Welcome!

Linux Containers Authors: Elizabeth White, Yeshim Deniz, Liz McMillan, Sematext Blog, Derek Weeks

Related Topics: Linux Containers

Linux Containers: Article

Variadic Functions: How They Contribute To Security Vulnerabilities and How To Fix Them

Variadic functions are implemented using either the ANSI C stdarg approach or, historically, the UNIX System V vararg approach

C/C++ language variadic functions are functions that accept a variable number of arguments. Variadic functions are implemented using either the ANSI C stdarg approach or, historically, the UNIX System V vararg approach. Both approaches require that the contract between the developer and user of the variadic function not be violated by the user.

Many of the formatted I/O functions in the ISO/IEC 9899:1999 C language standard (C99) such as printf() and scanf() are defined as variadic functions (including formatted output functions that operate on a multibyte characters [e.g., ASCII] and wide characters [e.g., UNICODE]).

These functions accept a fixed format string argument that specifies, among other things, the number and type of arguments that are expected. If the contents of the format string are incorrect (by error or by malicious intent), the resulting behavior of the function is undefined.

Incautious use of formatted I/O functions have led to numerous, exploitable vulnerabilities. The majority of these vulnerabilities occur when a potentially malicious user is able to control all or some portion of the format specification string as shown in the following program:

1.  #include <stdio.h>
2.  #include <string.h>
3.  void usage(char *pname) {
4.  char usageStr[1024];
5.  snprintf(usageStr, 1024, "Usage: %s <target>\n", pname);
6.  printf(usageStr);
7.  }
8.  int main(int argc, char * argv[]) {
9.    if (argc < 2) {
10.      usage(argv[0]);
11.      exit(-1);
12.    }
13. }

These vulnerabilities are often referred to as "format string" vulnerabilities. Exploits take a variety of forms, the most dangerous of which involves using the %n conversion specifier to overwrite memory and transfer control to arbitrary code of the attacker's choosing. The easiest way to prevent format string vulnerabilities is to ensure that the format string does not include characters from untrusted sources. Because of internationalization, however, format strings and message text are often moved into external catalogs or files that the program opens at runtime. An attacker can alter the values of the formats and strings in the program by modifying the contents of these files. The entire topic of formatted output is covered in detail in my book on Secure Coding in C/C++.

Format string vulnerabilities have been discovered in a variety of deployed C language programs, including:

  • The Washington University FTP daemon wu-ftpd that is shipped with many distributions of Linux and other UNIX operating systems (CA-2000-13).
  • The common desktop environment (CDE), an integrated graphical user interface that runs on UNIX and Linux operating systems (CA-2001-27).
  • Helix Player, and media players based on the Helix Player, including Real Player for Linux systems (VU#361181).
The following is an example of a variadic function implementation using ANSI stdarg:

1.  int average(int first, ...) {
2.   int count = 0, sum = 0, i = first;
3.   va_list marker;
4.   va_start(marker, first);
5.   while (i != -1) {
6.     sum += i;
7.     count++;
8.     i = va_arg(marker, int);
9.     }
10.   va_end(marker);
11.   return(sum ? (sum / count) : 0);
12. }

Variadic functions are declared using a partial parameter list followed by the ellipsis notation. The variadic average() function accepts a single, fixed integer argument followed by a variable argument list. Like other functions, the arguments to the variadic function are pushed on the calling stack.

Variadic functions are problematic for a number of reasons. The first and foremost is that the implementation has no real way of knowing how many arguments were passed (even though this information is available at compile time). The termination condition for the argument list is a contract between the programmers who implement the library function and the programmers who use the function in an application. In this implementation of the average() function, termination of the variable argument list is indicated by an argument whose value is -1. This means, for example, that average(5, -1, 2, -1) is 5, not 2, as the programmer might expect. Also, if the programmer calling the function neglects to provide this argument, the average() function will continue to process the next argument indefinitely until a -1 value is encountered or an exception occurs.

A second problem with variadic functions is a complete lack of type checking. In the case of formatted output functions, the type of the arguments is determined by the corresponding conversion specifier in the format string. For example, if a %d conversion specifier is encountered, the formatted output function assumes that the corresponding argument is an integer. If a %s is found, the corresponding argument is interpreted as a pointer to a string. This could result in a program fault, for example, if the corresponding argument was actually a small integer value.

