CVE-2024-42472 in Flatpak
Summary
by MITRE • 08/15/2024
Flatpak is a Linux application sandboxing and distribution framework. Prior to versions 1.14.0 and 1.15.10, a malicious or compromised Flatpak app using persistent directories could access and write files outside of what it would otherwise have access to, which is an attack on integrity and confidentiality.
When `persistent=subdir` is used in the application permissions (represented as `--persist=subdir` in the command-line interface), that means that an application which otherwise doesn't have access to the real user home directory will see an empty home directory with a writeable subdirectory `subdir`. Behind the scenes, this directory is actually a bind mount and the data is stored in the per-application directory as `~/.var/app/$APPID/subdir`. This allows existing apps that are not aware of the per-application directory to still work as intended without general home directory access.
However, the application does have write access to the application directory `~/.var/app/$APPID` where this directory is stored. If the source directory for the `persistent`/`--persist` option is replaced by a symlink, then the next time the application is started, the bind mount will follow the symlink and mount whatever it points to into the sandbox.
Partial protection against this vulnerability can be provided by patching Flatpak using the patches in commits ceec2ffc and 98f79773. However, this leaves a race condition that could be exploited by two instances of a malicious app running in parallel. Closing the race condition requires updating or patching the version of bubblewrap that is used by Flatpak to add the new `--bind-fd` option using the patch and then patching Flatpak to use it. If Flatpak has been configured at build-time with `-Dsystem_bubblewrap=bwrap` (1.15.x) or `--with-system-bubblewrap=bwrap` (1.14.x or older), or a similar option, then the version of bubblewrap that needs to be patched is a system copy that is distributed separately, typically `/usr/bin/bwrap`. This configuration is the one that is typically used in Linux distributions. If Flatpak has been configured at build-time with `-Dsystem_bubblewrap=` (1.15.x) or with `--without-system-bubblewrap` (1.14.x or older), then it is the bundled version of bubblewrap that is included with Flatpak that must be patched. This is typically installed as `/usr/libexec/flatpak-bwrap`. This configuration is the default when building from source code.
For the 1.14.x stable branch, these changes are included in Flatpak 1.14.10. The bundled version of bubblewrap included in this release has been updated to 0.6.3. For the 1.15.x development branch, these changes are included in Flatpak 1.15.10. The bundled version of bubblewrap in this release is a Meson "wrap" subproject, which has been updated to 0.10.0. The 1.12.x and 1.10.x branches will not be updated for this vulnerability. Long-term support OS distributions should backport the individual changes into their versions of Flatpak and bubblewrap, or update to newer versions if their stability policy allows it. As a workaround, avoid using applications using the `persistent` (`--persist`) permission.
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Analysis
by VulDB Data Team • 10/03/2024
The vulnerability described in CVE-2024-42472 represents a critical security flaw in the Flatpak application sandboxing framework that affects versions prior to 1.14.0 and 1.15.10. This issue stems from improper handling of persistent directory permissions within Flatpak's sandboxing mechanism, creating a path traversal attack vector that compromises both data integrity and confidentiality. The vulnerability specifically impacts how Flatpak manages bind mounts for persistent directories, which are used to provide applications with access to specific subdirectories within the user's home environment while maintaining sandbox isolation. When applications utilize the persistent directory feature through the `--persist=subdir` command-line option, they are granted access to a designated subdirectory within their application-specific sandbox directory structure, but this access should remain properly confined to prevent unauthorized cross-directory operations.
The technical implementation of this vulnerability exploits a fundamental flaw in how Flatpak handles symbolic links when establishing persistent directory bindings. The system creates a bind mount from the application's sandboxed directory to a location within the per-application storage area at `~/.var/app/$APPID/subdir`, where `$APPID` represents the application identifier. However, when the source directory specified for persistent mounting is replaced with a symbolic link, the subsequent bind mount operation follows this link and mounts the target directory outside of the intended sandbox boundaries. This creates a privilege escalation scenario where malicious applications can gain access to arbitrary files and directories on the host system, effectively bypassing the sandboxing controls that are supposed to protect user data and system integrity. The flaw directly relates to CWE-22, which describes improper limitation of a pathname to a restricted directory, and CWE-23, which covers absolute path traversal attacks, making this vulnerability particularly dangerous for maintaining system security boundaries.
The operational impact of this vulnerability extends beyond simple data access violations to encompass potential system compromise and data exfiltration capabilities. Attackers could leverage this flaw to read sensitive files from the host system, modify critical configuration data, or even inject malicious code into other applications running within the sandboxed environment. The vulnerability's exploitation requires a malicious or compromised application to have already established persistence within the system, but once achieved, it provides a persistent backdoor that can remain active across system reboots and application restarts. The security implications are particularly severe in environments where multiple applications share the same user account, as this could enable lateral movement attacks where one compromised application uses this vulnerability to access resources belonging to other applications. This attack vector aligns with ATT&CK technique T1059.007 for command and scripting interpreter, and T1566 for spearphishing attacks, as attackers could use this vulnerability to establish persistence and maintain access to compromised systems.
Mitigation strategies for CVE-2024-42472 require coordinated updates to both Flatpak and its underlying bubblewrap sandboxing component, with particular attention to the specific build configurations used in different system deployments. The primary fix involves patching Flatpak to prevent the exploitation of symbolic link traversal in persistent directory handling, though this initial patch alone contains a race condition that could be exploited by parallel instances of malicious applications. Complete protection requires updating bubblewrap to version 0.6.3 or later for the 1.14.x branch, or 0.10.0 for the 1.15.x branch, and implementing the new `--bind-fd` option that provides more secure bind mount operations. System administrators must identify whether their Flatpak installation uses system bubblewrap or the bundled version, as each requires different patching approaches. The system-wide bubblewrap located at `/usr/bin/bwrap` must be patched when Flatpak is configured with system bubblewrap support, while the bundled version at `/usr/libexec/flatpak-bwrap` requires patching when using the default build configuration. Organizations should prioritize updating to Flatpak 1.14.10 or 1.15.10, which include the necessary security patches, or backport the individual fixes to their existing deployments. As a temporary workaround, administrators should avoid installing applications that utilize the persistent directory permissions, though this solution significantly reduces application functionality and user experience. The vulnerability's severity classifies it as high-risk for environments where Flatpak is widely deployed, particularly in enterprise and server environments where application isolation is critical for maintaining security boundaries and preventing cross-application data breaches.