Summary
MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery
Impact
Vulnerability class:
- SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
- Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).
Who is impacted:
- Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).
Potential consequences:
- Add or modify SMTP recipients by injecting extra
RCPT TOcommands (mail redirection / data exfiltration). - Corrupt the SMTP transaction state (
RSET,NOOP, etc.) or attempt earlyDATAinjection (server-dependent). - In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
- Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.
Suggested remediation (high level):
- Reject
\rand\nin local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes. - Align quoted local-part parsing with RFC 5321’s
qtextSMTPandquoted-pairSMTPranges (no control characters).
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-30227? CVE-2026-30227 is a medium-severity security vulnerability in MimeKit (nuget), affecting versions <= 4.15.0. It is fixed in 4.15.1.
- Which versions of MimeKit are affected by CVE-2026-30227? MimeKit (nuget) versions <= 4.15.0 is affected.
- Is there a fix for CVE-2026-30227? Yes. CVE-2026-30227 is fixed in 4.15.1. Upgrade to this version or later.
- Is CVE-2026-30227 exploitable, and should I be worried? Whether CVE-2026-30227 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-30227 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-30227? Upgrade
MimeKitto 4.15.1 or later.