CVE-2026-7835

Executive Summary

CVE-2026-7835 is a format string argument mismatch vulnerability in Netatalk, an open-source implementation of the Apple Filing Protocol (AFP). The flaw affects versions 3.0.3 through 4.4.2 and allows a remote authenticated attacker to cause a minor denial of service (DoS) by sending crafted input that triggers incorrect format string processing. The vulnerability has a low CVSS score of 3.1, reflecting its limited impact (no data confidentiality or integrity loss) and the requirement for authentication. No public exploit code or proof-of-concept is currently available, and the vendor has not released a patch. The EPSS score is 0.0%, indicating a very low probability of exploitation in the wild.

Supply Chain & Package Security

Netatalk is not distributed via NPM, PyPI, Go, or Ruby registries. It is typically installed via Linux package managers (e.g., apt, yum) or compiled from source. Organizations using Netatalk should monitor official distribution channels for patch updates.

CVSS Breakdown & Threat Level

• CVSS Score: 3.1 (Low)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
• Attack Vector (AV): Network – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): Low – The attacker must have valid authentication credentials.
• User Interaction (UI): None – No user action is needed.
• Scope (S): Unchanged – The exploit does not affect other components.
• Confidentiality (C): None – No data exposure.
• Integrity (I): None – No data modification.
• Availability (A): Low – The vulnerability can cause a minor denial of service.

Threat Level: Low. The requirement for authentication and the limited impact (minor DoS) reduce the risk. However, in environments where Netatalk is exposed to untrusted users, the vulnerability could be used to disrupt file-sharing services.

Deep Dive & Mechanics

The vulnerability resides in the format string handling logic of Netatalk. When processing certain AFP requests, the software uses user-supplied input as a format string argument without proper validation. This leads to a format string argument mismatch, where the number or type of arguments passed to a format function (e.g., printf, syslog) does not match the format specifiers.

In a typical format string vulnerability, an attacker can read or write arbitrary memory. However, in this case, the mismatch only causes a minor denial of service, likely due to the specific code path and the way arguments are consumed. The crafted input triggers an incorrect format string processing that may cause the application to crash or hang, but does not allow memory corruption or code execution.

The vulnerable code is likely in the AFP request parsing or logging functions. For example, a function might call syslog(LOG_ERR, user_input) instead of syslog(LOG_ERR, "%s", user_input), where user_input contains format specifiers like %s or %n. The mismatch could cause the program to read from invalid memory locations, leading to a crash.

Vulnerable vs. Patched Code

Since no patch is available, the following is a hypothetical example of the vulnerable and patched code.

Vulnerable Code (Netatalk 3.0.3 - 4.4.2):

void log_error(const char *user_input) {
    // Vulnerable: user_input is used directly as format string
    syslog(LOG_ERR, user_input);
}

Patched Code (Hypothetical):

void log_error(const char *user_input) {
    // Patched: user_input is passed as an argument, not as format string
    syslog(LOG_ERR, "%s", user_input);
}

The fix ensures that user-supplied input is treated as a plain string, not as a format string, preventing any format specifier injection.

Exploitation & Lab Setup

Red Team

No public exploit code is available. To simulate the vulnerability in a lab environment:

1. Setup: Install Netatalk 4.4.2 on a Linux VM (e.g., Ubuntu 22.04). Configure AFP shares and create a test user.
2. Craft Payload: Use a tool like netatalk-client or a custom Python script to send AFP requests with format specifiers in fields like volume names or usernames.
3. Trigger: Authenticate and send a crafted request. For example, send a login request with a username containing %s%s%s%s%s%s%s%s%s%s to cause a crash.
4. Observe: Monitor the Netatalk process (e.g., afpd). If the process crashes or becomes unresponsive, the vulnerability is confirmed.

Example Python script (conceptual):

import socket

def trigger_dos(host, port, username, password):
    # Simplified AFP login with format string payload
    payload = b"\x00\x03\x00\x00\x00\x00\x00\x01"  # AFP header
    payload += b"\x00\x00\x00\x00"  # Command
    payload += username.encode() + b"\x00"
    payload += password.encode() + b"\x00"
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((host, port))
    s.send(payload)
    s.close()

trigger_dos("192.168.1.100", 548, "%s%s%s%s%s%s%s%s%s%s", "password")

Blue Team

• Detection: Monitor for crashes of the afpd process. Use process monitoring tools (e.g., systemd, monit) to detect unexpected terminations.
• Log Analysis: Check system logs for format string-related errors or unusual AFP request patterns.
• Network Monitoring: Look for AFP traffic containing repeated %s or other format specifiers in fields like usernames or volume names.
• Honeypot: Deploy a Netatalk honeypot to capture and analyze potential exploit attempts.

Indicators of Compromise (IOCs)

• Network IOCs:
  • AFP packets (port 548/tcp) containing format specifiers (e.g., %s, %x, %n) in username, password, or volume name fields.
  • Repeated AFP login attempts with unusual strings.
• Host IOCs:
  • Crashes of the afpd process.
  • Core dump files in /var/crash/ or /tmp/.
  • System log entries like afpd[PID]: format string error or afpd[PID]: segmentation fault.
• File IOCs:
  • Modified Netatalk configuration files (e.g., /etc/netatalk/afp.conf) containing format specifiers.

Detection Engineering

YARA Rule

rule CVE_2026_7835_Netatalk_FormatString {
    meta:
        description = "Detects format string patterns in Netatalk AFP traffic or logs"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-7835"
    strings:
        $fmt1 = /%[sdxn]/ 
        $fmt2 = /%[0-9]*[sdxn]/
    condition:
        any of them
}

Sigma Rule

title: Netatalk Format String DoS Attempt
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects AFP requests containing format specifiers indicative of CVE-2026-7835 exploitation attempts
author: Valters IT
date: 2025-04-10
logsource:
    category: network
    product: netatalk
detection:
    selection:
        - payload|contains: '%s'
        - payload|contains: '%x'
        - payload|contains: '%n'
    condition: selection
falsepositives:
    - Legitimate use of '%' in usernames (rare)
level: medium

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 548 (msg:"CVE-2026-7835 Netatalk Format String DoS Attempt"; flow:to_server,established; content:"|00|"; depth:1; content:"%s"; within:100; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Restrict network access to Netatalk services (port 548/tcp) using firewalls. Only allow trusted IPs.
• Short-term: Disable Netatalk if not required. If necessary, use application-level filtering to block requests containing format specifiers.
• Long-term: Monitor the Netatalk project for a patch. Once available, upgrade to the patched version immediately.
• Workaround: Modify the Netatalk source code to sanitize user input before passing it to format functions (e.g., use %s as the format string and pass user input as an argument). Recompile and deploy.

Valters IT Signature:
This report was generated by Valters IT DevSecOps AI. All information is provided for authorized security testing and defense purposes only.