CVE-2026-86823 in Newsletter Plugin
Summary
by MITRE • 09/16/2026
The Newsletter WordPress plugin before 9.3.7 does not validate the destination of the redirect performed after a public subscription action, allowing unauthenticated attackers to redirect users to arbitrary external sites and to disclose a subscriber token that grants access to that subscriber record's front-end actions.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in The Newsletter WordPress plugin prior to version 9.3.7 represents a critical Open Redirect flaw combined with sensitive information disclosure, stemming from insufficient validation of user-supplied input during the public subscription workflow. This security defect allows unauthenticated attackers to manipulate the redirect mechanism that occurs immediately after a user submits their email address for newsletter subscription. In standard web application architecture, redirections following form submissions are often used to provide feedback or navigate users to confirmation pages; however, when these destinations are not strictly validated against an allowlist of trusted domains, they become vectors for malicious activity. The plugin fails to sanitize the destination parameter passed during this process, enabling attackers to construct URLs that point to arbitrary external websites rather than legitimate internal endpoints.
From a technical perspective, this flaw exploits the lack of rigorous input validation on redirect targets, which is classified under CWE-601: URL Redirection to Untrusted Site (Open Redirect). By crafting a specific request payload containing a maliciously crafted destination parameter, an attacker can trick users into clicking links that appear legitimate but ultimately lead them to phishing sites or other harmful destinations. This manipulation not only compromises the integrity of the user experience but also facilitates social engineering attacks where victims may be lured into entering credentials on fraudulent pages hosted by the attacker. The absence of proper domain whitelisting means there is no mechanism to distinguish between safe internal redirects and dangerous external ones, leaving the application vulnerable to this class of attack across all public-facing instances running affected versions.
Beyond the immediate risk of open redirection, the vulnerability carries a significant secondary impact related to information disclosure. The exploitation of this flaw allows attackers to disclose subscriber tokens that are associated with specific user records within the plugin's database structure. These tokens serve as authentication mechanisms for accessing front-end actions and managing subscription preferences. When an attacker successfully exploits the redirect mechanism, they can intercept or manipulate these tokens during the submission process, effectively gaining unauthorized access to sensitive subscriber data. This disclosure violates principles of confidentiality and integrity outlined in CWE-209: Generation of Error Message Containing Sensitive Information, as it exposes internal identifiers that should remain confidential. The exposure of such tokens enables further exploitation scenarios where attackers can impersonate legitimate subscribers or perform actions on their behalf without valid credentials.
The operational impact of this vulnerability extends beyond individual user compromise to affect the broader trust model of the WordPress site hosting the plugin. Since the flaw is exploitable by unauthenticated users, it requires no prior access privileges, making it highly accessible to automated scanning tools and opportunistic attackers. The combination of open redirection and token disclosure creates a potent attack chain that can lead to account takeover, data exfiltration, or further lateral movement within the application environment. Organizations relying on this plugin for email marketing must recognize that the integrity of their subscriber base is compromised until remediation occurs. Attackers leveraging ATT&CK technique T1566.002: Spearphishing Link can utilize these crafted URLs to target specific individuals, increasing the likelihood of successful compromise due to the perceived legitimacy of the source domain before redirection occurs.
Mitigation strategies must prioritize immediate patching and defensive configuration changes. The primary remediation step is upgrading The Newsletter WordPress plugin to version 9.3.7 or later, where developers have implemented proper validation logic for redirect destinations. This typically involves implementing a strict allowlist approach that restricts redirects only to predefined internal paths and domains, rejecting any external URLs outright. Additionally, security headers such as Content-Security-Policy should be configured to mitigate potential cross-site scripting vectors that might accompany redirection attacks. For environments where immediate patching is not feasible, input validation at the web application firewall level can help block requests containing suspicious redirect parameters. Regular audits of third-party plugins are essential to ensure compliance with secure coding standards and to prevent similar vulnerabilities from persisting in production systems.