CWE-689: Permission Race Condition During Resource Copy

What is Permission Race Condition During Resource Copy?

• Overview: This vulnerability occurs when a program copies or clones a resource and delays setting permissions or access control until the copy is complete, temporarily exposing the resource to unauthorized access.

• Exploitation Methods:

• Attackers can exploit this vulnerability by accessing or altering the resource during the window when it is exposed.
• Common attack patterns include race conditions where attackers execute operations on the resource before the proper permissions are applied.

• Security Impact:

• Direct consequences include unauthorized access or modification of sensitive data.
• Potential cascading effects can involve privilege escalation or data corruption.
• Business impact may result in data breaches, loss of customer trust, and potential legal liabilities.

• Prevention Guidelines:

• Specific code-level fixes include setting permissions on the destination resource before initiating the copy process.
• Security best practices involve ensuring atomic operations where possible to eliminate race conditions.
• Recommended tools and frameworks include static analysis tools to detect race conditions and using programming languages or libraries that provide safer resource handling mechanisms.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, Perl

Affected Technologies: Not specified

Vulnerable Code Example

// This C code demonstrates a permission race condition vulnerability during file copy.
// The destination file is created with default permissions, which could allow unauthorized access
// during the copy process. A malicious user might read or modify the file before the copy is complete.

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

void copy_file(const char *src, const char *dest) {
    int src_fd, dest_fd;
    char buffer[1024];
    ssize_t bytes_read;

    // Open the source file for reading
    src_fd = open(src, O_RDONLY);
    if (src_fd < 0) {
        perror("open source");
        exit(EXIT_FAILURE);
    }

    // Create the destination file without setting restrictive permissions first
    dest_fd = open(dest, O_WRONLY | O_CREAT | O_TRUNC, 0644);  // Potential race condition
    if (dest_fd < 0) {
        perror("open destination");
        close(src_fd);
        exit(EXIT_FAILURE);
    }

    // Copy data from source to destination
    while ((bytes_read = read(src_fd, buffer, sizeof(buffer))) > 0) {
        if (write(dest_fd, buffer, bytes_read) != bytes_read) {
            perror("write");
            close(src_fd);
            close(dest_fd);
            exit(EXIT_FAILURE);
        }
    }

    // Close file descriptors
    close(src_fd);
    close(dest_fd);
}

How to fix Permission Race Condition During Resource Copy?

To fix the permission race condition, ensure that the file is created with restrictive permissions from the start. This can be achieved by temporarily setting the umask to a more restrictive mode or by explicitly specifying restrictive permissions when creating the file. This change prevents unauthorized access to the file during the copy operation.

Fixed Code Example

// This fixed C code eliminates the permission race condition by ensuring the destination file is created
// with restrictive permissions from the start. The umask is temporarily set to restrict permissions,
// ensuring only the owner can read and write to the file during the copy operation.

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>

void copy_file(const char *src, const char *dest) {
    int src_fd, dest_fd;
    char buffer[1024];
    ssize_t bytes_read;
    mode_t original_umask;

    // Open the source file for reading
    src_fd = open(src, O_RDONLY);
    if (src_fd < 0) {
        perror("open source");
        exit(EXIT_FAILURE);
    }

    // Set umask to restrict permissions temporarily
    original_umask = umask(0077);

    // Create the destination file with restrictive permissions
    dest_fd = open(dest, O_WRONLY | O_CREAT | O_TRUNC, 0600);
    if (dest_fd < 0) {
        perror("open destination");
        close(src_fd);
        exit(EXIT_FAILURE);
    }

    // Restore original umask
    umask(original_umask);

    // Copy data from source to destination
    while ((bytes_read = read(src_fd, buffer, sizeof(buffer))) > 0) {
        if (write(dest_fd, buffer, bytes_read) != bytes_read) {
            perror("write");
            close(src_fd);
            close(dest_fd);
            exit(EXIT_FAILURE);
        }
    }

    // Close file descriptors
    close(src_fd);
    close(dest_fd);
}

In the fixed code, the umask is temporarily set to 0077 before creating the destination file, ensuring it is created with permissions that only allow the owner to read and write. This prevents unauthorized access during the copy operation. The original umask is restored immediately after the file creation.