firewall (WAF) rules to detect and block requests with array parameter injection patterns
• Set up uptime monitoring for Adminer endpoints to detect sudden HTTP 500 error conditions

Monitoring Recommendations

• Enable verbose logging on the web server hosting Adminer to capture all POST request parameters
• Configure SIEM rules to correlate ?script=version endpoint access with subsequent HTTP 500 errors
• Implement alerting for any PHP TypeError exceptions in application logs
• Monitor for availability degradation of Adminer services as an early warning sign

How to Mitigate CVE-2026-25892

Immediate Actions Required

• Upgrade Adminer to version 5.4.2 immediately, which includes the fix for this vulnerability
• If immediate upgrade is not possible, restrict access to the ?script=version endpoint via web server configuration
• Review web server logs for any evidence of exploitation attempts
• Consider temporarily disabling the version check functionality until patching is complete

Patch Information

The vulnerability has been addressed in Adminer version 5.4.2. The fix is available through the GitHub Release v5.4.2. The specific commit addressing this issue can be reviewed at the GitHub Commit Update. Additional details about the vulnerability are documented in the GitHub Security Advisory GHSA-q4f2-39gr-45jh.

Workarounds

• Block POST requests to the ?script=version endpoint at the web server or reverse proxy level
• Implement origin validation at the web server level to only accept requests from adminer.org
• Deploy a web application firewall rule to reject requests containing version[] array parameters
• Temporarily disable or remove the version check functionality by modifying Adminer configuration

# Apache configuration to block vulnerable endpoint
<Location "/?script=version">
    <LimitExcept GET>
        Require all denied
    </LimitExcept>
</Location>