CVE-2026-28499 in leaf-kit
Summary
by MITRE • 03/18/2026
LeafKit is a templating language with Swift-inspired syntax. Prior to version 1.14.2, HTML escaping doesn't work correctly when a template prints a collection (Array / Dictionary) via `#(value)`. This can result in XSS, allowing potentially untrusted input to be rendered unescaped. Version 1.14.2 fixes the issue.
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Analysis
by VulDB Data Team • 03/22/2026
The vulnerability identified as CVE-2026-28499 affects LeafKit, a templating language designed with swift-inspired syntax that is commonly used in web applications for rendering dynamic content. This security flaw exists in versions prior to 1.14.2 and represents a critical concern for applications that process user input through templating systems. The issue manifests specifically when templates attempt to render collections such as arrays or dictionaries using the standard print syntax `#(value)`. The improper handling of HTML escaping in these scenarios creates a pathway for cross-site scripting attacks, where malicious input can be executed in the context of other users' browsers. The vulnerability stems from the templating engine's failure to properly sanitize and escape content when rendering collection data structures, effectively bypassing the intended security measures that should protect against malicious code injection.
The technical nature of this vulnerability aligns with CWE-79, which specifically addresses cross-site scripting flaws in web applications. When developers use LeafKit templates to display user-provided data through collection rendering, the templating engine fails to apply appropriate HTML escaping mechanisms to prevent malicious scripts from being executed. This represents a classic server-side template injection vulnerability where the templating system does not adequately protect against malicious input. The flaw is particularly dangerous because it affects the core templating functionality that many applications rely upon for displaying dynamic data, making it a widespread potential security concern across any application using affected versions of LeafKit. The vulnerability's impact is amplified because it affects common data structures that are frequently used in web applications, making it a high-value target for attackers seeking to exploit web application vulnerabilities.
The operational impact of this vulnerability extends beyond simple code execution, as it can enable attackers to perform a wide range of malicious activities including session hijacking, data theft, and defacement of web applications. When unescaped content is rendered in web pages, attackers can inject JavaScript payloads that can steal cookies, redirect users to malicious sites, or perform actions on behalf of authenticated users. The vulnerability particularly affects web applications that accept and display user-generated content through LeafKit templates, creating potential attack vectors for any application that processes user input through collection rendering mechanisms. This issue can be exploited by attackers who craft malicious input designed to inject script tags or other executable code that will be rendered unescaped in the browser. The consequences of exploitation can range from minor annoyances to complete system compromise, depending on the application's security posture and the privileges of the affected users.
The recommended mitigation strategy involves upgrading to LeafKit version 1.14.2 or later, which contains the necessary fixes to properly implement HTML escaping for collection rendering operations. Organizations should conduct thorough testing of their applications to ensure that the upgrade does not introduce any regressions in functionality while maintaining the security improvements. Additionally, developers should implement defensive programming practices such as input validation and sanitization at multiple layers of their applications, even when relying on templating systems that provide built-in escaping mechanisms. Security teams should also consider implementing automated scanning tools to identify any remaining instances of vulnerable code patterns within their codebases. The fix addresses the root cause by ensuring that all content rendered through the `#(value)` syntax is properly escaped regardless of the data type being displayed. This vulnerability serves as a reminder of the critical importance of proper input sanitization and output encoding in web applications, particularly when dealing with dynamic content rendering and user-provided data.