simplesamlphp/xml-security: Missing AES-GCM Authentication Tag Validation on Encrypted Nodes Allows for Unauthorized Decryption
Date: 2026-03
Affected: composer/simplesamlphp/xml-security, composer/simplesamlphp/xml-security
Attack vector: other
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-32600
CWE: CWE-354
CVSS: 8.2 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N)

### Summary XML nodes encrypted with either aes-128-gcm, aes-192-gcm, or aes-256-gcm lack validation of the authentication tag length. An attacker can use this to brute-force an authentication tag, recover the [GHASH key](https://en.wikipedia.org/wiki/Galois/Counter_Mode#:~:text=%29%20is%20the-,hash%20key,-%2C%20a%20string%20of), and decrypt the encrypted nodes. It also allows to forge arbitrary ciphertexts without knowing the encryption key. ### Details When decrypting with either aes-128-gcm, aes-192-gcm, or aes-256-gcm [here](https://github.com/robrichards/xmlseclibs/blob/2bdfd742624d739dfadbd415f00181b4a77aaf07/src/XMLSecurityKey.php#L467-L479), the $authTag is set from a substr(), but never has its length validated (it should be validated with something like strlen($authTag) == self::AUTHTAG_LENGTH). For that reason, a shorter than expected data blob will allow for the $authTag to have as short a tag as only one byte (see [PHP's documentation](https://www.php.net/manual/en/function.openssl-decrypt.php#:~:text=The%20length%20of%20the%20tag%20is%20not%20checked%20by%20the%20function.%20It%20is%20the%20caller%27s%20responsibility%20to%20ensure%20that%20the%20length%20of%20the%20tag%20matches%20the%20length%20of%20the%20tag%20retrieved%20when%20openssl_encrypt()%20has%20been%20called.%20Otherwise%20the%20decryption%20may%20succeed%20if%20the%20given%20tag%20only%20matches%20the%20start%20of%20the%20proper%20tag.)). See this example: ```php function test($data) { $iv


Mitigations:

Architectural layers (MAESTRO):