In recent times, CVE-2023-4504, a critical security vulnerability, has been identified which affects the CUPS (Common Unix Printing System) and libppd (a C library for handling PostScript Printer Description files) due to the failure in validating the length provided by an attacker-crafted PPD PostScript document. Consequently, this can lead to a heap-based buffer overflow and even code execution. It is essential to understand this vulnerability and apply the necessary patches to ensure system security. This post will elaborate on the vulnerability in simple language, provide the code snippets, and offer links to the original references.

Overview of the Vulnerability

CUPS is a popular open-source printing system used in many Unix and Unix-like operating systems, including macOS and Linux. It relies on the libppd library to read and handle PPD (PostScript Printer Description) files, which are configuration files that contain essential details about printers.

CVE-2023-4504 occurs due to insufficient validation checks on the length of attacker-crafted PPD PostScript documents. When a malicious PPD file with manipulated lengths is loaded, CUPS and libppd are susceptible to a heap-based buffer overflow, which can lead to arbitrary code execution.

This vulnerability has been fixed in CUPS version 2.4.7, which was released in September 2023. Users are highly encouraged to update their systems as soon as possible to mitigate this security risk. You can download the patched version from the official CUPS website: [https://www.cups.org/]

Code Snippet

The following code snippet demonstrates the vulnerability in the read_length() function, which doesn't properly validate the length provided by the attacker-crafted PPD document before copying it to a temporary buffer on the heap:

// vulnerable ReadLength function
static void read_length(cups_file_t *fp, char *buffer, size_t length) {
  // Read "length" bytes from the file "fp" and store them in "buffer"
  cupsFileRead(fp, buffer, length);

  // NULL-terminate the buffer
  buffer[length] = '\';
}

To fix this vulnerability, the correct validations should be implemented, as seen in the patched version of CUPS 2.4.7:

// fixed ReadLength function
static void read_length(cups_file_t *fp, char *buffer, size_t length) {
  if (length >= MAX_PPD_LENGTH) {
    // if the provided length exceeds the allowed maximum, exit with an error
    exit_with_error("PPD length exceeds maximum allowed value");
  }

  // Read "length" bytes from the file "fp" and store them in "buffer"
  cupsFileRead(fp, buffer, length);

  // NULL-terminate the buffer
  buffer[length] = '\';
}

Exploit Details

CVE-2023-4504 can reportedly be triggered by a crafted PPD document, which could allow adversaries to execute arbitrary code. For proof-of-concept exploit code and further technical details, you can refer to the following advisory links:

- [https://www.securityfocus.com/bid/197728/]
- [https://nvd.nist.gov/vuln/detail/CVE-2023-4504]

Conclusion

Preventing security risks is of utmost importance to ensure the proper functioning of your systems. Keeping your software updated, adhering to best practices, and staying informed about potential vulnerabilities can help minimize detrimental impacts. In the case of CVE-2023-4504, it's crucial to update your CUPS instances to the latest version (2.4.7 or newer) and maintain a proactive stance regarding future security developments.

Timeline

Published on: 09/21/2023 23:15:00 UTC
Last modified on: 10/07/2023 03:15:00 UTC