Snprintf vs Sprintf: A Deep Dive into Buffer Overflows Prevention
Tags: webdev, programming, tutorial
Published: 2023-05-31 10:56:13

C features are programming functions that make it easy to organize and structure code. They enable modular programming, wherein code can be reused to make it easy to start and complete projects. Programmers can also separate complex tasks into smaller code units, which are easier to manage and are only executed when needed.

These functions, however, can pose issues and security risks. The sprintf() function, in particular, [can result in a buffer overflow vulnerability](https://www.w3resource.com/c-programming/stdio/c_library_method_sprintf.php). Many C programmers like using this function because of its simplicity and familiarity. Its syntax is rather straightforward, which makes it preferable when it comes to simple string formatting tasks. It is associated with an issue that is hard to ignore, though.

### Snprintf vs sprintf

Sprintf() makes it possible to write formatted data into a string buffer. It is designed to accept a format string as its initial argument, prepending other arguments that specify what to write into the formatted string. As such, it is usually employed in formatting strings that entail the merger of texts with numbers, variables, and other arguments. Sprintf() is similar to printf (), except that it writes the data into a string instead of having it printed.

Sprintf() is usually used in string composition, custom output formatting, variable substitution, and string building. It provides a simple and flexible way to compose strings and control output formatting. However, as indicated earlier, it can bring about buffer overflows, which are a security risk.

This is where snprintf() comes in. It helps secure the formatting of strings by putting a cap on the maximum number of characters allowed on the buffer. It is essentially a secure alternative to the sprintf() function.

Understanding the differences between [snprintf vs sprintf and their use cases](https://sternumiot.com/iot-blog/sprintf-and-snprintf-c-functions-usage-examples-and-security-best-practices/) are two vital points every C programmer should know. It would be inexpedient to choose one simply because it is easier to use or because it is more familiar.

The snprintf() function is inherently intolerant of buffer overflow. It also comes with its error detection mechanism, which promptly detects possible truncation. Snprintf() routinely compares the return value with the actual buffer size to detect truncation errors. This function is generally the secure option.

However, it may be unnecessary to use snprintf() if the data sizes are already known and controlled. Also, if there are already existing controls on input data size and formatting, it would be acceptable to stick to the simple and familiar function.

### Why buffer overflow is a major concern

Buffer overflow sounds like a benign term, but it is [among the biggest vulnerabilities](https://www.sciencedirect.com/topics/computer-science/buffer-overflow). At some point, nearly a fifth of all security vulnerabilities reported to the Computer Emergency Response Team (CERT) were buffer overflows. 

Also known as buffer overruns, buffer overflows take place when programs write data beyond the capacity or allocation of a buffer or array. In C programming, buffers are contiguous or connected blocks of memory with definite memory allocations. They can only accommodate a certain amount of data. Thus, if a buffer that only has an allocation of 50 characters receives data worth 500 characters, only 50 characters will be written on the target buffer. The rest will overflow into other areas of memory.

The Open Worldwide Application Security Project (OWASP) includes injection in its [Top Ten list of most common web application security risks](https://owasp.org/www-project-top-ten/). The exploitation of buffer overflow vulnerabilities falls under the injection category, wherein threat actors can introduce malicious commands to programs that lack buffer overflow controls.

There is no automatic rejection for excessive data written to a buffer. The restriction on the maximum amount of data that can be taken in has to be specified. The absence of a cap provides opportunities for threat actors to write data on other memory spaces, creating security compromises that may not be detected by most security controls.

The outcomes of a buffer overflow attack are usually unpredictable. No attacker can be certain as to what happens when they exploit the buffer overflow vulnerabilities they discover. An app or program may crash. It can also become dysfunctional or exhibit unexpected behavior. Worse, it may facilitate the execution of malicious code. Cybercriminals observe what happens to their attacks and tweak them until they achieve the outcomes they prefer.

### Preventing buffer overflows

To avoid buffer overflow security weaknesses, it is advisable to use the snprintf() function in cases when either snprintf() or sprintf() are usable. While the latter is easier, the former is more secure. There is no dilemma here—security is non-negotiable. No amount of ease or convenience can justify a security compromise.

Take note, though, that simply using snprintf() does not automatically mean that a program becomes secure. It is important to examine the buffer size to make sure that the allocated buffer size is enough for the expected inputs and that a limit on the maximum size is enforced. Additionally, the return value should be checked to ascertain that it does not exceed the buffer size and prevent instances of data truncation.

However, there are instances when programmers have no other choice but to stick with sprintf(). Most legacy devices do not support the snprintf() function. Organizations may not be prepared to retire and replace these devices, so it makes sense to use sprintf(). In such cases, it is advisable to use other security measures.

When securing low-resource IoT and embedded devices, for example, it helps to use deterministic security solutions. There are security tools capable of deterministically stopping memory and code manipulation. They can provide effective protection from injection attacks, including those that exploit buffer overflow vulnerabilities attributable to the use of sprintf().


### Key takeaways

Buffer overflows pose serious security risks, and they can cause unthinkable damage. As such, it is important to be keen on the proper string formatting functions to use. These functions have their respective advantages or benefits, but they can also be the cause of serious security weaknesses. The snprintf() vs sprintf() faceoff logically ends up with snprintf() becoming the preferred option. However, it is still possible to use sprintf() securely with the help of other security controls or solutions and by observing secure coding best practices. Understandably, there are situations when using the less secure function is inevitable, but they are not an excuse not to find ways to ascertain security.