CVE-2026-30227: Jstedfast MimeKit CRLF Injection Vulnerability

CVE-2026-30227 Overview

CVE-2026-30227 is a CRLF injection vulnerability affecting MimeKit, a popular C# library used for creating and parsing messages using the Multipurpose Internet Mail Extension (MIME). Prior to version 4.15.1, the library fails to properly sanitize carriage return and line feed characters (\r\n) within SMTP envelope address local-parts when represented as quoted-strings. This non-compliance with RFC 5321 enables attackers to inject arbitrary SMTP commands or manipulate mail headers, potentially leading to email spoofing, unauthorized mail relay, or other mail-based attacks.

Critical Impact

Attackers who can influence a MailboxAddress value (MAIL FROM or RCPT TO) that is later serialized to an SMTP session can inject arbitrary SMTP commands such as RCPT TO, DATA, or RSET, potentially enabling unauthorized email delivery or header manipulation.

Affected Products

• jstedfast MimeKit versions prior to 4.15.1
• Applications using MailKit/MimeKit for SMTP message construction and delivery
• Any C# application implementing email functionality via the affected MimeKit versions

Discovery Timeline

• 2026-03-06 - CVE CVE-2026-30227 published to NVD
• 2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-30227

Vulnerability Analysis

The vulnerability exists in how MimeKit handles email address local-parts during SMTP session serialization. RFC 5321 explicitly defines the SMTP mailbox local-part grammar, where the qtextSMTP and quoted-pairSMTP character ranges specifically exclude control characters including CR (ASCII 13) and LF (ASCII 10). Since SMTP commands are terminated by <CRLF> sequences, allowing these characters within command arguments creates a dangerous injection point.

When an application allows user-controlled input to populate MailboxAddress objects (used for MAIL FROM or RCPT TO commands), an attacker can embed \r\n sequences within the quoted-string portion of the local-part. These sequences are not stripped or escaped during serialization, resulting in premature command termination and the ability to inject additional SMTP protocol commands.

Root Cause

The root cause is improper input validation within MimeKit's email address parsing and serialization logic. The library fails to validate that quoted-string local-parts conform to RFC 5321 grammar requirements, which explicitly prohibit CR and LF control characters. This violates the principle of strict adherence to protocol specifications and creates an injection vulnerability when untrusted data flows through the email address construction path.

Attack Vector

The attack requires an adversary to control or influence input that is used to construct a MailboxAddress object in an application using MimeKit. The exploitation scenario involves:

1. Attacker provides a malicious email address containing embedded CRLF sequences in the local-part
2. The application incorporates this address into a MailboxAddress object
3. When the message is sent via MailKit's SMTP client, the address is serialized without proper sanitization
4. The CRLF sequences terminate the current SMTP command prematurely
5. Additional attacker-controlled SMTP commands execute in the context of the authenticated session

This can result in SMTP command injection (injecting RCPT TO to add unauthorized recipients, DATA to inject message content, or RSET to reset state) and mail header injection for spoofing purposes.

The vulnerability is exploitable over the network when applications expose email functionality to users who can specify recipient addresses or sender information. See the GitHub Security Advisory for additional technical details.

Detection Methods for CVE-2026-30227

Indicators of Compromise

• Email addresses in application logs containing unusual control characters or encoded CRLF sequences (%0D%0A, \r\n)
• SMTP transaction logs showing unexpected command sequences or multiple commands per line
• Outbound email activity with recipients not matching intended application behavior
• Mail server logs indicating protocol errors or unexpected DATA commands

Detection Strategies

• Implement input validation logging to capture attempts to submit email addresses containing control characters
• Monitor SMTP transaction logs for anomalous command patterns or protocol violations
• Deploy application-layer firewalls configured to detect CRLF injection patterns in email-related traffic
• Utilize SentinelOne's behavioral analysis to detect applications exhibiting unexpected SMTP communication patterns

Monitoring Recommendations

• Enable detailed SMTP logging on mail servers to capture full command sequences
• Configure alerting for email addresses containing non-printable characters in application logs
• Monitor for unusual patterns in outbound email volume or recipient distribution
• Review application dependency versions to identify deployments using vulnerable MimeKit versions

How to Mitigate CVE-2026-30227

Immediate Actions Required

• Upgrade MimeKit to version 4.15.1 or later immediately
• Audit applications using MimeKit to identify user-controlled email address inputs
• Implement server-side input validation to reject email addresses containing CR or LF characters
• Review outbound email logs for signs of exploitation prior to patching

Patch Information

The vulnerability has been patched in MimeKit version 4.15.1. Organizations should update their NuGet package references to pull the patched version. Refer to the GitHub Security Advisory for official patch details and additional guidance.

Workarounds

• Implement strict input validation on all user-supplied email addresses before passing to MimeKit
• Reject any email local-parts containing CR (0x0D) or LF (0x0A) characters at the application layer
• Use allowlist-based validation for email address characters where possible
• Consider implementing an email address sanitization layer as defense-in-depth

# NuGet package update example
dotnet add package MimeKit --version 4.15.1
# Or update packages.config / .csproj to specify minimum version
# <PackageReference Include="MimeKit" Version="4.15.1" />