An insecure temporary file is a type of vulnerability that occurs when an application creates temporary files that can be accessed or modified by other users or processes on the same system. This can occur if the temporary file is created with overly relaxed file permissions or if the file is stored in a predictable location that can be easily accessed by an attacker.

Attackers can take advantage of this vulnerability by modifying the contents of the temporary file to bypass security controls or carry out other attacks on the system. For example, an attacker could modify a temporary file used by a web application to execute arbitrary code or steal sensitive information.

In this lesson, you will learn how insecure temporary file vulnerabilities work and how to protect your applications against them. We will begin by exploiting a vulnerability in a simple application. Then we will analyze the vulnerable code and explore some options for remediation and prevention.

So, what exactly is going on here?

In the example above, the application creates a temporary file named sensitive_data.csv in the /tmp directory and writes sensitive user data to it. However, the file is created with default permissions that allow anyone with access to the system to read or modify its contents.

Even though the file would only be accessible for a very short period of time, Alex was able to create a simple script to constantly monitor the location of that file and grab the contents as soon as it was created.

For reference, here's the vulnerable code snippet again, with comments:

The file is created, encrypted, sent to an S3 bucket, and then deleted, but at the time that the file contents are being encrypted and uploaded, the plaintext file is sitting in /tmp, allowing any users of the system to read the file.

The impact of this type of vulnerability varies considerably depending on the sensitivity of the file that can be accessed, and whether it can just be read or also written to. Some potential impacts include

Unauthorized access to sensitive data

Insecure temporary file vulnerabilities can allow attackers to read, modify, or delete sensitive data that is stored in temporary files. This can include personally identifiable information, financial data, or other confidential information that could be used for identity theft or other malicious purposes.

Malware installation

Attackers can use insecure temporary file vulnerabilities to install malware or other malicious code onto the system. For example, they can modify a temporary file used by a web application to execute arbitrary code or download a payload from a remote server.

System compromise

In some cases, attackers can use insecure temporary file vulnerabilities to gain access to the underlying operating system or other sensitive resources on the system. This can allow them to take control of the system, steal additional data, or carry out other attacks.

Below are a few tips to help prevent insecure temporary file vulnerabilities in your applications.

There is rarely a case where writing or reading from a local file is necessary, especially when coding in NodeJS, which is typically used to develop web applications. Temporary data storage can usually be achieved by simply holding data in a variable, and data persistence can be achieved by storing data in a database.

If you really need to write to a file, you should set the file permissions to be as restrictive as possible to avoid other users snooping or editing the contents.

The following example writes a file with 600 (chmod 600) permissions, meaning that only the owner of the file can read and write to it.

To learn more about insecure temporary file vulnerabilities, check out some other great content:

• The CWE listing can be found here
• Learn more about this vulnerability from OWASP