CVE-2026-27120 identifies a vulnerability in leaf-kit, a templating language with Swift-inspired syntax used primarily within the vapor web framework. The vulnerability stems from improper sanitization in the htmlEscaped function prior to version 1.4.1. Specifically, htmlEscaped only escapes HTML special characters if the extended grapheme clusters exactly match the special characters. Extended grapheme clusters are sequences of Unicode code points that appear as a single character. Attackers can craft extended grapheme clusters that include a special HTML character combined with additional characters, effectively bypassing the escaping mechanism. When such crafted input is injected into HTML attributes via leaf variables controlled by an attacker, it can result in cross-site scripting (XSS). This XSS vulnerability allows attackers to execute arbitrary JavaScript in the context of the victim's browser, potentially stealing session tokens, manipulating page content, or performing actions on behalf of the user. The vulnerability is classified under CWE-75 (Failure to Sanitize Special Elements into a Different Plane), CWE-79 (Improper Neutralization of Input During Web Page Generation), and CWE-87 (Improper Neutralization of Special Elements). The issue does not require authentication but does require user interaction to trigger the XSS payload. The vulnerability affects all leaf-kit versions prior to 1.4.1 and was publicly disclosed on February 20, 2026. No known exploits are currently in the wild. The vulnerability has a CVSS v3.1 base score of 6.1 (medium severity), reflecting network attack vector, low attack complexity, no privileges required, user interaction required, and partial confidentiality and integrity impact without availability impact. The vulnerability is fixed in leaf-kit version 1.4.1.

The primary impact of this vulnerability is the potential for cross-site scripting (XSS) attacks in web applications using vulnerable versions of leaf-kit. Successful exploitation can lead to the execution of arbitrary JavaScript in users' browsers, enabling attackers to steal sensitive information such as session cookies, perform actions on behalf of users, or manipulate web page content. This compromises the confidentiality and integrity of user data and can damage user trust and organizational reputation. While the vulnerability does not directly affect system availability, the indirect effects of XSS—such as phishing or malware distribution—can have broader security implications. Organizations relying on leaf-kit for templating in web applications are at risk, especially if user input is embedded in HTML attributes without additional sanitization. The vulnerability's medium severity indicates a significant but not critical risk, emphasizing the need for timely patching. Since exploitation requires user interaction, the risk can be mitigated somewhat by user awareness, but the presence of the vulnerability in widely deployed web applications could expose a large user base to attacks.

To mitigate this vulnerability, organizations should upgrade all instances of leaf-kit to version 1.4.1 or later, where the issue is fixed. In addition to upgrading, developers should implement defense-in-depth by applying additional input validation and output encoding on user-controlled data, especially when injecting variables into HTML attributes. Employing Content Security Policy (CSP) headers can help reduce the impact of potential XSS attacks by restricting script execution sources. Regular security code reviews and automated scanning for unsafe templating practices can identify similar issues early. Web application firewalls (WAFs) configured to detect and block XSS payloads may provide temporary protection but should not replace patching. Educating developers about the risks of improper escaping and the nuances of Unicode grapheme clusters can prevent future vulnerabilities. Finally, monitoring web application logs for suspicious activity and anomalous user behavior can aid in early detection of exploitation attempts.