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To sign an issue, the user must have view access to the issue and the issue must be editable (unless the Advanced Security option is enabled). When an issue is closed, it is possible to prevent new signatures by setting the issue to be read-only through workflow status property jira.issue.editable=false.
See this link for more information: https://confluence.atlassian.com/jirakb/issues-are-editable-in-closed-state-218268664.html
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