Format String Vulnerability

Jijith Rajan
Reviewed by
Neda Ali
Published on
20 Oct 2023
7 min read

A format string vulnerability is a type of software bug in which user input is utilized as the format argument for functions like printf, scanf, or others within the same family.

These format arguments contain various specifiers that dictate how data should be formatted and printed.

When an attacker gains control over the format argument provided to print for similar functions, they can potentially exploit the vulnerability to access and leak data.

This is due to the nature of these functions, which are variadic and retrieve data from the stack based on the format specifiers.

What is format string vulnerability?

The Format string attack is an attack through which the user input is run as a command by the web application.

Through this vulnerability, the attacker might run commands to extract information about the server, view the source code of the application.

The attacker can crash the application by executing malicious code to create a segmentation fault and many more.

The string can be divided into 3 parts :

  1. Format function - This includes printf, fprintf etc.

  2. Format string - This is the argument for format function

  3. Format string parameter - This defines the type of conversion.

Format string vulnerability is possible because the application failed to sufficiently sanitize the user input. Applications that use format function improperly are most vulnerable to this attack.

The format function is used to interpret formatting characters. Common formatting characters include %x (used to read stack data), %s (read process memory) and more.

The attacker can perform this attack through the following ways:

  • Enumerate process stack: The attacker uses %x and %p to view the stack organization of the application. Using this method, the attacker can leak sensitive information about the server.

  • Control execution flow: The attacker uses %n to overwrite the pointer variables used by the application. When the application calls these pointers, the pointer will send malicious code to the application.

  • Denial of service: The attacker uses %x followed by %x to crash the application and the server.


The below code is an example:

printf (usrName);

Using the above command, the attacker can extract the usernames present in the server.

What is the impact of format string vulnerability?

Format string vulnerabilities can have various impacts on software systems, depending on how they are exploited and the context in which they occur.

Here are some potential impacts of format string vulnerabilities:

1. Information disclosure

Attackers can use format string vulnerabilities to leak sensitive information from memory, such as passwords, encryption keys, or other confidential data.

By carefully crafting the format string, they can read data from the stack or other parts of memory.

2. Arbitrary code execution

In some cases, format string vulnerabilities can lead to arbitrary code execution.

Attackers can manipulate the format string to overwrite function pointers or return addresses on the stack, allowing them to execute malicious code of their choice.

3. Denial of Service (DoS)

A poorly handled format string can cause a program to crash or enter an infinite loop, leading to a denial-of-service condition. This can disrupt the availability of the software or system.

4. Data corruption

Format string vulnerabilities can corrupt data structures and variables in memory, leading to unexpected behavior, crashes, or data integrity issues.

5. Privilege escalation

If an attacker can manipulate format strings in a privileged context (e.g., within a system service or as a superuser), they may be able to escalate their privileges, gaining unauthorized access to system resources.

6. Exploitation of other vulnerabilities

Format string vulnerabilities can be used as part of a multi-stage attack.

An attacker may leverage a format string vulnerability to gain initial access and then exploit other vulnerabilities to achieve more significant compromises.

7. Stealthy attacks

Format string attacks can be challenging to detect because they often leave little or no trace in logs.

Attackers can use them for covert information gathering or persistence within a compromised system.

8. System compromise

In the worst-case scenario, a successful format string attack can lead to a complete compromise of the target system, allowing the attacker to take full control.

To mitigate format string vulnerabilities, developers should validate and sanitize user input, use safer alternatives like snprintf instead of vulnerable functions like printf, and implement strong input validation and access controls in their code.

Additionally, security testing and code reviews can help identify and address these vulnerabilities before they are exploited.

How can you prevent format string vulnerability?

Preventing and mitigating format string vulnerabilities requires a combination of secure coding practices, input validation, and system hardening.

Here are some steps to help prevent and mitigate format string vulnerabilities:

1. Use safe I/O functions

Instead of using vulnerable functions like printf and sprintf, use their safer counterparts like snprintf, which allows you to specify a maximum buffer size to prevent buffer overflows. Example:

  • cCopy code

  • snprintf(buffer, sizeof(buffer), "Format this safely: %s", user_input);

2. Sanitize user input

Validate and sanitize user input before it is used as a format string or any other part of a command. Ensure that user-provided data does not contain format specifiers (“%s”, “%n”, etc.).

3. Limit privileges

Run your software with the least privilege necessary. Avoid running code with elevated privileges whenever possible.

This reduces the potential impact of a successful format string attack.

4. Input validation

Implement strong input validation for all user-provided data. Reject input that doesn’t conform to expected patterns or contains unexpected characters.

5. Static analysis and code review

Use static code analysis tools and conduct code reviews to identify potential format string vulnerabilities during the development process.

6. Enable compiler warnings

Most modern compilers offer warnings for format string issues. Enable these warnings and treat them as errors in your build process.

7. Address warning flags

Make sure to address any compiler warnings related to format string vulnerabilities. These warnings often indicate potential issues that should be resolved.

8. Logging best practices

If you need to log user-generated data, avoid using user input directly in log messages. Instead, consider logging only sanitized or contextual information.

9. Use security libraries

Consider using security-focused libraries or functions designed to handle user input safely. For example, in web applications, use security libraries like OWASP’s ESAPI or input validation frameworks.

10. Runtime protections

Some runtime protection tools and techniques can detect and prevent format string attacks. These include Address Space Layout Randomization (ASLR) and stack canaries.

By following these best practices and maintaining a security-conscious mindset throughout the software development lifecycle, you can significantly reduce the risk of format string vulnerabilities in your applications.

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Written by
Jijith Rajan
Jijith Rajan
Cyber Security Engineer
Neda Ali
Neda Ali
Product Marketing Specialist
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