CVE-2026-72915 in Mastodon
Summary
by MITRE • 08/11/2026
Mastodon is a free, open-source social network server based on ActivityPub. From 4.6.0-beta.1 until 4.6.4 and 4.7.0-beta.1, any logged-in local user could use the show action in app/controllers/admin/collections_controller.rb to access personally identifying information about another local user in a collection because the controller used the general collection policy instead of the admin collection policy namespace. The exposed data included the other user's current email address and last-used IP address. This issue is fixed in versions 4.6.4 and 4.7.0-beta.1.
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Analysis
by VulDB Data Team • 08/11/2026
This vulnerability represents a critical access control flaw in the Mastodon social network server software that allowed authenticated users to bypass security restrictions and access sensitive personal information of other users. The issue stems from improper authorization checks within the administrative controller logic, specifically in the collections_controller.rb file where the show action was incorrectly utilizing a general collection policy rather than the more restrictive admin collection policy namespace. This misconfiguration created an exploitable path where any logged-in local user could retrieve personally identifiable information about other local users simply by leveraging the flawed access control mechanism. The vulnerability existed across multiple versions including 4.6.0-beta.1 through 4.6.4 and 4.7.0-beta.1, indicating a prolonged exposure window that could have been exploited by malicious actors with legitimate user accounts.
The technical implementation flaw manifests in how the application handles permission validation during collection access operations. When a user requests to view a specific collection through the show action, the system should enforce administrative privileges to ensure only authorized administrators can access potentially sensitive user data. However, the controller was incorrectly applying general collection policies that do not require elevated permissions, thereby allowing regular users to traverse the authorization boundary and access data they should not be permitted to view. This represents a clear violation of the principle of least privilege and demonstrates inadequate input validation and access control implementation within the application's security architecture. The flaw operates at the application layer with direct implications for user privacy and data protection protocols.
The operational impact of this vulnerability extends beyond simple information disclosure, as it exposed sensitive personal data including email addresses and IP address information that could be used for various malicious activities. Email addresses represent particularly valuable information for social engineering attacks, account takeover attempts, and targeted phishing campaigns. The last-used IP address information provides additional context about user behavior patterns and potential physical locations, further compromising user privacy and security. This vulnerability undermines the fundamental trust model of the social network platform and could enable coordinated attacks against specific users or groups within the community. The exposure affects all local users who are logged into the system, making it a widespread concern that impacts the entire user base rather than isolated accounts.
The fix implemented in versions 4.6.4 and 4.7.0-beta.1 addresses the core authorization issue by ensuring that the show action in the collections controller properly enforces admin collection policy namespace restrictions. This correction aligns with security best practices for implementing proper access controls and demonstrates the importance of maintaining strict separation between different privilege levels within application code. Organizations should consider this vulnerability as a case study in how improper authorization checks can create significant security risks even in open-source platforms where the code is publicly available. The remediation approach follows established security principles that emphasize the necessity of explicit access control validation for administrative functions and highlights the critical need for thorough code reviews focusing on privilege escalation paths.
This vulnerability aligns with several cybersecurity standards and frameworks including CWE-284 which addresses improper access control, and reflects patterns commonly seen in authorization bypass attacks. From an ATT&CK framework perspective, this issue maps to techniques involving privilege escalation and credential access through application-level flaws. The incident underscores the importance of implementing proper security controls during the software development lifecycle and demonstrates how seemingly minor configuration errors can have significant security implications. Organizations running Mastodon or similar platforms should prioritize updating to the fixed versions and conduct comprehensive security assessments of their deployment environments to ensure no similar flaws exist in related components. The vulnerability serves as a reminder that even open-source projects require continuous security monitoring and timely patch management to maintain user trust and platform integrity.