CVE-2017-16857 in Auto-Unapprove Plugin
Summary
by MITRE
It is possible to bypass the bitbucket auto-unapprove plugin via minimal brute-force because it is relying on asynchronous events on the back-end. This allows an attacker to merge any code into unsuspecting repositories. This affects all versions of the auto-unapprove plugin, however since the auto-unapprove plugin is not bundled with Bitbucket Server it does not affect any particular version of Bitbucket.
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Analysis
by VulDB Data Team • 12/12/2019
The vulnerability described in CVE-2017-16857 represents a critical security flaw in the Bitbucket auto-unapprove plugin that stems from improper handling of asynchronous event processing within the platform's backend infrastructure. This weakness allows attackers to exploit a timing window during which the plugin fails to properly validate merge operations, creating an opportunity for unauthorized code integration into target repositories. The issue specifically manifests when the plugin relies on asynchronous event mechanisms to process approval status changes, leaving a window where malicious actors can manipulate the system state before the proper validation occurs. This vulnerability affects all versions of the auto-unapprove plugin regardless of the Bitbucket Server version in use, as the plugin operates independently of the core Bitbucket platform and is installed as a separate add-on component.
The technical implementation of this vulnerability involves a race condition where the plugin's asynchronous event processing fails to maintain consistent state validation during merge operations. When a user attempts to merge code into a repository, the auto-unapprove plugin should verify that the merge request has proper approval status before allowing the operation to proceed. However, due to the asynchronous nature of the event handling mechanism, there exists a temporal gap where the system state may not accurately reflect the current approval status, allowing attackers to exploit this window through minimal brute-force attempts. The attacker can repeatedly attempt merge operations during this vulnerable period, eventually succeeding in bypassing the approval requirements through systematic trial and error. This type of vulnerability falls under the CWE-362 category of "Concurrent Execution using Shared Resource with Improper Synchronization" and aligns with ATT&CK technique T1078.004 for Valid Accounts and T1566.001 for Phishing for Information.
The operational impact of this vulnerability extends beyond simple code injection, as it fundamentally undermines the security controls that organizations rely upon to prevent unauthorized changes to their source code repositories. Attackers can exploit this weakness to merge malicious code into production repositories without proper authorization, potentially introducing backdoors, data exfiltration capabilities, or other harmful modifications that could compromise entire development pipelines. The vulnerability's persistence across all plugin versions indicates a fundamental architectural flaw rather than a simple implementation bug, making it particularly concerning for organizations that have deployed this plugin across multiple environments. Organizations using Bitbucket Server with the auto-unapprove plugin face significant risk of supply chain compromise, as attackers can leverage this vulnerability to inject malicious code into code repositories that may be used in automated build processes, CI/CD pipelines, or other critical development workflows. The lack of a direct correlation to specific Bitbucket Server versions means that the vulnerability exists in all environments where the plugin is installed, regardless of the underlying platform version, further amplifying its potential impact across diverse deployment scenarios.
Mitigation strategies for CVE-2017-16857 require both immediate and long-term approaches to address the core architectural weakness in the plugin's event handling mechanism. Organizations should immediately remove or disable the auto-unapprove plugin from their Bitbucket Server installations until a patched version is available, as the vulnerability cannot be effectively mitigated through configuration changes alone. The recommended approach involves implementing synchronous validation mechanisms that ensure proper state checking before allowing merge operations to proceed, eliminating the timing window that attackers exploit. Security teams should also implement additional monitoring and alerting around merge operations in repositories where this plugin was previously active, as unauthorized merges may not be immediately apparent. The vulnerability demonstrates the importance of proper event-driven system design and highlights the need for comprehensive testing of asynchronous operations in security-critical components. Organizations should consider implementing additional security controls such as webhook-based notifications, automated code review processes, and regular repository integrity checks to detect and prevent unauthorized changes. This vulnerability also emphasizes the critical importance of understanding the security implications of third-party add-ons and plugins, as these components can introduce vulnerabilities that affect the entire platform ecosystem. The issue serves as a reminder that even plugins that appear to be simple convenience tools can have significant security implications when not properly designed with security considerations in mind, particularly regarding state management and event synchronization.