CVE-2026-90616 in Flatpak
Summary
by MITRE • 09/12/2026
In Flatpak before 1.18.1, a malicious sandboxed app can obtain arbitrary read and write access to files on the host, which can be escalated to arbitrary code execution on the host, a different vulnerability than CVE-2026-76925. Flatpak creates a few app data directories (e.g., /var/cache, /var/data, /var/config, and /var/tmp) in every sandbox on every app launch where, in some cases, components of the path are attacker-controlled. Missing symlink protection can redirect the directories. Some of these directories are bind-mounted by Flatpak by passing the path (e.g., /home/user/.var/app/APP_ID/cache/tmp), which contains attacker-controlled directories (tmp) to bwrap --bind SRC DST. bwrap passes the path on to the kernel, which then follows symlinks. A malicious symlink can point to arbitrary locations on the host and it will become mounted inside the sandbox.
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Analysis
by VulDB Data Team • 09/12/2026
The vulnerability identified in Flatpak versions prior to 1.18.1 represents a critical failure in container isolation mechanisms that allows for privilege escalation from a confined application environment to the underlying host system. This flaw specifically arises during the initialization of app data directories, such as /var/cache, /var/data, /var/config, and /var/tmp, which are created within every sandbox upon each application launch. The core technical deficiency lies in the handling of path components that can be influenced by an attacker-controlled entity within the sandboxed environment. When Flatpak prepares to bind-mount these directories into the container using bubblewrap with commands like bwrap --bind SRC DST, it fails to adequately validate or sanitize the paths involved. Specifically, if any component of the source path is subject to manipulation by a malicious application, the system does not enforce strict symlink protection protocols before passing the path to the kernel for processing.
The operational mechanism of this exploitation relies on the behavior of the Linux kernel when handling bind mounts via bubblewrap. When bwrap receives a path that contains symlinks, it passes this path directly to the kernel without resolving or validating them against security constraints. The kernel subsequently follows these symbolic links during the mount operation. A malicious actor operating within a sandboxed application can create a symlink pointing to arbitrary locations on the host filesystem, such as sensitive configuration files, private user data, or system binaries. Because Flatpak binds this path into the container without verifying that the target of the symlink is restricted to safe directories, the attacker effectively gains read and write access to those host resources inside the sandbox environment. This bypasses the intended isolation boundaries established by Flatpak's security model.
The impact of this vulnerability extends beyond simple data exfiltration or modification. Once a malicious application has arbitrary read and write access to critical files on the host, it can escalate its privileges significantly. By modifying executable binaries, injecting code into trusted processes, or altering configuration files that control system behavior, an attacker can achieve arbitrary code execution with the permissions of the user running the Flatpak session. This transforms what might appear as a sandbox escape into a full compromise of the host machine's integrity and confidentiality. The distinction from CVE-2026-76925 highlights that this is a unique vector involving path traversal via bind mounts rather than other potential isolation failures, emphasizing the need for specific remediation strategies targeting filesystem handling in container runtimes.
From a classification perspective, this vulnerability aligns with CWE-59: Improper Link Resolution Before File Access, as it involves following symbolic links to access resources outside of intended boundaries. It also relates to CWE-284: Improper Access Control, since the sandbox fails to restrict file system operations appropriately for untrusted applications. In terms of attack tactics, this aligns with MITRE ATT&CK techniques related to Privilege Escalation and Defense Evasion, specifically how attackers leverage misconfigured container runtimes to break out of restricted environments. The failure to validate paths before bind mounting is a classic example of trusting user-controlled input in system-level operations without sufficient sanitization or canonicalization checks.
To mitigate this risk, users must upgrade Flatpak to version 1.18.1 or later, where the developers have implemented stricter validation for path components and enhanced symlink protection mechanisms within bubblewrap interactions. System administrators should also consider implementing additional security layers such as SELinux or AppArmor profiles that restrict bind mount operations even if the application layer fails. Regular auditing of sandboxed applications' permissions is recommended to ensure they adhere to the principle of least privilege, minimizing the potential impact if a similar vulnerability were discovered in future versions. Ensuring that all components of the Flatpak ecosystem are updated and configured correctly remains the primary defense against this class of container escape vulnerabilities.