CVE-2026-32313

xmlseclibs is a library written in PHP for working with XML Encryption and Signatures. Prior to 3.1.5, 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, and decrypt the encrypted nodes. It also allows to forge arbitrary ciphertexts without knowing the encryption key. This vulnerability is fixed in 3.1.5.

xmlseclibs is a library written in PHP for working with XML Encryption and Signatures. Prior to 3.1.5, 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, and decrypt the encrypted nodes. It also allows to forge arbitrary ciphertexts without knowing the encryption key. This vulnerability is fixed in 3.1.5.

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, 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, 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).

See this example:

function test($data) {
    $ivSize = 12;
    $tagSize = 16;

    $iv = substr($data, 0, $ivSize);
    $data = substr($data, $ivSize);
    $offset = 0 - $tagSize;
    $tag = substr($data, $offset);
    $ct = substr($data, 0, $offset);

    echo 'IV: "' . $iv . '"' . PHP_EOL;
    echo 'Tag: "' . $tag . '"' . PHP_EOL;
    echo 'CT: "' . $ct . '"' . PHP_EOL;
}

/* Outputs:
php > test('myNonceNoncet');
IV: "myNonceNonce"
Tag: "t"
CT: ""
php > test('myNonceNonceta');
IV: "myNonceNonce"
Tag: "ta"
CT: ""
php > test('myNonceNoncetag');
IV: "myNonceNonce"
Tag: "tag"
CT: ""
*/

With a legit ciphertext in hand, this is enough to recover the GHASH key. With that key, any authenticated tags can be computed offline which allows for decryption of the ciphertext and forgery of arbitrary ciphertexts.

PoC

1. Setup a server expecting XML with an encrypted assertion
   • Run this php script poc.php with php -S 127.0.0.1:8888 (taken from this saml test case)
   • The script expects this private key: sp-private-key.pem.
2. Create an XML document with an encrypted assertion (encrypted with aes-256-gcm)
   • Here is the SAMLResponse used in the video below: saml_response.txt

Note: The steps from 3 to 6 are implemented in this exploit script: nonce_reuse_with_fmt_val_oracle.py. You can run the script with sage -python nonce_reuse_with_fmt_val_oracle.py -s 'url-encoded_and_base64-encoded_samlresponse'

3. Take the content of the <xenc:CipherValue> node and apply the following modifications
   1. Base64-decode the content
   2. Take the first 12 bytes and save them as the nonce
   3. Take the last 16 bytes and save them as the tag
   4. Now brute-force the tag of an empty ciphertext
      1. Loop through all 256 possible byte values (let's call that byte_tag_attempt)
      2. Concatenate together the nonce and the byte_tag_attempt
      3. Base64-encode the result
      4. Replace the content of the <xenc:CipherValue> node with this…

xmlseclibs is a library written in PHP for working with XML Encryption and Signatures. Prior to 3.1.5, 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, and decrypt the encrypted nodes. It also allows to forge arbitrary ciphertexts without knowing the encryption key. This vulnerability is fixed in 3.1.5.

xmlseclibs is a library written in PHP for working with XML Encryption and Signatures. Prior to 3.1.5, 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, and decrypt the encrypted nodes. It also allows to forge arbitrary ciphertexts without knowing the encryption key. This vulnerability is fixed in 3.1.5.

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, 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, 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).

See this example:

function test($data) {
    $ivSize = 12;
    $tagSize = 16;

    $iv = substr($data, 0, $ivSize);
    $data = substr($data, $ivSize);
    $offset = 0 - $tagSize;
    $tag = substr($data, $offset);
    $ct = substr($data, 0, $offset);

    echo 'IV: "' . $iv . '"' . PHP_EOL;
    echo 'Tag: "' . $tag . '"' . PHP_EOL;
    echo 'CT: "' . $ct . '"' . PHP_EOL;
}

/* Outputs:
php > test('myNonceNoncet');
IV: "myNonceNonce"
Tag: "t"
CT: ""
php > test('myNonceNonceta');
IV: "myNonceNonce"
Tag: "ta"
CT: ""
php > test('myNonceNoncetag');
IV: "myNonceNonce"
Tag: "tag"
CT: ""
*/

With a legit ciphertext in hand, this is enough to recover the GHASH key. With that key, any authenticated tags can be computed offline which allows for decryption of the ciphertext and forgery of arbitrary ciphertexts.

PoC

1. Setup a server expecting XML with an encrypted assertion
   • Run this php script poc.php with php -S 127.0.0.1:8888 (taken from this saml test case)
   • The script expects this private key: sp-private-key.pem.
2. Create an XML document with an encrypted assertion (encrypted with aes-256-gcm)
   • Here is the SAMLResponse used in the video below: saml_response.txt

Note: The steps from 3 to 6 are implemented in this exploit script: nonce_reuse_with_fmt_val_oracle.py. You can run the script with sage -python nonce_reuse_with_fmt_val_oracle.py -s 'url-encoded_and_base64-encoded_samlresponse'

3. Take the content of the <xenc:CipherValue> node and apply the following modifications
   1. Base64-decode the content
   2. Take the first 12 bytes and save them as the nonce
   3. Take the last 16 bytes and save them as the tag
   4. Now brute-force the tag of an empty ciphertext
      1. Loop through all 256 possible byte values (let's call that byte_tag_attempt)
      2. Concatenate together the nonce and the byte_tag_attempt
      3. Base64-encode the result
      4. Replace the content of the <xenc:CipherValue> node with this…