CVE-2026-19862 in JetFormBuilder Plugin
Summary
by MITRE • 09/06/2026
The JetFormBuilder WordPress plugin before 3.6.5.2 does not validate or strip line breaks from address values it sources from submitted form fields before adding them to the headers of the e-mails it sends, allowing unauthenticated users to inject arbitrary e-mail headers, add hidden recipients and spoof the sender. Exploitation requires the site to be configured to take one of the message's addresses from a form field.
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Analysis
by VulDB Data Team • 09/06/2026
The vulnerability identified in JetFormBuilder versions prior to 3.6.5.2 represents a critical email header injection flaw rooted in insufficient input validation and sanitization mechanisms. This security defect allows unauthenticated attackers to manipulate the headers of emails generated by the plugin, leading to potential phishing attacks, spam distribution, or further exploitation of downstream systems. The core technical issue lies in how the plugin processes data submitted through form fields that are mapped to email address components such as sender addresses, recipient lists, or reply-to fields. Specifically, the software fails to strip line break characters from these input values before incorporating them into the SMTP header construction process. In standard email protocols defined by RFC 5322 and related standards, newline characters serve as delimiters between different header fields. When an attacker injects a carriage return followed by a line feed (CRLF) sequence into a form field that populates these address values, the mail server interprets this injection point as the end of one header and the beginning of another arbitrary header.
This lack of validation enables several malicious outcomes depending on how the WordPress site is configured to utilize JetFormBuilder forms. If the configuration allows any part of an email's addressing structure to be derived from user-supplied form data, an attacker can inject additional headers such as Bcc or Cc to add hidden recipients who will receive copies of the sensitive information submitted through the form without their knowledge. Furthermore, by injecting a From header with a spoofed address, attackers can make malicious emails appear to originate from trusted domains within the organization or legitimate service providers, significantly increasing the likelihood that recipients will trust and interact with the content. This capability effectively turns the vulnerable website into an open relay for phishing campaigns or spam operations, damaging the reputation of the domain hosting the WordPress site and potentially leading to blacklisting by major email providers.
The operational impact extends beyond immediate message manipulation. Successful exploitation can lead to data leakage if hidden recipients include external parties who capture form submissions containing personally identifiable information (PII) such as names, addresses, or financial details entered into the forms. Additionally, because this vulnerability requires no authentication, it is easily exploitable by automated bots scanning for vulnerable WordPress installations across the internet. The attack vector aligns with CWE-74 Improper Neutralization of Special Elements in Output Used by a Command, Program, Argument, Rule, or Directive and specifically maps to CWE-93 Improper Neutralization of CRLF Sequences (CRLF Injection) within the context of email protocols. In terms of offensive security frameworks like MITRE ATT&CK, this behavior corresponds to techniques involving Email Spoofing and potentially Obfuscated Files or Information if used in conjunction with other payloads delivered via the injected headers.
Mitigation strategies must prioritize immediate patching as well as defensive coding practices for developers maintaining similar plugins. The primary remediation is upgrading JetFormBuilder to version 3.6.5.2 or later, where this validation logic has been corrected to properly sanitize input data before header construction. For environments unable to upgrade immediately due to compatibility constraints, administrators should implement server-side filtering rules that block requests containing CRLF sequences in form submissions targeting email-related fields. Additionally, configuring the WordPress site to use fixed, hardcoded values for critical email headers rather than dynamic user-supplied inputs can eliminate this attack surface entirely. Security monitoring tools should be configured to detect anomalous outbound email patterns or high volumes of failed SMTP transactions that may indicate ongoing exploitation attempts against unpatched instances.