Welcome!

Linux Containers Authors: Elizabeth White, Pat Romanski, Stefan Bernbo, Liz McMillan, ManageEngine IT Matters

Related Topics: Linux Containers

Linux Containers: Article

Principles of Secure Programming

Applying basic security principles to programming

The purpose of this article is to show how basic security principles can help you develop programs that are harder for the bad guys to break. We'll examine a simple function that executes a command as though it were typed at the keyboard, exactly what the library function system does. But unlike many system implementations, we'll constrain what happens so the calling program can't trick it into executing some other program.

The system function takes a single argument: a character string with the command to be executed just as it would be typed at the keyboard. The function first invokes the Bourne shell, passing the command to that shell using the "–c" option. The shell then spawns the command. For example:

system("date")
invokes the command
/bin/sh –c "date"

This executes the program "date," which prints the date on the standard output.

Security Issues
Security issues arise when the program invoking the system function is a privileged program. The "privilege" may consist of having setuid and/or setgid privileges such as su or being able to run one of a specified set of programs such as a Web server serving CGI scripts. The attacker's goal is to trick the program into executing some other program, for example a version of date that's a command interpreter rather than just printing the date.

Problems arise because of the power of the Bourne shell as a command interpreter. That shell takes information from the environment, which consists of shell variables, file descriptors, signal-handling routines, and any other aspects of the process space that could affect program execution. For our purposes, we'll just consider environment variables.

One relevant environment variable is the PATH environment variable. When given a command that doesn't contain a '/' the Bourne shell treats the value of the PATH variable as a sequence of directory names. It looks in each directory in the given order for a program named "date" and executes the first one found. Suppose an attacker finds a setuid-to-root program that uses system to run the "date" command. The attacker can then copy the shell into a file named "date" in her current working directory, prepend "." to the list of directories in the value of PATH, and then execute the program. When system invokes the shell, it searches each directory in the value of PATH in order for a command named "date." The first directory searched will be the current working directory. The shell will find a program called "date" there and execute it, spawning the command interpreter, which will run with root privileges.

Our goal is to construct a version of system that's invulnerable to this kind of attack. Specifically, we want to guarantee that when the caller passes a command name to system, the user can't cause the program to execute a different program.

Applying the Principles
We'll apply two principles of secure design and implementation. They come from a paper by Jerome Saltzer and Michael Schroeder and are central to any security work. In practice, we would also consider the other six, but the two we'll use have more impact on the design and implementation of this particular function than the others.

Principle of Least Privilege
The first principle is the principle of least privilege. This principle, also called the need-to-know principle, says that a process should have the minimum privileges needed to perform its task. For this problem, this rule says that system should execute the command with the privileges of the user, not with those of root, if at all possible. As an example, were the privileged program to print the date and time by using system to run the command "date" as described above, there's no reason that "date" needs to be executed as root. It could just as easily be executed with the user's privileges. Hence the first step in our new system command would be to let the caller reset the privileges to those of the real user and group. Doing this means that the user can only compromise her own account - and as since she already has full access to it such a compromise is meaningless.

If the caller lets the user select one of a set of commands, then a different application of the principle of least privilege provides the required restriction. The program configuration should create a directory into which copies of the commands to be executed are placed. Then the program changes its notion of the root directory to that of the directory containing the commands. Even if the user can enter the name of a different command, only the authorized commands are accessible to the program. So only the authorized commands can be executed, and the user will get an error message. This is the technique that sendmail's restricted shell uses to ensure that sendmail only executes safe programs like procmail and vacation. Web servers should use this technique to ensure that commands to execute CGI programs can only execute the CGI programs in the Web server's directories.

Principle of Fail-Safe Defaults
The principle of fail-safe defaults says that access to resources and objects should be denied by default. If you need access to only one particular object, the usual approach of removing access to all other objects violates this principle. Instead, access to all objects should be removed, and then access privileges for that particular object should be explicitly granted. The distinction is subtle, but critical.

