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New Managed WAF rule for Next.js CVE-2025-29927.

Mar 22, 2025, 01:00 PM
Workers Pages WAF Rules

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
Enable the CVE-2025-29927 rule

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

Create a WAF rule (manual)

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.
Terminal window
(len(http.request.headers["x-middleware-subrequest"]) > 0)
  1. 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).
  2. Deploy the rule.
Next.js CVE-2025-29927 WAF rule

Next.js CVE-2025-29927

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.