CVE-2023-42119

Executive Summary

CVE-2023-42119 is an out-of-bounds read vulnerability in Exim’s dnsdb lookup functionality, accessible via the SMTP service on TCP port 25. The flaw allows a network-adjacent attacker to trigger a read past the end of an allocated buffer, potentially leaking sensitive memory contents. While the CVSS score is low (3.1) due to the high complexity of exploitation and limited direct impact, the vulnerability can be chained with other bugs to achieve remote code execution. No public proof-of-concept or patch is currently available, and the EPSS score is 0.0%, indicating very low exploitation activity in the wild.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
• Base Score: 3.1 (Low)
• Attack Vector: Adjacent Network (AV:A) – attacker must be on the same network segment.
• Attack Complexity: Low (AC:L) – no special conditions required.
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: Low (C:L) – limited information disclosure.
• Integrity Impact: None (I:N)
• Availability Impact: None (A:N)

Threat Level: Low. The vulnerability requires network adjacency and offers only limited information disclosure. However, when combined with other vulnerabilities, it could enable arbitrary code execution.

Deep Dive & Mechanics

The vulnerability resides in Exim’s dnsdb lookup, which is used to perform DNS-based database queries. The SMTP service processes user-supplied data without proper bounds checking, leading to an out-of-bounds read. Specifically, when handling a malformed DNS response or crafted input, the code reads beyond the allocated buffer, potentially leaking heap or stack data.

Root Cause: Lack of validation of the length of data returned from a DNS query. The code assumes the response fits within the buffer, but an attacker can force a shorter or malformed response, causing the read to exceed the buffer boundary.

Attack Scenario: An attacker on the same network sends a crafted SMTP command that triggers a dnsdb lookup. The malicious DNS response causes the out-of-bounds read, leaking memory contents. This information can then be used to bypass ASLR or leak credentials, facilitating further exploitation.

Vulnerable vs. Patched Code

Since no official patch is available, the vulnerable code pattern is described generically.

Vulnerable Code (Conceptual):

// dnsdb lookup function
char buffer[256];
int len = dns_lookup(query, buffer, sizeof(buffer));
// No check that len <= sizeof(buffer)
process_data(buffer, len);

Patched Code (Hypothetical):

char buffer[256];
int len = dns_lookup(query, buffer, sizeof(buffer));
if (len < 0 || len > sizeof(buffer)) {
    // Handle error
    return ERROR;
}
process_data(buffer, len);

The fix would involve validating the length returned by the DNS lookup before using the buffer.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Network adjacency to the target Exim server.
• Ability to send crafted SMTP commands and DNS responses.

Lab Setup:

1. Set up a local Exim server (vulnerable version) in a controlled environment.
2. Use a tool like scapy or a custom Python script to send a malicious DNS response.

Exploitation Steps:

1. Connect to the Exim SMTP port (25).
2. Send a crafted SMTP command that triggers a dnsdb lookup (e.g., dnslookup).
3. The Exim server performs a DNS query; respond with a malformed DNS packet that has a shorter data length than expected.
4. The out-of-bounds read leaks memory; capture the response.

Example Python Snippet (Conceptual):

import socket

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("target", 25))
s.send(b"EHLO attacker\r\n")
s.recv(1024)
# Trigger dnsdb lookup
s.send(b"dnslookup example.com\r\n")
# Malicious DNS response would be sent via a separate DNS server

Blue Team

Detection:

• Monitor for unusual SMTP commands, especially dnslookup or similar.
• Look for DNS responses with inconsistent lengths.
• Use network intrusion detection systems to flag out-of-bounds read patterns.

Hunting:

• Search logs for SMTP connections from unexpected network segments.
• Analyze DNS traffic for responses that are smaller than expected.

Indicators of Compromise (IOCs)

• Network:
  • SMTP connections from adjacent network segments.
  • DNS responses with data length less than the query’s expected size.
• Host:
  • Exim logs showing dnsdb lookup failures or errors.
  • Memory dumps containing leaked data from the Exim process.
• File:
  • No specific file-based IOCs.

Detection Engineering

YARA Rule

rule CVE_2023_42119_Exim_dnsdb_OOB_Read {
    meta:
        description = "Detects potential exploitation of CVE-2023-42119 in Exim dnsdb"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-42119"
    strings:
        $smtp_cmd = "dnslookup" ascii wide nocase
        $dns_error = "DNS lookup failed" ascii wide nocase
    condition:
        $smtp_cmd or $dns_error
}

Sigma Rule

title: Exim dnsdb Out-of-Bounds Read Attempt
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects SMTP commands that may trigger CVE-2023-42119
author: Valters IT
logsource:
    category: application
    product: exim
detection:
    selection:
        EventID: 1
        CommandLine|contains: 'dnslookup'
    condition: selection
falsepositives:
    - Legitimate DNS lookups
level: medium

Suricata Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET 25 (msg:"CVE-2023-42119 Exim dnsdb Out-of-Bounds Read Attempt"; flow:to_server,established; content:"dnslookup"; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

• Restrict SMTP access to trusted networks only (firewall rules).
• Disable the dnsdb lookup feature if not required.
• Monitor for unusual DNS responses.

Long-Term Fix:

• Apply a vendor patch when available.
• Upgrade to a patched version of Exim.
• Implement input validation for DNS response lengths.

Workarounds:

• Use network segmentation to limit adjacent network access.
• Deploy an IDS/IPS to detect exploitation attempts.

Valters IT
Securing Digital Frontiers