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SharePoint Workflow Security June 13, 2012

Filed under: History,Security,Task,Workflow — sladescross @ 10:02 pm


Information disclosure in task and workflow history lists

Because tasks and history list items can contain data about users and the actions they perform on documents, the items might disclose confidential information. For example, a promotion Approval workflow might collect feedback on its tasks that an organization wants only the workflow owner and each participant in the task to see.

Task and history lists are typical lists in a site. By default, therefore, all readers can view tasks and history items. Administrators and developers must determine the information that cannot be disclosed and decide whether to help secure task and history items that are created by the workflow.

Securing these items can be done in several ways. For example, administrators can set list-level permissions. If disclosure is to be private — that is, not publicly available but available to a specific group of people — administrators can create a new task or history list and set permissions for the list that are targeted to that group. If administrators do not want anyone to see history events on a workflow status page, they can remove view permissions to the workflow history list from which a status page pulls its information. Users who do not have permissions to view the history list itself, or any item on the list, will receive an Access Denied error when they open any status page that pulls data from that history list.

If finer restrictions are required, workflow developers can set per-item permissions when they create tasks or history items. The CreateTask activity has a SpecialPermissions property that gives only specified permissions to access the newly created task. The LogToHistoryList activity does not have such a property, so to set per-item permissions on history list items, administrators must use the object model (OM) in SharePoint Server 2010. Per-item permissions can affect performance negatively and should not be used unless they are necessary.

Tasks and history items do not have to be handled in the same manner. Administrators can mix and match list permissions and item-level permissions.

Spoofing and tampering attacks in the task and workflow history lists

Any contributor can modify tasks or history items if there are no restrictions on those lists. This means that malicious users can modify task descriptions to give participants incorrect instructions or to order participants to click malicious links. To change the perceived results of a process, malicious users also can add false or inaccurate history events or can modify history events to make them false or inaccurate.

As detailed earlier, task and history lists are normal lists in a site. By default, there are no permission restrictions on either task lists or history lists. To avoid spoofing and tampering attacks, administrators must determine the vulnerabilities that exist and either restrict access to columns in a list (for example, make vulnerable columns such as task descriptions read-only so that only the workflow can set them on item creation), set special permissions on the list, or set item-level permissions on the items in a list.

Security issues in the workflow history list

A key benefit of workflows is the ability to track process information to provide visibility into a process. The workflow history list is a repository for this information, where a workflow status page can search for data related to a workflow instance and can make this information available to users. Users can see all items to which they have access in the history list.

However, because the workflow history list tracks information, users might assume that it can be used as an audit trail for events. This is not the case: Workflow history is not a security feature. History lists are standard SharePoint lists that are used for storing events that are visible to any user and that have no special permissions associated with them. By default, users can modify and add events if they have edit and add permissions on the site. To audit events, use the SharePoint’s Audit Log feature. Only administrators can access this log and the log does not require additional work to protect it from tampering attacks.

To better protect the history list, administrators can restrict edit and add permissions to the list, so that only system account administrators (for example, workflow administrators) and list administrators can add items. List administrators must have add permissions to log “Terminate this workflow” events. If edit and add permissions are restricted on the history list, users still must be granted view permissions in order to see status information.

User-Impersonation Step type for declarative workflows

The User-Impersonation Step type can be used to run sections of declarative workflows by the person who authored the workflow rather than by the workflow’s initiator. Declarative means a model that you use to create the workflow and set the parameters for the workflow without writing any code.

In SharePoint Server 2010, declarative workflows always run in the user context of the workflow initiator unless an impersonation step is encountered. If an impersonation step is encountered, the declarative workflow is run in the context of the workflow associator. The default workflow tasks respect SharePoint permissions by impersonating the user who started a workflow when the workflow is run. This arrangement keeps things fairly safe in SharePoint Server 2010, but blocks many scenarios in which a workflow designer with high permission levels wants to author a powerful workflow that can be completed successfully by users who have lower permission levels.

Through a safe and scoped form of privilege elevation, site actions can be automated through workflow. This reduces the burden on a SharePoint site administrator. Automation of a high-security process is useful in publishing and approval scenarios in which existing actions are enabled to impersonate someone other than the workflow’s initiator.

The following are sample scenarios that demonstrate the User-Impersonation Step type:

  • Publish to a secure list
    Jackie has locked down the Pages document library for the public face of her SharePoint site. She has set up an Approval workflow that, by using Microsoft SharePoint Designer 2010, submits content from site contributors for approval. Jackie puts her workflow actions in an impersonation step so that the workflow actions will always impersonate her, a site administrator, as the author of the workflow.
    When Connie (a contributor) posts a content draft to the Pages library of the site, and tries to publish her article, that action causes Jackie’s Approval workflow to start so that the post can be reviewed and approved. Tasks are sent out to the approvers in the workflow on behalf of Connie. Upon review and approval by the approvers, the system sets the moderation status of the post to “Approved”, even though Connie does not have permission to approve pages.
  • Granting permissions to users
    Joanne has set up a workflow in SharePoint Designer 2010 that uses a user-impersonation action “Add User to Group” to grant Design permissions to her site. Because the workflow uses an impersonation scope, the action of adding a user to the group will always be performed on Joanne’s behalf.
    The rest of the workflow lets contributors visit the site and complete a form to log their access request to a list.
    For example, a separate user, Olivier, receives a task when Connie, a user, logs a request, and when he approves the task, Connie is added to the Designer group for the site even though neither Olivier nor Connie has Manage Lists permissions on Joanne’s site.
  • Templates and taking ownership
    William created several workflows in SharePoint Designer 2010 and saved them as templates for reuse across the company, but he soon leaves the company. His account is removed, his administrator status is revoked, and now the SharePoint Designer 2010 workflows that William created fail to complete due to the loss of William’s permissions.
    A parent SharePoint site administrator, John, can intervene for each workflow, without having to re-create the workflows in SharePoint Designer 2010. John takes ownership of the administrative symptoms in each broken template. After doing this, secure publishing and access granting now occur under John’s name instead of William’s — and nothing else has changed.

The following are workflow actions that can be impersonated:

  •             Set Content Approval Status (as Owner)
  •             Create List Item (as Owner)
  •             Update List Item (as Owner)
  •             Delete List Item (as Owner)
  •             Add/Remove/Set/Inherit List Item Permissions (as Owner)

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