CVE-2021-39178

XSS in Image Optimization API for Next.js

• Affected: All of the following must be true to be affected
  • Next.js between version 10.0.0 and 11.1.0
  • The next.config.js file has images.domains array assigned
  • The image host assigned in images.domains allows user-provided SVG
• Not affected: The next.config.js file has images.loader assigned to something other than default
• Not affected: Deployments on Vercel are not affected

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-2021-39178 has a CVSS score of 7.5 (High). 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 (11.1.1); upgrading removes the vulnerable code path.

Kodem intelligence

Next.js v11.1.1

1. What is CVE-2021-39178? CVE-2021-39178 is a high-severity cross-site scripting (XSS) vulnerability in next (npm), affecting versions >= 10.0.0, < 11.1.1. It is fixed in 11.1.1. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2021-39178? CVE-2021-39178 has a CVSS score of 7.5 (High). 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.
3. Which versions of next are affected by CVE-2021-39178? next (npm) versions >= 10.0.0, < 11.1.1 is affected.
4. Is there a fix for CVE-2021-39178? Yes. CVE-2021-39178 is fixed in 11.1.1. Upgrade to this version or later.
5. Is CVE-2021-39178 exploitable, and should I be worried? Whether CVE-2021-39178 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
6. What actually determines whether CVE-2021-39178 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.
7. How do I fix CVE-2021-39178? Upgrade next to 11.1.1 or later.