CVE-2023-4504 - Heap Buffer Overflow in CUPS/libppd – Full Exploit Analysis & Remediation

CVE-2023-4504 is a recent security vulnerability impacting CUPS (Common UNIX Printing System) and the associated libppd library. This bug exposes users to remote code execution risks and denial of service, triggered by a logic error in validating the length of attacker-supplied data inside PPD PostScript files. Patched in September 2023 with the release of CUPS 2.4.7, it’s a poignant reminder: Always validate input length, especially from files users (or attackers) can upload.

In this long read, we’ll break down what happened, how attackers could exploit it, show you code snippets for a clearer grasp, and guide you towards fixes. All sources are linked for your further research.

Affected: CUPS < 2.4.7, libppd < 2..b1

- Fixed: CUPS 2.4.7 (Release Notes)

Severity: High (heap buffer overflow, possible code execution)

Short Summary:
CUPS and libppd failed to check length fields in the "PostScript Printer Description" (PPD) file format. An attacker could craft a malicious PPD file and upload it (perhaps as a new printer driver), causing memory corruption and, potentially, running code on the server.

CVE database entry:

CVE-2023-4504 at NVD

GitHub CUPS Advisory:

Github Security Advisory - GHSA-4p9w-v4c6-6m98
- Release notes / patch:
CUPS 2.4.7 Announcement

libppd fix:

libppd 2.b1 Changelog

Root Cause

PPD files describe printer capabilities in text. CUPS reads them during queue setup or updates, sometimes as root. The vulnerable code used an attacker-supplied length to copy a chunk of data into a heap buffer—without checking if the buffer was large enough.

Let’s see a simplified version similar to the vulnerable logic

// Hypothetical unsafe code, for explanation
int length = atoi(user_ppd_line + offset);
char *buf = malloc(256);
memcpy(buf, user_ppd_line + data_start, length); // user controls 'length'!

Here, an attacker crafts a line in the PPD file so length is larger than 256. memcpy will then write past the heap buffer, corrupting memory.

#### Excerpt from the actual fix (libppd commit):

// New safe version
if (length >=  && length < 256) {
    memcpy(buf, user_ppd_line + data_start, length);
}

Exploiting the Bug

Exploitation hinges on user (or attacker) ability to upload/cause CUPS to parse a malicious PPD. Common vectors:

Once the overflow is triggered, attackers might control certain pointers or overwrite adjacent memory. On some systems (where CUPS runs as root), this could be catastrophic—allowing privilege escalation or arbitrary code execution.

1. Craft Malicious PPD File

*PPD-Adobe: "4.3"
*%...
*LongCustomField: "AAAAAAAA....AAAA"    // 1024+ 'A's to overflow

3. Watch for Crash/Exploitation

On vulnerable installations (CUPS < 2.4.7), this should cause CUPS (or its child process) to crash, possibly opening the door for code execution depending on heap state, security hardening, etc.

Minimal PoC C (educational, not weaponized)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
    char *buf = malloc(256);
    char data[1024];
    memset(data, 'A', sizeof(data));
    // Emulate attacker-controlled size
    memcpy(buf, data, 1024); // OVERFLOW!
    printf("If you see this, buffer overflow may have occurred.\n");
    free(buf);
    return ;
}

In the real CUPS/libppd, the buffer/fill amount comes from the PPD file data.

Upgrade immediately

- To CUPS 2.4.7 or later (download).
- If you use libppd: upgrade to 2.b1+ (download).

Short-term workarounds

- Only allow trusted admins to upload/modify PPD/printer configurations
- Isolate CUPS (AppArmor/SELinux/containers) if possible

Final Thoughts

CVE-2023-4504 highlights how simple input validation problems can become devastating, especially when exposed through file formats and code that run as privileged users. Always validate user-supplied data and upgrade as soon as possible.

Timeline

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