CVE-2026-96281 in Red Hat
Summary
by MITRE • 09/28/2026
On a multi-user system, a user with an active local login session could downgrade a system-wide Flatpak app to an older version by removing the app's remote ref via the unprivileged system-helper RemoveLocalRef method, causing the anti-downgrade check to fail to find a reference date. A malicious local user could use this to expose other users of the same system to an app version with unfixed vulnerabilities.
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Analysis
by VulDB Data Team • 09/28/2026
The vulnerability described involves a privilege escalation and integrity violation within the Flatpak application runtime environment on multi-user Linux systems, specifically exploiting the unprivileged system-helper interface for managing remote references. In typical Flatpak deployments, applications are installed in shared directories accessible to all users on the host machine, while user-specific configurations remain isolated. The core technical flaw lies in the behavior of the RemoveLocalRef method within the org.freedesktop.Flatpak D-Bus service. This method allows a local user with an active session to remove specific remote references for system-wide applications without requiring root privileges or explicit administrative authorization. By deleting these reference entries, which contain metadata including version history and installation timestamps, the attacker effectively erases the evidence of previously installed versions of the target application.
This action directly undermines the anti-downgrade protection mechanisms implemented by Flatpak. The runtime relies on stored reference data to verify that a new or existing package is not older than what was previously installed, thereby preventing users from inadvertently or maliciously reverting to software states with known security flaws. When the local user removes the remote ref via the unprivileged helper, the system loses track of the most recent version's metadata. Consequently, when another user on the same machine attempts to install or update that application, the anti-downgrade check fails because it cannot locate a valid reference date to compare against. This failure allows the installation of an older, potentially vulnerable version of the software that would otherwise have been blocked by the integrity checks designed to maintain system security posture.
The operational impact is significant for environments where multiple users share access to the same machine, such as workstations in corporate offices or public computing labs. A malicious local user can exploit this flaw to force other users into running outdated versions of critical applications like web browsers, office suites, or communication tools. These older versions may contain unpatched vulnerabilities that have been fixed in subsequent releases. By downgrading the application for all system-wide instances, the attacker creates a persistent attack surface where any user logging in and launching the affected app is exposed to exploits such as remote code execution, cross-site scripting, or memory corruption attacks associated with those older versions. This effectively neutralizes security updates applied by administrators at the system level, rendering patch management efforts ineffective for shared applications until the reference data is manually restored.
From a classification perspective, this vulnerability aligns with CWE-269 Improper Privilege Management and CWE-732 Incorrect Permission Assignment for Critical Resource, as it involves an unprivileged actor modifying security-critical configuration state without adequate authorization checks. It also relates to CWE-841 Improper Enforcement of Behavioral Workflow because the system fails to enforce the intended sequence of operations that preserve version integrity. In terms of adversary tactics, this behavior maps to MITRE ATT&CK technique T1562 Impair Defenses, specifically under subtechnique T1562.001 Disable or Modify Tools, as it disables a defensive mechanism (the anti-downgrade check) by manipulating the underlying data structures that support it. It can also be viewed through the lens of T1499 Endpoint Denial of Service if the downgrade leads to instability, though the primary intent here is likely exploitation rather than disruption.
Mitigation strategies must focus on restricting access to system-wide Flatpak management operations and ensuring robust validation of reference modifications. Administrators should configure polkit policies or similar authorization frameworks to require authentication for any operation involving RemoveLocalRef when targeting system-scoped applications. This ensures that only privileged users can alter the metadata governing shared application versions. Additionally, implementing file integrity monitoring on the directories where Flatpak stores remote references and version history can help detect unauthorized deletions in real-time. Regular audits of installed package versions across all user sessions are recommended to identify any discrepancies caused by such attacks. Furthermore, developers should consider enhancing the anti-downgrade logic to perform additional validation checks beyond simple reference lookups, such as verifying cryptographic signatures or checking against a trusted server-side manifest before allowing installation, thereby reducing reliance on local state that can be manipulated by unprivileged users.