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docs: document allowedHosts SSR requirement in v21 update guide#68403

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erkamyaman wants to merge 1 commit intoangular:mainfrom
erkamyaman:docs-adev-v21-ssr-allowed-hosts
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docs: document allowedHosts SSR requirement in v21 update guide#68403
erkamyaman wants to merge 1 commit intoangular:mainfrom
erkamyaman:docs-adev-v21-ssr-allowed-hosts

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The GHSA-x288-3778-4hhx patch requires allowedHosts on CommonEngine or SSR silently falls back to CSR. Add a checklist item to the v21 update guide.

fixes #68391

PR Checklist

Please check if your PR fulfills the following requirements:

PR Type

What kind of change does this PR introduce?

  • Bugfix
  • Feature
  • Code style update (formatting, local variables)
  • Refactoring (no functional changes, no api changes)
  • Build related changes
  • CI related changes
  • Documentation content changes
  • angular.dev application / infrastructure changes
  • Other... Please describe:

What is the current behavior?

The Angular update guide does not mention that security patch GHSA-x288-3778-4hhx makes allowedHosts mandatory on CommonEngine. Apps that upgrade without setting it see SSR silently fall back to CSR — HTTP 200 with an empty <app-root></app-root> and only a console.error that's easy to miss.

Issue Number: #68391

What is the new behavior?

The update checklist now includes a step for upgrades that cross v21, instructing SSR users to set allowedHosts on CommonEngine in server.ts and pointing to security advisory GHSA-x288-3778-4hhx as the rationale.

Does this PR introduce a breaking change?

  • Yes
  • No

Other information

Addresses ask (a) from the issue. The other two asks, an ng update ACTION REQUIRED warning and a more visible runtime fallback, involve CLI migration scripts and @angular/ssr runtime behavior, so they're out of scope for this docs PR.

The GHSA-x288-3778-4hhx patch requires `allowedHosts` on
`CommonEngine` or SSR silently falls back to CSR. Add a checklist
item to the v21 update guide.
@pullapprove pullapprove Bot requested a review from josephperrott April 27, 2026 19:26
@angular-robot angular-robot Bot added the area: docs Related to the documentation label Apr 27, 2026
@ngbot ngbot Bot added this to the Backlog milestone Apr 27, 2026
'The `lastSuccessfulNavigation` property on the Router has been converted to a signal. To get its value, you now need to invoke it as a function: `router.lastSuccessfulNavigation()`.',
},
{
possibleIn: 2100,
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This is actually not possible in 21.0.0, it became available in 21.1.5. So we can't include it in the v21 update guide, it will need to be in v22 as our update guide typically only shows instructions for major versions.

While its possible to define support for 21.1.5 as the target version in the update guide I don't think that we would want to have it included here, from an update guide perspective this should be in v22. If you want the action text to include that its available as of 21.1.5 I think that is fine as well.

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I would have said that this is actually okay. The update guide itself recommands running ng update @angular/core@21 @angular/cli@21 which will update to 21.2+.

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SSR silently falls back to CSR after security patch (GHSA-x288-3778-4hhx) — allowedHosts not mentioned in migration guide

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