Welcome!

Linux Containers Authors: Liz McMillan, James Carlini, Elizabeth White, Vaibhaw Pandey, Pat Romanski

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
    "Space Monkey by Vivent Smart Home is a product that is a distributed cloud-based edge storage network. Vivent Smart Home, our parent company, is a smart home provider that places a lot of hard drives across homes in North America," explained JT Olds, Director of Engineering, and Brandon Crowfeather, Product Manager, at Vivint Smart Home, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
    SYS-CON Events announced today that Conference Guru has been named “Media Sponsor” of the 22nd International Cloud Expo, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. A valuable conference experience generates new contacts, sales leads, potential strategic partners and potential investors; helps gather competitive intelligence and even provides inspiration for new products and services. Conference Guru works with conference organizers to pass great deals to gre...
    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 develop...
    In his Opening Keynote at 21st Cloud Expo, John Considine, General Manager of IBM Cloud Infrastructure, led attendees through the exciting evolution of the cloud. He looked at this major disruption from the perspective of technology, business models, and what this means for enterprises of all sizes. John Considine is General Manager of Cloud Infrastructure Services at IBM. In that role he is responsible for leading IBM’s public cloud infrastructure including strategy, development, and offering m...
    "Evatronix provides design services to companies that need to integrate the IoT technology in their products but they don't necessarily have the expertise, knowledge and design team to do so," explained Adam Morawiec, VP of Business Development at Evatronix, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
    To get the most out of their data, successful companies are not focusing on queries and data lakes, they are actively integrating analytics into their operations with a data-first application development approach. Real-time adjustments to improve revenues, reduce costs, or mitigate risk rely on applications that minimize latency on a variety of data sources. In his session at @BigDataExpo, Jack Norris, Senior Vice President, Data and Applications at MapR Technologies, reviewed best practices to ...
    Widespread fragmentation is stalling the growth of the IIoT and making it difficult for partners to work together. The number of software platforms, apps, hardware and connectivity standards is creating paralysis among businesses that are afraid of being locked into a solution. EdgeX Foundry is unifying the community around a common IoT edge framework and an ecosystem of interoperable components.
    Large industrial manufacturing organizations are adopting the agile principles of cloud software companies. The industrial manufacturing development process has not scaled over time. Now that design CAD teams are geographically distributed, centralizing their work is key. With large multi-gigabyte projects, outdated tools have stifled industrial team agility, time-to-market milestones, and impacted P&L stakeholders.
    "Akvelon is a software development company and we also provide consultancy services to folks who are looking to scale or accelerate their engineering roadmaps," explained Jeremiah Mothersell, Marketing Manager at Akvelon, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
    "IBM is really all in on blockchain. We take a look at sort of the history of blockchain ledger technologies. It started out with bitcoin, Ethereum, and IBM evaluated these particular blockchain technologies and found they were anonymous and permissionless and that many companies were looking for permissioned blockchain," stated René Bostic, Technical VP of the IBM Cloud Unit in North America, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Conventi...
    In his session at 21st Cloud Expo, Carl J. Levine, Senior Technical Evangelist for NS1, will objectively discuss how DNS is used to solve Digital Transformation challenges in large SaaS applications, CDNs, AdTech platforms, and other demanding use cases. Carl J. Levine is the Senior Technical Evangelist for NS1. A veteran of the Internet Infrastructure space, he has over a decade of experience with startups, networking protocols and Internet infrastructure, combined with the unique ability to it...
    22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, NY, and co-located with the 1st DXWorld Expo will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud ...
    "Cloud Academy is an enterprise training platform for the cloud, specifically public clouds. We offer guided learning experiences on AWS, Azure, Google Cloud and all the surrounding methodologies and technologies that you need to know and your teams need to know in order to leverage the full benefits of the cloud," explained Alex Brower, VP of Marketing at Cloud Academy, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clar...
    Gemini is Yahoo’s native and search advertising platform. To ensure the quality of a complex distributed system that spans multiple products and components and across various desktop websites and mobile app and web experiences – both Yahoo owned and operated and third-party syndication (supply), with complex interaction with more than a billion users and numerous advertisers globally (demand) – it becomes imperative to automate a set of end-to-end tests 24x7 to detect bugs and regression. In th...
    "MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
    Coca-Cola’s Google powered digital signage system lays the groundwork for a more valuable connection between Coke and its customers. Digital signs pair software with high-resolution displays so that a message can be changed instantly based on what the operator wants to communicate or sell. In their Day 3 Keynote at 21st Cloud Expo, Greg Chambers, Global Group Director, Digital Innovation, Coca-Cola, and Vidya Nagarajan, a Senior Product Manager at Google, discussed how from store operations and ...
    "There's plenty of bandwidth out there but it's never in the right place. So what Cedexis does is uses data to work out the best pathways to get data from the origin to the person who wants to get it," explained Simon Jones, Evangelist and Head of Marketing at Cedexis, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
    SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5–7, 2018, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buye...
    SYS-CON Events announced today that Telecom Reseller has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Telecom Reseller reports on Unified Communications, UCaaS, BPaaS for enterprise and SMBs. They report extensively on both customer premises based solutions such as IP-PBX as well as cloud based and hosted platforms.
    It is of utmost importance for the future success of WebRTC to ensure that interoperability is operational between web browsers and any WebRTC-compliant client. To be guaranteed as operational and effective, interoperability must be tested extensively by establishing WebRTC data and media connections between different web browsers running on different devices and operating systems. In his session at WebRTC Summit at @ThingsExpo, Dr. Alex Gouaillard, CEO and Founder of CoSMo Software, presented ...