Every time a variadic function consumes an argument, an internal argument pointer is incremented to reference the next argument on the stack. If there is some type confusion, it is possible that the argument pointer is incorrectly incremented. This happens less than you might imagine on a 32-bit architecture such as the 32-bit Intel Architecture (IA-32) because almost all arguments (including addresses, char, short, int, and long int) use four bytes. However, conversion specifiers such as a, A, e, E, f, F, g, or G are used to output a 64-bit floating-point number, thereby incrementing the argument pointer by 8.

The standard C formatted output functions need modifications to print 64-bit integer and pointer values in hexadecimal. The %x modifier will only print out the first 32 bits of the value that is passed to it and increment the internal argument pointer by 4 bytes. To print out a 64-bit pointer, the ANSI C %p directive needs to be used rather than %x or %u. To print 64-bit integers, you need to use the one size specifier.

Solutions
One property of format string exploits is that the number of arguments referenced by the attacker's format string is greater than the arguments in the call to the formatted output function. Unfortunately, there is currently no mechanism by which a variadic function implementation can determine the number of arguments (or preferably the number of bytes) passed, so it is impossible to determine when this limit has been exceeded. If such a mechanism existed, variadic functions (such as printf()) could be implemented in such a way as to prevent most format string vulnerabilities.

More Stories By Robert Seacord

Robert C. Seacord is a senior vulnerability analyst at the CERT/Coordination Center (CERT/CC) at the Software Engineering Institute (SEI) in Pittsburgh, PA, and author of Secure Coding in C and C++ (Addison-Wesley, 2005). An eclectic technologist, Robert is coauthor of two previous books, Building Systems from Commercial Components (Addison-Wesley, 2002) and Modernizing Legacy Systems (Addison-Wesley, 2003).

Comments (1) View Comments

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.


Most Recent Comments
LinuxWorld News Desk 12/06/05 04:18:44 PM EST

LinuxWorld Feature - Variadic Functions: How They Contribute To Security Vulnerabilities and How To Fix Them. C/C++ language variadic functions are functions that accept a variable number of arguments. Variadic functions are implemented using either the ANSI C stdarg approach or, historically, the UNIX System V vararg approach. Both approaches require that the contract between the developer and user of the variadic function not be violated by the user.