To see this, consider the problem of ensuring the user's PATH environment variable is set appropriately. The naive approach is to search the environment for the PATH environment variable and check that its value is acceptable. This leads to two problems. First, what happens if the value is not acceptable? In this case, the value must be replaced. Second, what happens if there are multiple occurrences of the variable? The values of all must be checked and found satisfactory, or all but one must be deleted.

A second approach is to require that the program use the full path name of the program. So the invocation of the system call would be:

system("/bin/date")

This causes the shell to ignore the PATH setting. Unfortunately, this approach is also flawed.

Environment variables other than PATH affect the executed program. For some versions of the Bourne shell, the value of the environment variable IFS is a string of characters that the shell treats as word separators. (This is particularly useful when a shell script is reading lines from the password file, for example.) In such a shell, the following command prints files X and Y:

IFS="/$IFS"; export IFS; cat/x/y

because the shell sees the "/" character as a word separator, which lets the user thwart the use of a full path name as described above. All she need do is set IFS in her environment to include the "/" character and then create a program called "bin" in her current working directory. She then changes her PATH environment variable to look in the current working directory first. When she runs the command, the privileged program invokes the above system function. The subordinate shell reads the argument of system as having two words, "bin" followed by the argument "date." Hence the user's program "bin" will be executed and the shell will pass "date" to it as an argument.

Again the programmer can try to prevent this by setting IFS explicitly in the environment:

system("IFS=\" \t\n\"; export IFS; /bin/date")

As tempting as this approach is, it suffers from two problems. The first is that the attacker can easily defeat it by adding "I" to the IFS variable. Then the shell sees this as adding the environment variable FS to the environment. The second problem arises when the attacker doesn't do this. There are now two occurrences of the IFS environment variable in the environment. Which one is used? That turns out to be implementation-dependent: some versions of the shell use the first (the user's), and others use the second (the one defined in the system argument.

Following the principle of fail-safe defaults offers a simple answer to all this. First, create an empty environment for the shell. Then add preset, safe values of PATH, IFS, and any other needed environment variables to that environment. Finally, set the shell's environment to be the newly created one. Doing so makes the user's environment irrelevant to the system function and the shell it calls. The shell never refers to the user's environment. The shell only uses the newly created safe environment.

Now the order in which the shell evaluates the variables in the environment is irrelevant, because there is only one occurrence of each variable in the environment. If the user adds "/" to the value of IFS in her environment, or alters the value of the PATH environment variable, the shell ignores those changes because it never sees the values of those variables. It only sees the ones defined in the environment set up by the program.

Conclusion
Programming with security in mind is critical for today's programs. This style of programming requires a methodical approach, not one in which various tricks are used without understanding how and why they work. The problem is that tricks only apply to certain situations, and can only be used effectively if those situations arise. But the principles of secure design and implementation apply always, and dramatically improve both the quality and the security of the programs and systems they are applied to.

Recommended Reading

  • J. Saltzer and M. Schroeder, "The Protection of Information in Computer Systems," Proceedings of the IEEE 63 (9) pp. 1278-1308 (September 1975). This paper first enunciated the principles and discussed them thoroughly in a variety of contexts. A must read for anyone doing design and/or implementation in the field of computer security.
  • B. Kernighan and P. Plauger, The Elements of Programming Style, McGraw-Hill Book Co., Reading, MA (1974). The principles described in this book lead to a clear and readable programming style. Their emphasis on simplicity and clarity parallels principles in security. Highly recommended.
  • M. Graff and K. Van Wyk, Secure Coding: Principles and Practices, O'Reilly and Associates, Sebastopol, CA (June 2003). This book describes security through the lifecycle of a program or system. An excellent high-level view of how to write code that emphasizes security.
  • J. Viega and G. McGraw, Building Secure Software: How to Avoid Security Problems the Right Way, Addison-Wesley Publishing Co., Boston, MA (2002). This book discusses both principles and practice, drawing most of its examples from Unix and Linux systems. Another must read for Unix and Linux programmers.
  • M. Howard and D. LeBlanc, Writing Secure Code, Microsoft Press, Redmond, WA (2001). Similar to Viega and McGraw but focusing on Windows platforms, this book shows the application of principles to a different environment. A must read for Windows developers, and a worthwhile read for Unix and Linux programmers interested in a different programming environment.
  • A. Stavely, Towards Zero-Defect Programming, Addison-Wesley Publishing Co., Reading, MA (1998). Although focused on correctness more than security, its ideas can be readily adapted to security. Its mix of formalism and informality is refreshing.
  • More Stories By Matt Bishop

