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Medium, CVSS 6.9amended

Protocol-Relative URL Injection via Single Backslash Bypass in Angular SSR

CVE-2026-33397GHSA-vfx2-hv2g-xj5f

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (@angular/ssr 22.0.0-next.2, 21.2.3, 20.3.21), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm @angular/ssr

    Affected
    >= 22.0.0-next.0, < 22.0.0-next.2>= 21.0.0-next.0, < 21.2.3>= 20.0.0-next.0, < 20.3.21
    Fixed in
    20.3.21, 22.0.0-next.2, 21.2.3

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N · score source
EPSS
  • CVE-2026-33397: 0.26% estimated exploitation activity in the next 30 days; percentile 17.79 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

An Open Redirect vulnerability exists in @angular/ssr due to an incomplete fix for CVE-2026-27738. While the original fix successfully blocked multiple leading slashes (e.g., ///), the internal validation logic fails to account for a single backslash (\) bypass. When an Angular SSR application is deployed behind a proxy that passes the X-Forwarded-Prefix header: - An attacker provides a value starting with a single backslash (e.g., \evil.com). - The internal validation failed to flag the single backslash as invalid. - The application prepends a leading forward slash, resulting in a Location header containing /\evil.com. - Modern browsers interpret the /\ sequence as //, treating it as a protocol-relative URL and redirecting the user to the attacker-controlled domain. Furthermore, the response lacks the Vary: X-Forwarded-Prefix header, allowing the malicious redirect to be stored in intermediate caches (Web Cache Poisoning). Impact This vulnerability allows attackers to conduct…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-vfx2-hv2g-xj5f

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-vfx2-hv2g-xj5f

Agent brief

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# Patch Next advisory brief: CVE-2026-33397

Advisory: Protocol-Relative URL Injection via Single Backslash Bypass in Angular SSR
IDs: CVE-2026-33397, GHSA-vfx2-hv2g-xj5f
Record: https://patchnext.com/advisories/cve-2026-33397 (revision 3, source checked 26 Oct 2026, 12:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @angular/ssr 22.0.0-next.2, 21.2.3, 20.3.21. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @angular/ssr: affected >= 22.0.0-next.0, < 22.0.0-next.2; >= 21.0.0-next.0, < 21.2.3; >= 20.0.0-next.0, < 20.3.21; fixed in 20.3.21, 22.0.0-next.2, 21.2.3

## Suggested inspection (read-only)
- `npm ls @angular/ssr --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @angular/ssr.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-vfx2-hv2g-xj5f (OSV record GHSA-vfx2-hv2g-xj5f lists affected ranges: npm @angular/ssr >=22.0.0-next.0 =21.0.0-next.0 =20.0.0-next.0 <20.3.21 (fixed))
- Verified: https://github.com/advisories/GHSA-vfx2-hv2g-xj5f (GitHub-reviewed advisory GHSA-vfx2-hv2g-xj5f lists affected ranges: npm @angular/ssr >=22.0.0-next.0 =21.0.0-next.0 =20.0.0-next.0 <20.3.21)

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