@ThingsExpo Stories
The Internet of Things is clearly many things: data collection and analytics, wearables, Smart Grids and Smart Cities, the Industrial Internet, and more. Cool platforms like Arduino, Raspberry Pi, Intel's Galileo and Edison, and a diverse world of sensors are making the IoT a great toy box for developers in all these areas. In this Power Panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists discussed what things are the most important, which will have the most profound e...
Keeping pace with advancements in software delivery processes and tooling is taxing even for the most proficient organizations. Point tools, platforms, open source and the increasing adoption of private and public cloud services requires strong engineering rigor - all in the face of developer demands to use the tools of choice. As Agile has settled in as a mainstream practice, now DevOps has emerged as the next wave to improve software delivery speed and output. To make DevOps work, organization...
My team embarked on building a data lake for our sales and marketing data to better understand customer journeys. This required building a hybrid data pipeline to connect our cloud CRM with the new Hadoop Data Lake. One challenge is that IT was not in a position to provide support until we proved value and marketing did not have the experience, so we embarked on the journey ourselves within the product marketing team for our line of business within Progress. In his session at @BigDataExpo, Sum...
Web Real-Time Communication APIs have quickly revolutionized what browsers are capable of. In addition to video and audio streams, we can now bi-directionally send arbitrary data over WebRTC's PeerConnection Data Channels. With the advent of Progressive Web Apps and new hardware APIs such as WebBluetooh and WebUSB, we can finally enable users to stitch together the Internet of Things directly from their browsers while communicating privately and securely in a decentralized way.
DevOps is often described as a combination of technology and culture. Without both, DevOps isn't complete. However, applying the culture to outdated technology is a recipe for disaster; as response times grow and connections between teams are delayed by technology, the culture will die. A Nutanix Enterprise Cloud has many benefits that provide the needed base for a true DevOps paradigm.
What sort of WebRTC based applications can we expect to see over the next year and beyond? One way to predict development trends is to see what sorts of applications startups are building. In his session at @ThingsExpo, Arin Sime, founder of WebRTC.ventures, will discuss the current and likely future trends in WebRTC application development based on real requests for custom applications from real customers, as well as other public sources of information,
In his General Session at 17th Cloud Expo, Bruce Swann, Senior Product Marketing Manager for Adobe Campaign, explored the key ingredients of cross-channel marketing in a digital world. Learn how the Adobe Marketing Cloud can help marketers embrace opportunities for personalized, relevant and real-time customer engagement across offline (direct mail, point of sale, call center) and digital (email, website, SMS, mobile apps, social networks, connected objects).
With the introduction of IoT and Smart Living in every aspect of our lives, one question has become relevant: What are the security implications? To answer this, first we have to look and explore the security models of the technologies that IoT is founded upon. In his session at @ThingsExpo, Nevi Kaja, a Research Engineer at Ford Motor Company, will discuss some of the security challenges of the IoT infrastructure and relate how these aspects impact Smart Living. The material will be delivered i...
"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.
SYS-CON Events announced today that Ocean9will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Ocean9 provides cloud services for Backup, Disaster Recovery (DRaaS) and instant Innovation, and redefines enterprise infrastructure with its cloud native subscription offerings for mission critical SAP workloads.
Your homes and cars can be automated and self-serviced. Why can't your storage? From simply asking questions to analyze and troubleshoot your infrastructure, to provisioning storage with snapshots, recovery and replication, your wildest sci-fi dream has come true. In his session at @DevOpsSummit at 20th Cloud Expo, Dan Florea, Director of Product Management at Tintri, will provide a ChatOps demo where you can talk to your storage and manage it from anywhere, through Slack and similar services ...
In his session at @ThingsExpo, Eric Lachapelle, CEO of the Professional Evaluation and Certification Board (PECB), will provide an overview of various initiatives to certifiy the security of connected devices and future trends in ensuring public trust of IoT. Eric Lachapelle is the Chief Executive Officer of the Professional Evaluation and Certification Board (PECB), an international certification body. His role is to help companies and individuals to achieve professional, accredited and worldw...
SYS-CON Events announced today that Linux Academy, the foremost online Linux and cloud training platform and community, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Linux Academy was founded on the belief that providing high-quality, in-depth training should be available at an affordable price. Industry leaders in quality training, provided services, and student certification passes, its goal is to c...
"delaPlex is a software development company. We do team-based outsourcing development," explained Mark Rivers, COO and Co-founder of delaPlex Software, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
SYS-CON Events announced today that SoftLayer, an IBM Company, has been named “Gold Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York, New York. SoftLayer, an IBM Company, provides cloud infrastructure as a service from a growing number of data centers and network points of presence around the world. SoftLayer’s customers range from Web startups to global enterprises.
SYS-CON Events announced today that Technologic Systems Inc., an embedded systems solutions company, will exhibit at SYS-CON's @ThingsExpo, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Technologic Systems is an embedded systems company with headquarters in Fountain Hills, Arizona. They have been in business for 32 years, helping more than 8,000 OEM customers and building over a hundred COTS products that have never been discontinued. Technologic Systems’ pr...
SYS-CON Events announced today that CA Technologies has been named “Platinum Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY, and the 21st International Cloud Expo®, which will take place October 31-November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. CA Technologies helps customers succeed in a future where every business – from apparel to energy – is being rewritten by software. From ...
In his keynote at @ThingsExpo, Chris Matthieu, Director of IoT Engineering at Citrix and co-founder and CTO of Octoblu, focused on building an IoT platform and company. He provided a behind-the-scenes look at Octoblu’s platform, business, and pivots along the way (including the Citrix acquisition of Octoblu).
SYS-CON Events announced today that Loom Systems will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Founded in 2015, Loom Systems delivers an advanced AI solution to predict and prevent problems in the digital business. Loom stands alone in the industry as an AI analysis platform requiring no prior math knowledge from operators, leveraging the existing staff to succeed in the digital era. With offices in S...
SYS-CON Events announced today that HTBase will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. HTBase (Gartner 2016 Cool Vendor) delivers a Composable IT infrastructure solution architected for agility and increased efficiency. It turns compute, storage, and fabric into fluid pools of resources that are easily composed and re-composed to meet each application’s needs. With HTBase, companies can quickly prov...