CVE-2025-49912 is a Stored Cross-site Scripting (XSS) vulnerability found in the Nks Email Subscription Popup plugin, specifically versions up to and including 1.2.26. The vulnerability stems from improper neutralization of input during web page generation, meaning that user-supplied data is not correctly sanitized or encoded before being embedded into the HTML output. This allows an attacker who can submit crafted input to have that input persistently stored and later executed in the context of other users’ browsers when they view the affected pages. The attack vector is network-based, but exploitation requires the attacker to have some level of authenticated access (PR:H) and user interaction (UI:R), such as tricking a user into clicking a malicious link or submitting malicious data. The vulnerability affects confidentiality, integrity, and availability to a limited extent, as malicious scripts can steal session cookies, manipulate page content, or perform actions on behalf of the victim. The scope is changed (S:C), indicating that the vulnerability can affect resources beyond the initially vulnerable component. The CVSS 3.1 base score is 5.9, categorizing it as medium severity. No public exploits have been reported yet, but the presence of stored XSS in a widely used plugin poses a risk for phishing, session hijacking, and defacement attacks. The vulnerability was reserved in June 2025 and published in October 2025, with no patches currently linked, indicating that remediation may still be pending or in progress.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on the Nks Email Subscription Popup plugin on their websites. Stored XSS can lead to session hijacking, allowing attackers to impersonate legitimate users, including administrators, potentially leading to unauthorized access and data leakage. It can also facilitate phishing attacks by injecting malicious content into trusted web pages, undermining user trust and brand reputation. Additionally, attackers could manipulate website content or distribute malware, affecting availability and integrity. Organizations in sectors with high regulatory scrutiny, such as finance, healthcare, and e-commerce, may face compliance risks if customer data is compromised. The need for authenticated access reduces the attack surface but does not eliminate risk, as insider threats or compromised accounts can be leveraged. The vulnerability’s presence in a plugin used for email subscriptions also raises concerns about the security of customer contact information and potential abuse for spam or social engineering campaigns.

To mitigate this vulnerability, European organizations should first monitor the vendor’s communications for official patches and apply them promptly once available. In the interim, restrict access to the plugin’s administrative interfaces to trusted personnel only and enforce strong authentication mechanisms, including multi-factor authentication. Implement web application firewall (WAF) rules to detect and block typical XSS payloads targeting the affected plugin. Conduct thorough input validation and output encoding on all user-supplied data within the application, especially in areas related to email subscription inputs. Regularly audit user privileges to minimize the number of accounts with the ability to submit or approve content that could be exploited. Employ Content Security Policy (CSP) headers to limit the execution of unauthorized scripts. Additionally, educate users and administrators about the risks of social engineering and the importance of cautious interaction with suspicious links or inputs. Finally, consider alternative plugins with better security track records if patching is delayed.