CVE-2021-32829

ZStack is open source IaaS(infrastructure as a service) software aiming to automate datacenters, managing resources of compute, storage, and networking all by APIs. Affected versions of ZStack REST API are vulnerable to post-authentication Remote Code Execution (RCE) via bypass of the Groovy shell sandbox. The REST API exposes the GET zstack/v1/batch-queries?script endpoint which is backed up by the BatchQueryAction class. Messages are represented by the APIBatchQueryMsg, dispatched to the QueryFacadeImpl facade and handled by the BatchQuery class. The HTTP request parameter script is mapped to the APIBatchQueryMsg.script property and evaluated as a Groovy script in BatchQuery.query the evaluation of the user-controlled Groovy script is sandboxed by SandboxTransformer which will apply the restrictions defined in the registered (sandbox.register()) GroovyInterceptor. Even though the sandbox heavily restricts the receiver types to a small set of allowed types, the sandbox is non effective at controlling any code placed in Java annotations and therefore vulnerable to meta-programming escapes. This issue leads to post-authenticated remote code execution. For more details see the referenced GHSL-2021-065. This issue is patched in versions 3.8.21, 3.10.8, and 4.1.0.

Weakness

The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.

Affected Software

Extended Description

Assuming a user with a given identity, authorization is the process of determining whether that user can access a given resource, based on the user’s privileges and any permissions or other access-control specifications that apply to the resource. When access control checks are incorrectly applied, users are able to access data or perform actions that they should not be allowed to perform. This can lead to a wide range of problems, including information exposures, denial of service, and arbitrary code execution.

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

• https://blog.orange.tw/2019/02/abusing-meta-programming-for-unauthenticated-rce.html
• https://github.com/zstackio/zstack/security/advisories/GHSA-6xgq-7rqg-x3q5
• https://securitylab.github.com/advisories/GHSL-2021-065-zstack/