CVE-2002-2261 in Sendmail
Summary
by MITRE
Sendmail 8.9.0 through 8.12.6 allows remote attackers to bypass relaying restrictions enforced by the check_relay function by spoofing a blank DNS hostname.
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Analysis
by VulDB Data Team • 08/29/2025
The vulnerability identified as CVE-2002-2261 represents a significant security flaw in Sendmail versions 8.9.0 through 8.12.6 that directly impacts the email server's ability to enforce relay restrictions. This weakness allows remote attackers to circumvent critical access controls that are designed to prevent unauthorized relay of email messages through the server, potentially enabling spam relay and malicious email delivery.
The technical flaw resides in the check_relay function's handling of DNS hostname validation. When Sendmail processes email relay requests, it typically performs DNS lookups to verify the authenticity of sender hostnames and enforce relay restrictions based on these validations. However, this particular vulnerability occurs when attackers spoof a blank DNS hostname, which the vulnerable Sendmail versions fail to properly validate or reject. The flaw essentially allows attackers to bypass the normal hostname verification process by presenting a malformed or empty hostname that the system interprets as valid, thus permitting relay operations that should have been blocked.
This vulnerability operates under the broader category of improper input validation and authentication bypass mechanisms, which aligns with CWE-20 for improper input validation and CWE-287 for improper authentication. The operational impact of this vulnerability is substantial as it enables attackers to use the compromised Sendmail server as an open relay for sending spam emails, conducting phishing campaigns, or delivering malicious payloads. The attack vector is particularly concerning because it requires no local access or authentication credentials, making it a remote exploit that can be executed from anywhere on the internet.
The security implications extend beyond simple spam relay capabilities, as this vulnerability can be leveraged to establish persistent attack infrastructure through compromised mail servers. Attackers can exploit this flaw to create a network of open relays that can be used for large-scale email campaigns, potentially leading to reputation damage for the affected organizations and contributing to the overall spam ecosystem. The vulnerability affects the fundamental security model of Sendmail's relay protection mechanisms, undermining the trust model that email servers rely upon for secure message delivery.
Organizations should implement immediate mitigations including upgrading to Sendmail versions that have addressed this vulnerability, typically versions beyond 8.12.6, and configuring additional relay restrictions that do not solely depend on DNS hostname validation. Network administrators should also implement proper monitoring of relay activities and consider implementing additional authentication mechanisms such as SMTP AUTH or IP-based access controls. The ATT&CK framework categorizes this vulnerability under T1190 for Exploit Public-Facing Application, with potential techniques including T1078 for Valid Accounts and T1566 for Phishing, as attackers can leverage compromised servers for broader campaign operations.