CVE-2026-114127 in Astro
Summary
by MITRE • 10/21/2026
Astro is a web framework for content-driven websites. From 7.1.0 until 7.3.3, Astro's server-side View Transition CSS generator does not keep string animation properties within their generated declarations. An application that passes attacker-controlled data into properties such as duration, delay, easing, direction, fillMode, or animation names can allow CSS delimiters to terminate the declaration and add attacker-selected rules to the server-rendered page. The generated stylesheet still escapes literal less-than characters, so the injected content cannot terminate the style element or directly inject HTML or JavaScript. The injected CSS can conceal or reposition content, add generated text, and otherwise misrepresent the interface; some applications may also expose data already present in selectable DOM attributes through conditional resource loads. This issue is fixed in 7.3.3.
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Analysis
by VulDB Data Team • 10/21/2026
The vulnerability identified within Astro versions ranging from 7.1.0 to 7.3.3 constitutes a server-side CSS injection flaw rooted in the framework's handling of View Transition animations. As a web framework designed for content-driven sites, Astro generates stylesheets dynamically on the server side to support smooth visual transitions between page states. The core technical deficiency lies in the sanitization logic applied to string animation properties such as duration, delay, easing, direction, fillMode, and specific animation names. When an application developer passes user-controlled or attacker-influenced data into these properties without adequate validation, the framework fails to properly escape CSS delimiters like semicolons or closing braces within those strings. This oversight allows an adversary to terminate the intended declaration prematurely and inject arbitrary CSS rules directly into the server-rendered stylesheet.
From a technical perspective, this flaw is classified under CWE-79, which covers Cross-site Scripting (XSS), although it manifests specifically as CSS Injection rather than direct script execution. The attack vector relies on the ability to manipulate input fields that are mapped to animation properties in the View Transition API. Because the generated stylesheet escapes literal less-than characters, the attacker cannot break out of the style element to inject HTML tags or execute JavaScript directly via standard XSS vectors. However, this limitation does not render the vulnerability harmless; instead, it shifts the impact from code execution to visual manipulation and data exfiltration through CSS-based side channels. The injected styles can alter the rendering engine's behavior by concealing critical interface elements, repositioning content to deceive users, or adding generated text that misrepresents the application's functionality.
The operational impact of this vulnerability extends beyond mere aesthetic disruption. In many modern web applications, visual presentation is tightly coupled with data visibility and access control mechanisms. By manipulating CSS properties such as display, opacity, position, or z-index through injection, an attacker can hide security warnings, overlays, or authentication prompts while maintaining the appearance that they are present. Furthermore, sophisticated attacks leveraging this flaw can exploit conditional resource loads based on DOM attributes. If sensitive data is stored in selectable DOM elements and exposed via CSS selectors for styling purposes, an attacker might construct malicious rules to trigger requests to external servers controlled by them, effectively exfiltrating information through beaconing techniques or other HTTP-based side channels. This aligns with the ATT&CK technique of Client-side Rendering Manipulation, where attackers alter how content is presented to users without modifying the underlying HTML structure directly.
Mitigation strategies for this vulnerability primarily involve upgrading to Astro version 7.3.3 or later, which addresses the sanitization logic in the server-side View Transition CSS generator. For applications that cannot immediately upgrade, developers must implement strict input validation and output encoding on all data passed into animation properties. This includes ensuring that special characters like semicolons, commas, and parentheses are properly escaped before being interpolated into style declarations. Additionally, implementing Content Security Policy (CSP) headers can help mitigate the impact by restricting where stylesheets can be loaded from and preventing unauthorized external resource loads, thereby reducing the effectiveness of data exfiltration attempts via CSS-based side channels. Regular security audits focusing on dynamic stylesheet generation are also recommended to identify similar patterns in other parts of the application stack.