Changelog

Multiple security vulnerabilities were disclosed by the React team and Vercel affecting React Server Components and Next.js. These include denial of service, middleware and proxy bypass, server-side request forgery, cross-site scripting, and cache poisoning issues across a range of severity levels.

We strongly recommend updating your application and its dependencies immediately. Patched versions are available for React (react-server-dom-webpack, react-server-dom-parcel, and react-server-dom-turbopack 19.0.6, 19.1.7, and 19.2.6) and Next.js (15.5.16 and 16.2.5).

Cloudflare WAF rules deployed in response to prior React Server Component CVEs (CVE-2025-55184 ↗︎ and CVE-2026-23864 ↗︎) already provide coverage for the newly disclosed denial-of-service vulnerabilities. These rules are enabled by default with a Block action for all customers using the Cloudflare Managed Ruleset, including Free plan customers using the Free Managed Ruleset.

The existing rules detect the underlying attack patterns generically. As a result, they apply to the new CVE-2026-23870 ↗︎ denial-of-service vulnerability in Server Components and the corresponding Next.js advisory GHSA-8h8q-6873-q5fj ↗︎.

Cloudflare is investigating whether WAF rules can be safely and effectively deployed for three of the high-severity advisories: CVE-2026-23870 ↗︎ / GHSA-8h8q-6873-q5fj ↗︎, GHSA-267c-6grr-h53f ↗︎, and GHSA-mg66-mrh9-m8jx ↗︎. If it is possible to create a managed WAF rule that mitigates these CVEs and does not potentially break application behavior, Cloudflare will add additional managed WAF rules. These rules will be announced through the WAF changelog. Because these vulnerabilities were shared with Cloudflare with minimal advance notice, we are still investigating what WAF mitigations are possible.

Several of the disclosed vulnerabilities are not possible to block in WAF. We strongly recommend updating your applications so they are not purely reliant on WAF mitigations.

Customers on Pro, Business, or Enterprise plans should ensure that Managed Rules are enabled.

Vinext: Vinext ↗︎ is a Vite plugin that reimplements the Next.js API surface. Vinext's latest release is not vulnerable to any of the disclosed CVEs. Vinext's architecture differs from stock Next.js in ways that sidestep the affected code paths. For example, it does not implement the PPR resume protocol, does not expose Pages Router data-route endpoints, and strips internal headers such as x-nextjs-data at request boundaries. As an extra layer of defense, we added a React 19.2.6 or later requirement when running vinext init (PR #1118 ↗︎, PR #1112 ↗︎) to prevent accidentally running a vulnerable version of React with Vinext.

OpenNext on Cloudflare: OpenNext is an adapter that lets you deploy Next.js apps to the Cloudflare Workers platform. OpenNext itself is not directly vulnerable to the React denial-of-service CVE, but users must update the Next.js version in their application. The OpenNext team has updated the adapter to further harden against these vectors and released a new version of the Cloudflare adapter. Test fixtures and examples have been updated to use patched versions (PR #1255 ↗︎).

Update: Mon Mar 24th, 11PM UTC: Next.js has made further changes to address a smaller vulnerability introduced in the patches made to its middleware handling. Users should upgrade to Next.js versions 15.2.4, 14.2.26, 13.5.10 or 12.3.6. If you are unable to immediately upgrade or are running an older version of Next.js, you can enable the WAF rule described in this changelog as a mitigation.

Update: Mon Mar 24th, 8PM UTC: Next.js has now backported the patch for this vulnerability ↗︎ to cover Next.js v12 and v13. Users on those versions will need to patch to 13.5.9 and 12.3.5 (respectively) to mitigate the vulnerability.

Update: Sat Mar 22nd, 4PM UTC: We have changed this WAF rule to opt-in only, as sites that use auth middleware with third-party auth vendors were observing failing requests.

We strongly recommend updating your version of Next.js (if eligible) to the patched versions, as your app will otherwise be vulnerable to an authentication bypass attack regardless of auth provider.

This rule is opt-in only for sites on the Pro plan or above in the WAF managed ruleset.

To enable the rule:

1. Head to Security > WAF > Managed rules in the Cloudflare dashboard for the zone (website) you want to protect.
2. Click the three dots next to Cloudflare Managed Ruleset and choose Edit
3. Scroll down and choose Browse Rules
4. Search for CVE-2025-29927 (ruleId: 34583778093748cc83ff7b38f472013e)
5. Change the Status to Enabled and the Action to Block. You can optionally set the rule to Log, to validate potential impact before enabling it. Log will not block requests.
6. Click Next
7. Scroll down and choose Save

This will enable the WAF rule and block requests with the x-middleware-subrequest header regardless of Next.js version.

For users on the Free plan, or who want to define a more specific rule, you can create a Custom WAF rule to block requests with the x-middleware-subrequest header regardless of Next.js version.

To create a custom rule:

1. Head to Security > WAF > Custom rules in the Cloudflare dashboard for the zone (website) you want to protect.
2. Give the rule a name - e.g. next-js-CVE-2025-29927
3. Set the matching parameters for the rule match any request where the x-middleware-subrequest header exists per the rule expression below.

(len(http.request.headers["x-middleware-subrequest"]) > 0)

4. Set the action to 'block'. If you want to observe the impact before blocking requests, set the action to 'log' (and edit the rule later).
5. Deploy the rule.

We've made a WAF (Web Application Firewall) rule available to all sites on Cloudflare to protect against the Next.js authentication bypass vulnerability ↗︎ (CVE-2025-29927) published on March 21st, 2025.

Note: This rule is not enabled by default as it blocked requests across sites for specific authentication middleware.

• This managed rule protects sites using Next.js on Workers and Pages, as well as sites using Cloudflare to protect Next.js applications hosted elsewhere.
• This rule has been made available (but not enabled by default) to all sites as part of our WAF Managed Ruleset and blocks requests that attempt to bypass authentication in Next.js applications.
• The vulnerability affects almost all Next.js versions, and has been fully patched in Next.js 14.2.26 and 15.2.4. Earlier, interim releases did not fully patch this vulnerability.
• Users on older versions of Next.js (11.1.4 to 13.5.6) did not originally have a patch available, but this the patch for this vulnerability and a subsequent additional patch have been backported to Next.js versions 12.3.6 and 13.5.10 as of Monday, March 24th. Users on Next.js v11 will need to deploy the stated workaround or enable the WAF rule.

The managed WAF rule mitigates this by blocking external user requests with the x-middleware-subrequest header regardless of Next.js version, but we recommend users using Next.js 14 and 15 upgrade to the patched versions of Next.js as an additional mitigation.