    Matt Bishop is a professor in the Department of Computer Science at the University of California at Davis. A recognized expert in vulnerability analysis, secure systems/software design, network security, access control, authentication, and UNIX security, Bishop also works to improve computer security instruction. He is the author of Computer Security: Art and Science and Introduction to Computer Security (Addison-Wesley).

    Comments (0)

    Share your thoughts on this story.

    Add your comment
    You must be signed in to add a comment. Sign-in | Register

    In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


    @ThingsExpo Stories
    There will be new vendors providing applications, middleware, and connected devices to support the thriving IoT ecosystem. This essentially means that electronic device manufacturers will also be in the software business. Many will be new to building embedded software or robust software. This creates an increased importance on software quality, particularly within the Industrial Internet of Things where business-critical applications are becoming dependent on products controlled by software. Qua...
    In addition to all the benefits, IoT is also bringing new kind of customer experience challenges - cars that unlock themselves, thermostats turning houses into saunas and baby video monitors broadcasting over the internet. This list can only increase because while IoT services should be intuitive and simple to use, the delivery ecosystem is a myriad of potential problems as IoT explodes complexity. So finding a performance issue is like finding the proverbial needle in the haystack.
    Machine Learning helps make complex systems more efficient. By applying advanced Machine Learning techniques such as Cognitive Fingerprinting, wind project operators can utilize these tools to learn from collected data, detect regular patterns, and optimize their own operations. In his session at 18th Cloud Expo, Stuart Gillen, Director of Business Development at SparkCognition, discussed how research has demonstrated the value of Machine Learning in delivering next generation analytics to imp...
    Large scale deployments present unique planning challenges, system commissioning hurdles between IT and OT and demand careful system hand-off orchestration. In his session at @ThingsExpo, Jeff Smith, Senior Director and a founding member of Incenergy, will discuss some of the key tactics to ensure delivery success based on his experience of the last two years deploying Industrial IoT systems across four continents.
    The 19th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Digital Transformation, Microservices and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportuni...
    The Internet of Things will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform. In his session at @ThingsExpo, Craig Sproule, CEO of Metavine, demonstrated how to move beyond today's coding paradigm and shared the must-have mindsets for removing complexity from the develo...
    Basho Technologies has announced the latest release of Basho Riak TS, version 1.3. Riak TS is an enterprise-grade NoSQL database optimized for Internet of Things (IoT). The open source version enables developers to download the software for free and use it in production as well as make contributions to the code and develop applications around Riak TS. Enhancements to Riak TS make it quick, easy and cost-effective to spin up an instance to test new ideas and build IoT applications. In addition to...
    Internet of @ThingsExpo, taking place November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with the 19th International Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world and ThingsExpo Silicon Valley Call for Papers is now open.
    IoT is rapidly changing the way enterprises are using data to improve business decision-making. In order to derive business value, organizations must unlock insights from the data gathered and then act on these. In their session at @ThingsExpo, Eric Hoffman, Vice President at EastBanc Technologies, and Peter Shashkin, Head of Development Department at EastBanc Technologies, discussed how one organization leveraged IoT, cloud technology and data analysis to improve customer experiences and effi...
    "We've discovered that after shows 80% if leads that people get, 80% of the conversations end up on the show floor, meaning people forget about it, people forget who they talk to, people forget that there are actual business opportunities to be had here so we try to help out and keep the conversations going," explained Jeff Mesnik, Founder and President of ContentMX, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
