21 Making Images More Secure
Security is of increasing concern for embedded devices. Consider the issues and problems discussed in just this sampling of work found across the Internet:
“Security Risks of Embedded Systems“ by Bruce Schneier
“Internet Census 2012“ by Carna Botnet
“Security Issues for Embedded Devices“ by Jake Edge
When securing your image is of concern, there are steps, tools, and variables that you can consider to help you reach the security goals you need for your particular device. Not all situations are identical when it comes to making an image secure. Consequently, this section provides some guidance and suggestions for consideration when you want to make your image more secure.
Because the security requirements and risks are different for every type of device, this section cannot provide a complete reference on securing your custom OS. It is strongly recommended that you also consult other sources of information on embedded Linux system hardening and on security.
21.1 General Considerations
There are general considerations that help you create more secure images. You should consider the following suggestions to make your device more secure:
Scan additional code you are adding to the system (e.g. application code) by using static analysis tools. Look for buffer overflows and other potential security problems.
Pay particular attention to the security for any web-based administration interface.
Web interfaces typically need to perform administrative functions and tend to need to run with elevated privileges. Thus, the consequences resulting from the interface’s security becoming compromised can be serious. Look for common web vulnerabilities such as cross-site-scripting (XSS), unvalidated inputs, and so forth.
As with system passwords, the default credentials for accessing a web-based interface should not be the same across all devices. This is particularly true if the interface is enabled by default as it can be assumed that many end-users will not change the credentials.
Ensure you can update the software on the device to mitigate vulnerabilities discovered in the future. This consideration especially applies when your device is network-enabled.
Regularly scan and apply fixes for CVE security issues affecting all software components in the product, see “Checking for Vulnerabilities”.
Regularly update your version of Poky and OE-Core from their upstream developers, e.g. to apply updates and security fixes from stable and LTS branches.
Ensure you remove or disable debugging functionality before producing the final image. For information on how to do this, see the “Considerations Specific to the OpenEmbedded Build System” section.
Ensure you have no network services listening that are not needed.
Remove any software from the image that is not needed.
Enable hardware support for secure boot functionality when your device supports this functionality.
21.2 Security Flags
The Yocto Project has security flags that you can enable that help make
your build output more secure. The security flags are in the
meta/conf/distro/include/security_flags.inc file in your
Source Directory (e.g.
Depending on the recipe, certain security flags are enabled and disabled by default.
Use the following line in your
local.conf file or in your custom
distribution configuration file to enable the security compiler and
linker flags for your build:
21.3 Considerations Specific to the OpenEmbedded Build System
You can take some steps that are specific to the OpenEmbedded build system to make your images more secure:
Ensure “debug-tweaks” is not one of your selected IMAGE_FEATURES. When creating a new project, the default is to provide you with an initial
local.conffile that enables this feature using the EXTRA_IMAGE_FEATURES variable with the line:
EXTRA_IMAGE_FEATURES = "debug-tweaks"
To disable that feature, simply comment out that line in your
local.conffile, or make sure IMAGE_FEATURES does not contain “debug-tweaks” before producing your final image. Among other things, leaving this in place sets the root password as blank, which makes logging in for debugging or inspection easy during development but also means anyone can easily log in during production.
It is possible to set a root password for the image and also to set passwords for any extra users you might add (e.g. administrative or service type users). When you set up passwords for multiple images or users, you should not duplicate passwords.
To set up passwords, use the extrausers class, which is the preferred method. For an example on how to set up both root and user passwords, see the “extrausers” section.
When adding extra user accounts or setting a root password, be cautious about setting the same password on every device. If you do this, and the password you have set is exposed, then every device is now potentially compromised. If you need this access but want to ensure security, consider setting a different, random password for each device. Typically, you do this as a separate step after you deploy the image onto the device.
Consider enabling a Mandatory Access Control (MAC) framework such as SMACK or SELinux and tuning it appropriately for your device’s usage. You can find more information in the meta-selinux layer.
21.4 Tools for Hardening Your Image
The Yocto Project provides tools for making your image more secure. You
can find these tools in the
meta-security layer of the
Yocto Project Source Repositories.