CVE-2026-33204

SimpleJWT is a simple JSON web token library written in PHP. Prior to version 1.1.1, an unauthenticated attacker can perform a Denial of Service via JWE header tampering when PBES2 algorithms are used. Applications that call JWE::decrypt() on attacker-controlled JWEs using PBES2 algorithms are affected. This issue has been patched in version 1.1.1.

SimpleJWT is a simple JSON web token library written in PHP. Prior to version 1.1.1, an unauthenticated attacker can perform a Denial of Service via JWE header tampering when PBES2 algorithms are used. Applications that call JWE::decrypt() on attacker-controlled JWEs using PBES2 algorithms are affected. This issue has been patched in version 1.1.1.

Summary

An unauthenticated attacker can perform a Denial of Service via JWE header tampering when PBES2 algorithms are used.
Applications that call JWE::decrypt() on attacker-controlled JWEs using PBES2 algorithms are affected.

Details

PHP version: PHP 8.4.11 SimpleJWT version: v1.1.0

The relevant portion of the vulnerable implementation is shown below (PBES2.php):

<?php
/* ... SNIP ... */
class PBES2 extends BaseAlgorithm implements KeyEncryptionAlgorithm {
    use AESKeyWrapTrait;

    /** @var array<string, mixed> $alg_params */
    static protected $alg_params = [
        'PBES2-HS256+A128KW' => ['hash' => 'sha256'],
        'PBES2-HS384+A192KW' => ['hash' => 'sha384'],
        'PBES2-HS512+A256KW' => ['hash' => 'sha512']
    ];

    /** @var truthy-string $hash_alg */
    protected $hash_alg;

    /** @var int $iterations */
    protected $iterations = 4096;
    
    /* ... SNIP ... */

    /**
     * Sets the number of iterations to use in PBKFD2 key generation.
     *
     * @param int $iterations number of iterations
     * @return void
     */
    public function setIterations(int $iterations) {
        $this->iterations = $iterations;
    }
    
    /* ... SNIP ... */

    /**
     * {@inheritdoc}
     */
    public function decryptKey(string $encrypted_key, KeySet $keys, array $headers, ?string $kid = null): string {
        /** @var SymmetricKey $key */
        $key = $this->selectKey($keys, $kid);
        if ($key == null) {
            throw new CryptException('Key not found or is invalid', CryptException::KEY_NOT_FOUND_ERROR);
        }
        if (!isset($headers['p2s']) || !isset($headers['p2c'])) {
            throw new CryptException('p2s or p2c headers not set', CryptException::INVALID_DATA_ERROR);
        }

        $derived_key = $this->generateKeyFromPassword($key->toBinary(), $headers);
        return $this->unwrapKey($encrypted_key, $derived_key, $headers);
    }
    
    /* ... SNIP ... */

    /**
     * @param array<string, mixed> $headers
     */
    private function generateKeyFromPassword(string $password, array $headers): string {
        $salt = $headers['alg'] . "\x00" . Util::base64url_decode($headers['p2s']);
        /** @var int<0, max> $length */
        $length = intdiv($this->getAESKWKeySize(), 8);

        return hash_pbkdf2($this->hash_alg, $password, $salt, $headers['p2c'], $length, true);
    }
}
?>

The security flaw lies in the lack of input validation when handling JWEs that uses PBES2.
A "sanity ceiling" is not set on the iteration count, which is the parameter known in the JWE specification as p2c (RFC7518).
The library calls decryptKey() with the untrusted input $headers which then use the PHP function hash_pbkdf2() with the user-supplied value $headers['p2c'].

This results in an algorithmic complexity denial-of-service (CPU exhaustion) because the PBKDF2 iteration count is fully attacker-controlled.
Because the header is processed before successful decryption and authentication, the attack can be triggered using an invalid JWE, meaning authentication is not required.

Proof of Concept

Spin up a simple PHP server which accepts a JWE as input and tries to decrypt the user supplied JWE.

mkdir simplejwt-poc
cd simplejwt-poc
composer install
composer require kelvinmo/simplejwt
php -S localhost:8000

The content of index.php:

<?php

require __DIR__ . '/vendor/autoload.php';

$set = SimpleJWT\Keys\KeySet::createFromSecret('secret123');

$uri = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);
$method = $_SERVER['REQUEST_METHOD'];

if ($uri === '/encrypt' && $method === 'GET') {
    // Note $headers['alg'] and $headers['enc'] are required
    $headers = ['alg' => 'PBES2-HS256+A128KW', 'enc' => 'A256CBC-HS512'];
   …

SimpleJWT is a simple JSON web token library written in PHP. Prior to version 1.1.1, an unauthenticated attacker can perform a Denial of Service via JWE header tampering when PBES2 algorithms are used. Applications that call JWE::decrypt() on attacker-controlled JWEs using PBES2 algorithms are affected. This issue has been patched in version 1.1.1.