    With 15% of enterprises adopting a hybrid IT strategy, you need to set a plan to integrate hybrid cloud throughout your infrastructure. In his session at 18th Cloud Expo, Steven Dreher, Director of Solutions Architecture at Green House Data, discussed how to plan for shifting resource requirements, overcome challenges, and implement hybrid IT alongside your existing data center assets. Highlights included anticipating workload, cost and resource calculations, integrating services on both sides...
    Manufacturers are embracing the Industrial Internet the same way consumers are leveraging Fitbits – to improve overall health and wellness. Both can provide consistent measurement, visibility, and suggest performance improvements customized to help reach goals. Fitbit users can view real-time data and make adjustments to increase their activity. In his session at @ThingsExpo, Mark Bernardo Professional Services Leader, Americas, at GE Digital, discussed how leveraging the Industrial Internet a...
    Big Data engines are powering a lot of service businesses right now. Data is collected from users from wearable technologies, web behaviors, purchase behavior as well as several arbitrary data points we’d never think of. The demand for faster and bigger engines to crunch and serve up the data to services is growing exponentially. You see a LOT of correlation between “Cloud” and “Big Data” but on Big Data and “Hybrid,” where hybrid hosting is the sanest approach to the Big Data Infrastructure pro...
    "My role is working with customers, helping them go through this digital transformation. I spend a lot of time talking to banks, big industries, manufacturers working through how they are integrating and transforming their IT platforms and moving them forward," explained William Morrish, General Manager Product Sales at Interoute, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
    A critical component of any IoT project is what to do with all the data being generated. This data needs to be captured, processed, structured, and stored in a way to facilitate different kinds of queries. Traditional data warehouse and analytical systems are mature technologies that can be used to handle certain kinds of queries, but they are not always well suited to many problems, particularly when there is a need for real-time insights.
    The best-practices for building IoT applications with Go Code that attendees can use to build their own IoT applications. In his session at @ThingsExpo, Indraneel Mitra, Senior Solutions Architect & Technology Evangelist at Cognizant, provided valuable information and resources for both novice and experienced developers on how to get started with IoT and Golang in a day. He also provided information on how to use Intel Arduino Kit, Go Robotics API and AWS IoT stack to build an application tha...
    IoT generates lots of temporal data. But how do you unlock its value? You need to discover patterns that are repeatable in vast quantities of data, understand their meaning, and implement scalable monitoring across multiple data streams in order to monetize the discoveries and insights. Motif discovery and deep learning platforms are emerging to visualize sensor data, to search for patterns and to build application that can monitor real time streams efficiently. In his session at @ThingsExpo, ...
    You think you know what’s in your data. But do you? Most organizations are now aware of the business intelligence represented by their data. Data science stands to take this to a level you never thought of – literally. The techniques of data science, when used with the capabilities of Big Data technologies, can make connections you had not yet imagined, helping you discover new insights and ask new questions of your data. In his session at @ThingsExpo, Sarbjit Sarkaria, data science team lead ...
    Extracting business value from Internet of Things (IoT) data doesn’t happen overnight. There are several requirements that must be satisfied, including IoT device enablement, data analysis, real-time detection of complex events and automated orchestration of actions. Unfortunately, too many companies fall short in achieving their business goals by implementing incomplete solutions or not focusing on tangible use cases. In his general session at @ThingsExpo, Dave McCarthy, Director of Products...
    WebRTC is bringing significant change to the communications landscape that will bridge the worlds of web and telephony, making the Internet the new standard for communications. Cloud9 took the road less traveled and used WebRTC to create a downloadable enterprise-grade communications platform that is changing the communication dynamic in the financial sector. In his session at @ThingsExpo, Leo Papadopoulos, CTO of Cloud9, discussed the importance of WebRTC and how it enables companies to focus...