An issue was discovered in Xen through 4.14.x. In the Ocaml xenstored implementation, the internal representation of the tree has special cases for the root node, because this node has no parent. Unfortunately, permissions were not checked for certain operations on the root node. Unprivileged guests can get and modify permissions, list, and delete the root node. (Deleting the whole xenstore tree is a host-wide denial of service.) Achieving xenstore write access is also possible. All systems using oxenstored are vulnerable. Building and using oxenstored is the default in the upstream Xen distribution, if the Ocaml compiler is available. Systems using C xenstored are not vulnerable.
Weakness
The product does not perform an authorization check when an actor attempts to access a resource or perform an action.
Affected Software
| Name |
Vendor |
Start Version |
End Version |
| Xen |
Xen |
* |
4.14.0 (including) |
| Xen |
Ubuntu |
bionic |
* |
| Xen |
Ubuntu |
esm-apps/focal |
* |
| Xen |
Ubuntu |
esm-infra/bionic |
* |
| Xen |
Ubuntu |
esm-infra/xenial |
* |
| Xen |
Ubuntu |
focal |
* |
| Xen |
Ubuntu |
groovy |
* |
| Xen |
Ubuntu |
hirsute |
* |
| Xen |
Ubuntu |
impish |
* |
| Xen |
Ubuntu |
trusty |
* |
| Xen |
Ubuntu |
xenial |
* |
Potential Mitigations
- Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries.
- Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
- Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
- For example, consider using authorization frameworks such as the JAAS Authorization Framework [REF-233] and the OWASP ESAPI Access Control feature [REF-45].
- For web applications, make sure that the access control mechanism is enforced correctly at the server side on every page. Users should not be able to access any unauthorized functionality or information by simply requesting direct access to that page.
- One way to do this is to ensure that all pages containing sensitive information are not cached, and that all such pages restrict access to requests that are accompanied by an active and authenticated session token associated with a user who has the required permissions to access that page.
References