VulnerabilityPublished 6 Mar 2026Updated 9 Mar 2026

CVE-2026-30227: jstedfast MimeKit · CRLF Injection

Affects jstedfast MimeKitSource CVE Database V5

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Not known to be exploited · CVSS 6.9 Medium : upgrade to 4.15.1 when convenient.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 4.0 base score

6.9Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 1.1% chance of exploitation in 30 days – higher than 66% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 4.15.1

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: Low
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 6 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade MimeKit and mimekit to 4.15.1.

Affected software

Product	Affected	Fixed in
jstedfastMimeKit	Affected <4.15.1	Fixed in 4.15.1
NuGetmimekit`pkg:nuget/mimekit`Other threats	Affected <4.15.1	Fixed in 4.15.1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Organizations should immediately upgrade MimeKit to version 4.15.1 or later, where the vulnerability is patched. Developers using MimeKit should audit their code to ensure that any user-controllable input used in MailboxAddress fields is properly validated and sanitized to prevent injection of CRLF characters. Implement strict input validation on email address fields, rejecting any input containing control characters or non-compliant syntax per RFC 5321. Employ application-layer filters or SMTP proxies that can detect and block anomalous SMTP command sequences indicative of injection attempts. Conduct thorough testing of email handling workflows to verify that injected commands cannot be executed. Monitor email logs for unusual SMTP commands or session resets that may indicate exploitation attempts. Finally, maintain awareness of MimeKit updates and security advisories to promptly address future vulnerabilities.

Description

MimeKit is a C# library which may be used for the creation and parsing of messages using the Multipurpose Internet Mail Extension (MIME), as defined by numerous IETF specifications. Prior to version 4.15.1, a CRLF injection vulnerability in MimeKit allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session. RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous. This issue has been patched in version 4.15.1.

Impact

The vulnerability can allow attackers to inject arbitrary SMTP commands into the mail transmission process, potentially enabling unauthorized mail delivery, mail spoofing, or disruption of mail services. This can undermine the integrity and reliability of email communications for organizations using vulnerable MimeKit versions. Attackers could manipulate recipient addresses or mail content, leading to phishing, spam propagation, or denial of service by resetting SMTP sessions. Since MimeKit is used in various mail-related applications and services, the impact can extend to any organization relying on these for email processing, including enterprises, service providers, and software vendors. The medium severity rating reflects the significant but not critical risk, given the lack of authentication requirements and the potential for remote exploitation. However, the scope depends on the extent to which vulnerable versions are deployed in production environments.

Summary

MimeKit is a widely used C# library for creating and parsing MIME messages according to IETF standards. Prior to version 4.15.1, MimeKit contained a CRLF injection vulnerability (CWE-93) identified as CVE-2026-30227. The vulnerability arises because MimeKit improperly neutralizes CRLF sequences in the SMTP envelope address local-part when it is formatted as a quoted-string. According to RFC 5321, the local-part of an SMTP mailbox must not contain carriage return (CR, ASCII 13) or line feed (LF, ASCII 10) characters, as SMTP commands are delimited by CRLF sequences. By injecting these characters, an attacker can manipulate the SMTP session by injecting additional SMTP commands such as RCPT TO, DATA, or RSET. This can lead to unauthorized mail delivery commands or mail header injection, potentially allowing spoofing or disruption of mail flow. Exploitation requires the attacker to influence the MailboxAddress value that MimeKit serializes during SMTP communication. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The issue was addressed and fixed in MimeKit version 4.15.1 by properly sanitizing and rejecting CRLF sequences in the local-part of mailbox addresses. No known exploits are reported in the wild as of now.

CWE-93Improper Neutralization of CRLF Sequences (CRLF Injection) (opens in a new tab)
Base weaknessOWASP Top 10 A05:2025: Injection (opens in a new tab)
The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.
Implementation
Avoid using CRLF as a special sequence.
Implementation
Appropriately filter or quote CRLF sequences in user-controlled input.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
OWASP ASVS 4.0.3
V1.5.1 (opens in a new tab)
Attack patterns
Command Delimiters (opens in a new tab)
Web Server Logs Tampering (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/jstedfast/MimeKit/security/advisories/GHSA-g7hc-96xr-gvvx (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 14 Mar 2026, 19:42 UTC

WeaknessesCWE-93

Technical details

References

Community reviews

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