CVE-2026-7836

Executive Summary

CVE-2026-7836 is a low-severity vulnerability in the Netatalk open-source implementation of the Apple Filing Protocol (AFP). The flaw resides in the hextoint macro, which is used to convert hexadecimal characters to integers. Due to improper handling of uppercase hexadecimal characters (A-F), an incorrect calculation occurs, allowing a remote authenticated attacker to cause limited data modification via crafted hexadecimal input. The vulnerability affects Netatalk versions 2.0.0 through 4.4.2. No public exploit code or patch is currently available. The EPSS score is 0.0%, indicating a very low probability of exploitation in the wild.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
• Base Score: 3.1 (Low)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – attacker must be authenticated to the AFP service.
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: Low (I:L) – limited data modification possible.
• Availability Impact: None (A:N)

Threat Level: Low. The vulnerability requires authentication and only allows limited data modification. However, in environments where AFP is exposed to untrusted users, it could be used to corrupt specific data structures.

Deep Dive & Mechanics

The hextoint macro in Netatalk is defined to convert a single hexadecimal character (0-9, A-F, a-f) to its integer value. The macro likely uses a simple subtraction or lookup table. The bug is in the handling of uppercase characters (A-F). Instead of correctly mapping 'A' to 10, 'B' to 11, etc., the macro performs an incorrect calculation, possibly due to a missing offset or incorrect arithmetic.

For example, a typical implementation might be:

#define hextoint(c) ((c) >= '0' && (c) <= '9' ? (c) - '0' : (c) >= 'A' && (c) <= 'F' ? (c) - 'A' + 10 : (c) >= 'a' && (c) <= 'f' ? (c) - 'a' + 10 : 0)

If the uppercase branch is flawed (e.g., missing + 10), then 'A' would be converted to 0 instead of 10, leading to incorrect parsing of hexadecimal strings.

This macro is used in various places within Netatalk, such as parsing AFP options, volume configurations, or authentication tokens. An authenticated attacker could send crafted hexadecimal input that, when processed by the flawed macro, results in unintended data being written or modified. The impact is limited because the attacker must be authenticated and the modification is constrained to the context where the macro is used.

Vulnerable vs. Patched Code

Vulnerable Code (hypothetical):

#define hextoint(c) ((c) >= '0' && (c) <= '9' ? (c) - '0' : (c) >= 'A' && (c) <= 'F' ? (c) - 'A' : (c) >= 'a' && (c) <= 'f' ? (c) - 'a' + 10 : 0)

In this flawed version, uppercase letters are converted to 0-5 instead of 10-15.

Patched Code (hypothetical):

#define hextoint(c) ((c) >= '0' && (c) <= '9' ? (c) - '0' : (c) >= 'A' && (c) <= 'F' ? (c) - 'A' + 10 : (c) >= 'a' && (c) <= 'f' ? (c) - 'a' + 10 : 0)

The patched version correctly adds 10 for uppercase letters.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to a Netatalk server (version 2.0.0 to 4.4.2) with AFP enabled.
• Valid credentials for an AFP user.

Exploitation Steps:

1. Identify the AFP service (port 548/tcp) and authenticate.
2. Send a crafted AFP request that includes a hexadecimal string with uppercase letters (e.g., "FF").
3. The flawed hextoint macro will incorrectly parse the uppercase characters, potentially causing the server to misinterpret the data.
4. This could lead to limited data modification, such as altering volume settings or file metadata.

Example (conceptual):

# Using a tool like afpfs-ng or custom script
afp_client -s server -u user -p pass -c "set_volume_option:name=Test,hex=FF"

The hex=FF would be parsed incorrectly, potentially setting a different value.

Blue Team

Detection:

• Monitor AFP logs for unusual authentication attempts or malformed requests.
• Look for patterns of hexadecimal strings that are all uppercase (e.g., "ABCDEF") in AFP commands.
• Use network monitoring to detect anomalous AFP traffic.

Hunting:

• Search for Netatalk servers running versions 2.0.0 through 4.4.2.
• Check for any unexpected changes in AFP volume configurations or file metadata.

Indicators of Compromise (IOCs)

• Network: AFP traffic (port 548/tcp) containing hexadecimal strings with uppercase letters (e.g., "0xABCDEF").
• Logs: Netatalk logs showing errors related to hex parsing or unexpected configuration changes.
• File System: Unexplained modifications to AFP volume settings or file metadata.

Detection Engineering

YARA Rule

rule CVE_2026_7836_Netatalk_hextoint {
    meta:
        description = "Detects potential exploitation of CVE-2026-7836 in Netatalk"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-7836"
    strings:
        $hex_upper = /[A-F]{2,}/  // Two or more uppercase hex characters
        $afp_cmd = "AFP" nocase
    condition:
        $afp_cmd and #hex_upper > 5  // More than 5 uppercase hex groups
}

Sigma Rule

title: CVE-2026-7836 Netatalk hextoint Exploitation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects AFP traffic with excessive uppercase hexadecimal characters, indicating potential exploitation of CVE-2026-7836.
author: Valters IT
date: 2025-04-10
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        dest_port: 548
        payload|contains: 
            - '0x'
            - '0X'
    condition: selection
falsepositives:
    - Legitimate AFP clients using uppercase hex
level: low

Suricata Rule

alert tcp any any -> any 548 (msg:"CVE-2026-7836 Netatalk hextoint Exploitation Attempt"; content:"|41 42 43 44 45 46|"; distance:0; within:6; sid:1000001; rev:1;)

Note: This rule looks for the hex bytes of "ABCDEF" in AFP traffic.

Remediation & Mitigation

Immediate Actions:

1. Restrict Access: Limit AFP access to trusted users only. Use firewall rules to restrict port 548/tcp.
2. Monitor: Enable detailed logging on Netatalk servers and monitor for anomalous activity.
3. Workaround: If possible, disable AFP and use alternative file sharing protocols (e.g., SMB, NFS).

Long-term Fix:

• Apply the official patch once released by the Netatalk project.
• Upgrade to a patched version when available.
• Consider using a different AFP implementation if Netatalk remains unpatched.

Valters IT Signature This report was generated by Valters IT Cyber Threat Intelligence Division. For questions, contact threatintel@valtersit.com.