CVE-2026-23829 identifies a CRLF injection vulnerability in the Mailpit SMTP server component prior to version 1.28.3. Mailpit is an email testing tool and API widely used by developers to simulate SMTP interactions. The vulnerability stems from an insufficient regular expression used to validate the 'RCPT TO' and 'MAIL FROM' email address fields. Specifically, the regex intended to filter out control characters fails to exclude carriage return (\r) and line feed (\n) characters when used inside a character class, allowing these characters to be injected into email addresses. An attacker can exploit this by crafting email addresses containing \r or \n sequences, which the SMTP server then interprets as header delimiters, enabling injection or corruption of arbitrary SMTP headers. This can lead to manipulation of email metadata, such as adding malicious headers, altering routing information, or bypassing security filters that rely on header integrity. The vulnerability is remotely exploitable without requiring authentication or user interaction, increasing its risk profile. Although it does not directly disclose sensitive information or cause denial of service, the integrity impact can facilitate further attacks like phishing or spoofing. The issue was addressed in Mailpit version 1.28.3 by correcting the regular expression to properly exclude CRLF characters from email address validation. No known exploits are currently reported in the wild, but the vulnerability's nature warrants prompt remediation in affected environments.

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.

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.