Summary
Description
In affected versions of the Next.js SDK, simultaneous requests that trigger a nonce retry may cause the proxy cache fetcher to perform improper lookups for the token request results.
Which Projects are Affected?
Users are affected if they meet all of the following preconditions:
- Applications using the auth0/nextjs-auth0 SDK, versions 4.12.0 to 4.17.0, and
- Applications using the proxy handler /me/* and /my-org/* with DPoP enabled.
Affected product and versions
Auth0/nextjs-auth0 v4.12.0 to 4.17.0
Resolution
Upgrade Auth0/nextjs-auth0 version to v4.18.0 or greater
Acknowledgements
Okta would like to thank Reynaldo Immanuel for their discovery and responsible disclosure.
Impact
Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing. Typical impact: TOCTOU exploits, data corruption, or privilege escalation.
CVE-2026-40155 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low 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 (4.18.0); 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-2026-40155? CVE-2026-40155 is a medium-severity race condition vulnerability in @auth0/nextjs-auth0 (npm), affecting versions >= 4.12.0, <= 4.17.0. It is fixed in 4.18.0. Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing.
- How severe is CVE-2026-40155? CVE-2026-40155 has a CVSS score of 5.4 (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 @auth0/nextjs-auth0 are affected by CVE-2026-40155? @auth0/nextjs-auth0 (npm) versions >= 4.12.0, <= 4.17.0 is affected.
- Is there a fix for CVE-2026-40155? Yes. CVE-2026-40155 is fixed in 4.18.0. Upgrade to this version or later.
- Is CVE-2026-40155 exploitable, and should I be worried? Whether CVE-2026-40155 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-2026-40155 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-2026-40155? Upgrade
@auth0/nextjs-auth0to 4.18.0 or later.