CVE-2020-28017: Exim Buffer Overflow Vulnerability

CVE-2020-28017 Overview

CVE-2020-28017 is an Integer Overflow vulnerability in Exim mail transfer agent versions prior to 4.94.2 that can lead to a Buffer Overflow condition. The vulnerability exists in the receive_add_recipient function and can be triggered via an e-mail message with an extremely large number of recipients (approximately fifty million). While the vulnerability is remotely exploitable over the network without authentication, practical exploitation may be difficult due to the significant resource consumption required to trigger the condition.

Critical Impact

This Integer Overflow to Buffer Overflow vulnerability in Exim's recipient handling could potentially allow remote attackers to execute arbitrary code or cause denial of service on vulnerable mail servers.

Affected Products

• Exim versions before 4.94.2

Discovery Timeline

• 2021-05-06 - CVE-2020-28017 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-28017

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The flaw resides in Exim's receive_add_recipient function, which handles the processing of email recipients during message reception. When an attacker sends an email with an extraordinarily large number of recipients (approximately fifty million), an integer overflow condition occurs during recipient counting or memory allocation calculations.

The integer overflow subsequently leads to a buffer overflow condition, where insufficient memory is allocated to hold the recipient data. This memory corruption can potentially be leveraged for arbitrary code execution, though the practical exploitation difficulty is noted due to the extreme resource requirements needed to deliver fifty million recipients to trigger the vulnerability.

Root Cause

The root cause is improper handling of integer arithmetic when processing recipient counts in the receive_add_recipient function. The code fails to properly validate or check for integer overflow conditions when calculating buffer sizes or recipient counts, allowing values to wrap around and result in undersized buffer allocations.

Attack Vector

The attack is conducted over the network (SMTP protocol) without requiring authentication or user interaction. An attacker would need to:

1. Establish an SMTP connection to a vulnerable Exim server
2. Construct a specially crafted email message with approximately fifty million recipients
3. Transmit this message to trigger the integer overflow condition in receive_add_recipient
4. The integer overflow leads to buffer overflow, potentially enabling code execution or denial of service

The attack complexity is considered low from a technical standpoint, though the practical requirements of transmitting such a large message present challenges. For detailed technical analysis, refer to the Exim Security Advisory CVE-2020-28017.

Detection Methods for CVE-2020-28017

Indicators of Compromise

• Unusually large SMTP sessions with excessive RCPT TO commands
• Abnormal memory consumption on Exim mail server processes
• Exim process crashes or unexpected restarts potentially indicating exploitation attempts
• Log entries showing rejected messages with extremely high recipient counts

Detection Strategies

• Monitor SMTP traffic for sessions with abnormally high recipient counts (threshold alerting for messages exceeding reasonable recipient limits)
• Implement rate limiting on RCPT TO commands per session to detect and block abuse
• Deploy network intrusion detection signatures targeting excessive SMTP recipient enumeration patterns
• Audit Exim version information across infrastructure to identify vulnerable installations

Monitoring Recommendations

• Enable verbose SMTP logging to capture recipient counts per message
• Configure alerting for Exim process memory usage anomalies
• Monitor for Exim service crashes or unexpected restarts that may indicate exploitation attempts
• Track SMTP connection duration and command counts for statistical anomaly detection

How to Mitigate CVE-2020-28017

Immediate Actions Required

• Upgrade Exim to version 4.94.2 or later immediately
• Review and implement recipient limits in Exim configuration to restrict maximum recipients per message
• Consider deploying network-level controls to rate-limit SMTP traffic
• Audit systems for evidence of exploitation attempts in mail logs

Patch Information

The vulnerability has been addressed in Exim version 4.94.2 and later releases. Organizations should upgrade to the latest stable version of Exim to remediate this vulnerability. Additional details and technical analysis are available in the Exim Security Advisory CVE-2020-28017.

Workarounds

• Configure Exim's recipients_max option to enforce a reasonable limit on recipients per message
• Implement SMTP-level rate limiting to restrict the number of RCPT TO commands per connection
• Deploy upstream firewall or mail gateway rules to drop connections exhibiting abnormal recipient patterns
• Consider running Exim in a sandboxed or containerized environment to limit potential impact

# Exim configuration example - Add to exim.conf
# Limit maximum recipients per message to prevent exploitation
recipients_max = 100

# Enable stricter SMTP controls
smtp_accept_max_per_host = 10
smtp_receive_timeout = 5m