VulnerabilityPublished 18 Jan 2026Updated 20 Jan 2026

CVE-2026-23829: axllent mailpit · CRLF Injection

Affects axllent mailpitSource CVE Database V5

Exposed hosts seen by OffSeq Mirage · public detection template (Nuclei) · CVSS 5.3 Medium : upgrade to 1.28.3 when convenient.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 3.1 base score

5.3Medium

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

EPSS: 1.4% chance of exploitation in 30 days – higher than 72% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.28.3

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: Low
Availability: None

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
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 on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 18 Jan 2026
Detectable: evidence found.
nuclei template: Mailpit < 1.28.2 - SMTP CRLF Injection
Weaponised: 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: evidence found.
1 exposed host found; no attack traffic recorded

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), OffSeq Mirage (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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 mailpit and axllent/mailpit to 1.28.3.

Affected software

Product	Affected	Fixed in
axllentmailpit	Affected <1.28.3	Fixed in 1.28.3
GitHubaxllent/mailpit`pkg:github/axllent/mailpit`Other threats	Affected <1.28.3	Fixed in 1.28.3

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

AI-suggested

To mitigate CVE-2026-23829, European organizations should immediately upgrade all Mailpit instances to version 1.28.3 or later, where the vulnerability is fixed. Beyond patching, organizations should implement strict network segmentation to restrict access to Mailpit servers, ensuring only trusted developer and QA personnel can interact with these tools. Input validation should be enforced at multiple layers, including any custom scripts or integrations that interact with Mailpit, to prevent injection of control characters. Monitoring and logging of SMTP interactions within Mailpit should be enhanced to detect anomalous header injections or unusual email address formats. Security teams should review email testing workflows to ensure no sensitive data or production credentials are used in vulnerable environments. Additionally, organizations should educate developers about the risks of using outdated testing tools and incorporate vulnerability scanning into their software development lifecycle to catch such issues early. If Mailpit is exposed externally, consider deploying web application firewalls or SMTP proxies that can sanitize inputs and block suspicious payloads. Finally, maintain an inventory of all development tools and their versions to ensure timely patch management.

Description

Mailpit is an email testing tool and API for developers. Prior to version 1.28.3, Mailpit's SMTP server is vulnerable to Header Injection due to an insufficient Regular Expression used to validate `RCPT TO` and `MAIL FROM` addresses. An attacker can inject arbitrary SMTP headers (or corrupt existing ones) by including carriage return characters (`\r`) in the email address. This header injection occurs because the regex intended to filter control characters fails to exclude `\r` and `\n` when used inside a character class. Version 1.28.3 fixes this issue.

Impact

For European organizations, the primary impact of this vulnerability lies in the potential manipulation of email headers within development or testing environments using Mailpit. This can undermine the integrity of email simulations, potentially allowing attackers to craft deceptive emails that bypass security controls or facilitate phishing campaigns. While Mailpit is primarily a testing tool and not typically used in production email delivery, compromised testing environments can lead to inaccurate security assessments or inadvertent exposure to malicious payloads. Organizations relying on Mailpit for internal development or QA processes may face risks of internal phishing or social engineering if attackers leverage this vulnerability to inject malicious headers. Additionally, if Mailpit instances are exposed to untrusted networks, attackers could exploit this vulnerability to interfere with email testing workflows or inject misleading data into logs and reports. The vulnerability does not affect confidentiality or availability directly but poses a moderate risk to the integrity of email-related processes. European entities with stringent compliance requirements around email security and software development lifecycle integrity should prioritize patching to maintain trustworthiness of their testing environments.

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).
CWE-150Improper Neutralization of Escape, Meta, or Control Sequences (opens in a new tab)
Variant weakness
The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as escape, meta, or control character sequences when they are sent to a downstream component.
Developers should anticipate that escape, meta and control characters/sequences will be injected/removed/manipulated in the input vectors of their product. Use an appropriate combination of denylists and allowlists to ensure only valid, expected and appropriate input is processed by the system.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
Implementation · Output Encoding
While it is risky to use dynamically-generated query strings, code, or commands that mix control and data together, sometimes it may be unavoidable. Properly quote arguments and escape any special characters within those arguments.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
Server Side Request Forgery Prevention (opens in a new tab)
Unvalidated Redirects and Forwards (opens in a new tab)
Cross Site Scripting Prevention (opens in a new tab)
OWASP ASVS 4.0.3
V1.5.1 (opens in a new tab)
V5.2.2 (opens in a new tab)
Attack patterns
Email Injection (opens in a new tab)
Using Meta-characters in E-mail Headers to Inject Malicious Payloads (opens in a new tab)
Web Server Logs Tampering (opens in a new tab)
Log Injection-Tampering-Forging (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 26 Jan 2026, 19:57 UTC

Public exploits and detectiona Nuclei detection template

WeaknessesCWE-93, CWE-150

Technical details

References

Community reviews

Related threats