CVE-2023-51765 Overview
CVE-2023-51765 is an SMTP smuggling vulnerability affecting Sendmail versions through 8.17.2 that allows remote attackers to inject email messages with spoofed MAIL FROM addresses. This vulnerability enables attackers to bypass SPF (Sender Policy Framework) protection mechanisms by exploiting differences in how Sendmail and other email servers handle end-of-data sequences in SMTP communications.
The vulnerability arises because Sendmail supports the <LF>.<CR><LF> sequence as an end-of-data indicator, while many other popular email servers do not recognize this non-standard sequence. This inconsistency creates an opportunity for attackers to "smuggle" additional SMTP commands through intermediary mail servers, effectively injecting spoofed emails that appear to originate from trusted domains.
Critical Impact
Remote attackers can bypass SPF email authentication mechanisms to send spoofed emails that appear to originate from legitimate domains, potentially enabling phishing attacks, business email compromise, and reputation damage to affected organizations.
Affected Products
- Sendmail through version 8.17.2
- FreeBSD (all affected versions with bundled Sendmail)
- Red Hat Enterprise Linux 8.0 and 9.0
Discovery Timeline
- 2023-12-24 - CVE-2023-51765 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-51765
Vulnerability Analysis
This SMTP smuggling vulnerability exploits a protocol interpretation inconsistency in Sendmail's handling of the end-of-data sequence during SMTP transactions. According to RFC 5321, the standard end-of-data sequence is <CR><LF>.<CR><LF>, but Sendmail's implementation also accepts the non-standard <LF>.<CR><LF> sequence in certain configurations.
When Sendmail acts as an inbound mail server behind another mail transfer agent (MTA), attackers can craft specially formatted email messages that exploit this parsing difference. The upstream MTA may not recognize the non-standard sequence as an end-of-data marker, allowing the attacker to inject additional SMTP commands that Sendmail will process as a separate, legitimate email transaction.
This attack directly undermines SPF validation because the smuggled email inherits the authentication context of the legitimate connection, allowing attackers to send emails that pass SPF checks despite originating from unauthorized sources.
Root Cause
The root cause of CVE-2023-51765 lies in Sendmail's permissive parsing of SMTP end-of-data sequences. Specifically, Sendmail accepts <LF>.<CR><LF> (bare line feed followed by a period and CRLF) as a valid end-of-data indicator, deviating from the strict RFC 5321 specification that mandates only <CR><LF>.<CR><LF>.
This implementation decision was likely made for backward compatibility with non-conformant mail clients, but it creates a security gap when Sendmail operates in environments with other MTAs that strictly follow the RFC specification. The vulnerability is configuration-dependent and primarily affects deployments where Sendmail receives mail from other servers that do not sanitize non-standard line endings.
Attack Vector
The attack is conducted remotely over the network through standard SMTP communication (typically port 25). An attacker connects to an upstream mail server that forwards messages to a vulnerable Sendmail instance and sends a crafted email containing the <LF>.<CR><LF> sequence within the message body.
The upstream MTA interprets this as part of the message content and forwards it to Sendmail. However, Sendmail treats the sequence as an end-of-data marker, processing everything after it as new SMTP commands. This allows the attacker to inject a completely separate email with a spoofed sender address that bypasses SPF validation.
The attack requires no authentication and can be executed by any party capable of sending emails to the target mail infrastructure. The exploitation technique was publicly disclosed at the CCC Congress 2023 Event and detailed in the SEC Consult Blog Post on SMTP.
SENDMAIL LICENSE
-The following license terms and conditions apply, unless a redistribution
-agreement or other license is obtained from Sendmail, Inc., 6475 Christie
-Ave, Third Floor, Emeryville, CA 94608, USA, or by electronic mail at
-license@sendmail.com.
+The following license terms and conditions apply, unless a redistribution
+agreement or other license is obtained from Proofpoint, Inc., 892
+Ross Street, Sunnyvale, CA, 94089, USA, or by electronic mail at
+sendmail-license@proofpoint.com.
License Terms:
Source: FreeBSD Source Code Commit
Detection Methods for CVE-2023-51765
Indicators of Compromise
- Emails passing SPF checks but originating from unexpected or suspicious source IP addresses
- SMTP logs showing unusual end-of-data sequences or multiple DATA commands within a single connection
- Increased volume of emails with mismatched envelope sender (MAIL FROM) and header From addresses
- Reports of phishing emails appearing to originate from your organization's legitimate domains
Detection Strategies
- Implement SMTP transaction logging that captures raw protocol data including line ending sequences
- Deploy email security gateways with SMTP smuggling detection capabilities that inspect for non-standard <LF>.<CR><LF> sequences
- Configure DMARC with strict alignment policies and enable forensic reporting to detect spoofed emails
- Monitor for SPF "pass" results on emails that fail other authentication checks or originate from suspicious sources
Monitoring Recommendations
- Enable verbose SMTP logging on Sendmail servers to capture detailed transaction information
- Configure alerts for emails with domain mismatches between envelope and header addresses
- Monitor DMARC aggregate and forensic reports for signs of domain spoofing attacks
- Track connection patterns for SMTP sessions with abnormal DATA command sequences
How to Mitigate CVE-2023-51765
Immediate Actions Required
- Upgrade Sendmail to version 8.18 or later, which resolves this vulnerability with the o option in srv_features
- Apply vendor-provided security patches for your operating system distribution (FreeBSD, RHEL, Debian, SUSE)
- Review Sendmail configuration to ensure strict RFC compliance for end-of-data handling
- Implement DMARC policies with p=reject to prevent spoofed emails from being delivered
Patch Information
The vulnerability is resolved in Sendmail version 8.18 and later releases. The fix introduces strict handling of end-of-data sequences when the o option is configured in srv_features. Organizations running older versions should upgrade or apply distribution-specific patches.
Vendor advisories and patches are available from:
Workarounds
- Configure upstream MTAs to normalize line endings and strip bare LF characters before forwarding to Sendmail
- Deploy an email security gateway that sanitizes SMTP traffic for non-standard sequences
- Implement strict DMARC policies (p=reject) to mitigate the impact of successful spoofing attempts
- Consider placing Sendmail behind a compliant MTA that enforces RFC 5321 line ending requirements
# Sendmail 8.18+ configuration to enable strict SMTP handling
# Add 'o' to srv_features in sendmail.mc to enforce strict end-of-data handling
define(`confSRV_FEATURES', `o')dnl
# Rebuild and restart Sendmail after configuration changes
m4 sendmail.mc > sendmail.cf
systemctl restart sendmail
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