SimpleJWT is a simple JSON web token library written in PHP. Prior to version 1.1.1, an unauthenticated attacker can perform a Denial of Service via JWE header tampering when PBES2 algorithms are used. Applications that call JWE::decrypt() on attacker-controlled JWEs using PBES2 algorithms are affected. This issue has been patched in version 1.1.1.

Summary

An unauthenticated attacker can perform a Denial of Service via JWE header tampering when PBES2 algorithms are used.
Applications that call JWE::decrypt() on attacker-controlled JWEs using PBES2 algorithms are affected.

Details

PHP version: PHP 8.4.11 SimpleJWT version: v1.1.0

The relevant portion of the vulnerable implementation is shown below (PBES2.php):

<?php
/* ... SNIP ... */
class PBES2 extends BaseAlgorithm implements KeyEncryptionAlgorithm {
    use AESKeyWrapTrait;

    /** @var array<string, mixed> $alg_params */
    static protected $alg_params = [
        'PBES2-HS256+A128KW' => ['hash' => 'sha256'],
        'PBES2-HS384+A192KW' => ['hash' => 'sha384'],
        'PBES2-HS512+A256KW' => ['hash' => 'sha512']
    ];

    /** @var truthy-string $hash_alg */
    protected $hash_alg;

    /** @var int $iterations */
    protected $iterations = 4096;
    
    /* ... SNIP ... */

    /**
     * Sets the number of iterations to use in PBKFD2 key generation.
     *
     * @param int $iterations number of iterations
     * @return void
     */
    public function setIterations(int $iterations) {
        $this->iterations = $iterations;
    }
    
    /* ... SNIP ... */

    /**
     * {@inheritdoc}
     */
    public function decryptKey(string $encrypted_key, KeySet $keys, array $headers, ?string $kid = null): string {
        /** @var SymmetricKey $key */
        $key = $this->selectKey($keys, $kid);
        if ($key == null) {
            throw new CryptException('Key not found or is invalid', CryptException::KEY_NOT_FOUND_ERROR);
        }
        if (!isset($headers['p2s']) || !isset($headers['p2c'])) {
            throw new CryptException('p2s or p2c headers not set', CryptException::INVALID_DATA_ERROR);
        }

        $derived_key = $this->generateKeyFromPassword($key->toBinary(), $headers);
        return $this->unwrapKey($encrypted_key, $derived_key, $headers);
    }
    
    /* ... SNIP ... */

    /**
     * @param array<string, mixed> $headers
     */
    private function generateKeyFromPassword(string $password, array $headers): string {
        $salt = $headers['alg'] . "\x00" . Util::base64url_decode($headers['p2s']);
        /** @var int<0, max> $length */
        $length = intdiv($this->getAESKWKeySize(), 8);

        return hash_pbkdf2($this->hash_alg, $password, $salt, $headers['p2c'], $length, true);
    }
}
?>

The security flaw lies in the lack of input validation when handling JWEs that uses PBES2.
A "sanity ceiling" is not set on the iteration count, which is the parameter known in the JWE specification as p2c (RFC7518).
The library calls decryptKey() with the untrusted input $headers which then use the PHP function hash_pbkdf2() with the user-supplied value $headers['p2c'].

This results in an algorithmic complexity denial-of-service (CPU exhaustion) because the PBKDF2 iteration count is fully attacker-controlled.
Because the header is processed before successful decryption and authentication, the attack can be triggered using an invalid JWE, meaning authentication is not required.

Proof of Concept

Spin up a simple PHP server which accepts a JWE as input and tries to decrypt the user supplied JWE.

mkdir simplejwt-poc
cd simplejwt-poc
composer install
composer require kelvinmo/simplejwt
php -S localhost:8000

The content of index.php:

<?php

require __DIR__ . '/vendor/autoload.php';

$set = SimpleJWT\Keys\KeySet::createFromSecret('secret123');

$uri = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);
$method = $_SERVER['REQUEST_METHOD'];

if ($uri === '/encrypt' && $method === 'GET') {
    // Note $headers['alg'] and $headers['enc'] are required
    $headers = ['alg' => 'PBES2-HS256+A128KW', 'enc' => 'A256CBC-HS512'];
   …