CVE-2021-32674 in Zope
Summary
by MITRE • 06/09/2021
Zope is an open-source web application server. This advisory extends the previous advisory at https://github.com/zopefoundation/Zope/security/advisories/GHSA-5pr9-v234-jw36 with additional cases of TAL expression traversal vulnerabilities. Most Python modules are not available for using in TAL expressions that you can add through-the-web, for example in Zope Page Templates. This restriction avoids file system access, for example via the 'os' module. But some of the untrusted modules are available indirectly through Python modules that are available for direct use. By default, you need to have the Manager role to add or edit Zope Page Templates through the web. Only sites that allow untrusted users to add/edit Zope Page Templates through the web are at risk. The problem has been fixed in Zope 5.21 and 4.6.1. The workaround is the same as for https://github.com/zopefoundation/Zope/security/advisories/GHSA-5pr9-v234-jw36: A site administrator can restrict adding/editing Zope Page Templates through the web using the standard Zope user/role permission mechanisms. Untrusted users should not be assigned the Zope Manager role and adding/editing Zope Page Templates through the web should be restricted to trusted users only.
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Analysis
by VulDB Data Team • 06/11/2021
The vulnerability identified as CVE-2021-32674 affects Zope, an open-source web application server that serves as a foundation for building dynamic web applications. This security flaw represents an extension of previously identified TAL expression traversal vulnerabilities, specifically targeting the template attribute language implementation within Zope's page templates. The vulnerability operates through a sophisticated mechanism that exploits indirect module access pathways, creating a vector for privilege escalation and code execution. Organizations utilizing Zope web applications face potential security risks when untrusted users possess the ability to modify page templates through web interfaces, as this creates opportunities for malicious actors to leverage available Python modules that bypass standard security restrictions.
The technical implementation of this vulnerability stems from the way Zope handles TAL expressions within page templates, where certain Python modules that are normally restricted from direct use can be accessed indirectly through other available modules. This indirect access bypasses the intended security controls that typically prevent file system access and other dangerous operations through the 'os' module and similar components. The vulnerability specifically targets the distinction between modules that are available for direct use versus those that are restricted, creating a scenario where untrusted users can potentially execute arbitrary code through carefully crafted TAL expressions. This issue manifests when users with appropriate permissions can add or edit Zope Page Templates through the web interface, effectively creating a path for privilege escalation attacks.
The operational impact of this vulnerability extends beyond simple code execution to encompass broader security implications for organizations running Zope-based applications. Attackers with access to template modification capabilities can potentially gain unauthorized access to system resources, execute malicious code, and perform actions that would normally require elevated privileges. The vulnerability particularly affects organizations that permit untrusted users to modify web content, as the default security model requires Manager role privileges for template modifications. This creates a significant risk for web applications where user permissions are not properly segmented, potentially allowing attackers to escalate privileges and gain access to sensitive system information. The vulnerability's impact is amplified when organizations fail to implement proper role-based access controls and user permission restrictions.
Organizations can address this vulnerability through multiple mitigation strategies that align with established security frameworks and best practices. The recommended solution involves upgrading to Zope versions 5.21 or 4.6.1, which contain the necessary patches to resolve the TAL expression traversal issues. Additionally, administrators should implement restrictive user permission mechanisms that prevent untrusted users from adding or editing page templates through web interfaces. This approach directly addresses the root cause by ensuring that only trusted users possess the necessary privileges to modify template content. The mitigation strategy aligns with the principle of least privilege and follows security guidelines that emphasize proper access control implementation. Organizations should also consider implementing additional monitoring and auditing measures to detect unauthorized template modifications and maintain compliance with security standards such as those outlined in the CWE taxonomy for code injection vulnerabilities and the ATT&CK framework for privilege escalation techniques.