Description
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.
EPSS Score:
0%
Comprehensive Technical Analysis of EUVD-2024-39634
1. Vulnerability Assessment and Severity Evaluation
The vulnerability described in EUVD-2024-39634 affects Flatpak, a Linux application sandboxing and distribution framework. The issue arises from the way Flatpak handles persistent directories, allowing a malicious or compromised application to access and write files outside its intended scope. This vulnerability compromises both the integrity and confidentiality of the system.
Severity Evaluation:
- CVSS Base Score: 10.0
- CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
The high CVSS score indicates a critical vulnerability. The attack vector (AV:N) is network-based, requiring low complexity (AC:L) and no privileges (PR:N) or user interaction (UI:N). The impact on confidentiality and integrity is high (C:H/I:H), with no direct impact on availability (A:N). The scope change (S:C) indicates that the vulnerability affects components beyond the security scope of the vulnerable component.
2. Potential Attack Vectors and Exploitation Methods
Attack Vectors:
- Symlink Attack: An attacker could replace the source directory for the
persistent/--persistoption with a symlink. When the application is started, the bind mount follows the symlink, mounting the targeted directory into the sandbox. - Race Condition: If two instances of a malicious app run in parallel, they could exploit a race condition to gain unauthorized access to files.
Exploitation Methods:
- Symlink Manipulation: By manipulating symlinks, an attacker can redirect the bind mount to sensitive directories, allowing unauthorized access and modification.
- Parallel Execution: Running multiple instances of a malicious app can exploit the race condition, leading to unauthorized file access.
3. Affected Systems and Software Versions
Affected Versions:
- Flatpak versions prior to 1.14.10 and 1.15.10.
- Specifically, Flatpak 1.15.0 and versions below 1.15.10.
Systems:
- Linux distributions using Flatpak for application sandboxing and distribution.
- Systems where Flatpak is configured with either system or bundled versions of bubblewrap.
4. Recommended Mitigation Strategies
Immediate Mitigation:
- Avoid Using
persistentPermission: As a workaround, avoid using applications that require thepersistent(--persist) permission.
Patching:
- Flatpak Updates: Update to Flatpak 1.14.10 or 1.15.10, which include patches for the vulnerability.
- Bubblewrap Updates: Ensure that the version of bubblewrap used by Flatpak is updated to 0.6.3 (for Flatpak 1.14.10) or 0.10.0 (for Flatpak 1.15.10).
Long-Term Mitigation:
- Backporting: Long-term support OS distributions should backport the necessary changes into their versions of Flatpak and bubblewrap or update to newer versions if stability policies allow.
5. Impact on European Cybersecurity Landscape
The vulnerability poses a significant risk to the European cybersecurity landscape, particularly for organizations and individuals relying on Flatpak for application sandboxing. The potential for unauthorized access to sensitive files can lead to data breaches, loss of confidentiality, and integrity issues. Given the widespread use of Flatpak in Linux distributions, the impact could be extensive, affecting both personal and enterprise environments.
6. Technical Details for Security Professionals
Technical Overview:
- Persistent Directories: The
persistent=subdiroption allows applications to write to a specific subdirectory within the application directory, which is bind-mounted to the sandbox. - Symlink Vulnerability: If the source directory is replaced by a symlink, the bind mount follows the symlink, potentially exposing sensitive directories.
- Race Condition: The race condition arises when multiple instances of a malicious app run in parallel, exploiting the timing of bind mount operations.
Patch Details:
- Flatpak Patches: Commits ceec2ffc and 98f79773 provide partial protection but leave a race condition.
- Bubblewrap Patch: The
--bind-fdoption in bubblewrap is required to close the race condition. This involves updating bubblewrap to version 0.6.3 or 0.10.0 and configuring Flatpak to use the updated bubblewrap.
Configuration:
- System Bubblewrap: If Flatpak is configured with
-Dsystem_bubblewrap=bwrapor--with-system-bubblewrap=bwrap, the system copy of bubblewrap (typically/usr/bin/bwrap) needs to be patched. - Bundled Bubblewrap: If Flatpak is configured with
-Dsystem_bubblewrap=or--without-system-bubblewrap, the bundled version of bubblewrap (typically/usr/libexec/flatpak-bwrap) needs to be patched.
References:
Conclusion
The vulnerability in Flatpak poses a critical risk to systems relying on its sandboxing capabilities. Immediate patching and careful configuration management are essential to mitigate the risk. Organizations should prioritize updating to the patched versions of Flatpak and bubblewrap to ensure the security and integrity of their systems.