CWE-560: Use of umask() with chmod-style Argument

What is Use of umask() with chmod-style Argument?

• Overview: The vulnerability occurs when the umask() function is called with an argument intended for chmod(), leading to incorrect file permission settings.

• Exploitation Methods:

Attackers can exploit this vulnerability by accessing files with inadequate permissions, potentially leading to unauthorized access or modification.
Common attack patterns include gaining access to sensitive files that should have been protected by stricter permissions.

• Security Impact:

Direct consequences include unintended access to files, allowing unauthorized read, write, or execute operations.
Potential cascading effects include data leaks, file corruption, or privilege escalation.
Business impact might involve data breaches, loss of customer trust, and legal ramifications due to non-compliance with data protection regulations.

• Prevention Guidelines:

Specific code-level fixes include ensuring umask() is called with the correct argument format, distinct from chmod().
Security best practices involve conducting thorough code reviews and using static analysis tools to identify incorrect umask() usage.
Recommended tools and frameworks include linters or static code analysis tools that can detect improper use of file permission functions.

Corgea can automatically detect and fix Use of umask() with chmod-style Argument in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C

Affected Technologies: Not specified

Vulnerable Code Example

#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>

int main() {
    // Incorrectly using umask() with a chmod-style argument
    // This sets the umask to 0644, which is incorrect because umask should
    // specify permissions to be masked (disabled), not granted.
    umask(0644);

    printf("Umask incorrectly set to 0644.\n");
    return 0;
}

How to fix Use of umask() with chmod-style Argument?

To fix the use of umask() with a chmod-style argument, remember that umask() specifies which permission bits should be blocked, not which should be allowed. In contrast, chmod() specifies the permissions to be enabled. For example, to ensure that new files have at least rw-r--r-- permissions by default, use umask(0022). Here's the breakdown:

0 - Do not mask any of the owner's permissions (read, write, execute).
0 - Do not mask any of the group's read permissions.
2 - Mask the group's write permission.
2 - Mask the other users' write permission.

Fixed Code Example

#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>

int main() {
    // Correctly using umask() with a mask that specifies what permissions to block
    // This sets the umask to 0022, allowing files to be created with 755 permissions
    // by default, which translates to rw-r--r-- for new files.
    umask(0022);

    printf("Umask correctly set to 0022.\n");
    return 0;
}

In this fixed example, umask(0022) correctly sets the mask to disallow group and others from writing to new files, while allowing read and execute permissions. This results in safer file creation behavior in most UNIX-like systems, ensuring that files are not inadvertently created with overly permissive access rights.