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

> Attackers can redirect users to malicious sites via a backslash bypass in Angular SSR when deployed behind a proxy, potentially enabling cache poisoning.

- **Severity:** Medium 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Mar 2026
- **Updated:** 30 Mar 2026
- **Also known as:** GHSA-vfx2-hv2g-xj5f

## What is CVE-2026-33397?

CVE-2026-33397 is a medium-severity vulnerability in angular-cli, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2026-33397?

6 open source packages ship an affected version, including @angular/ssr. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@angular/ssr` | npm | From 20.0.0-next.0 | 20.3.21 |
| `@angular/ssr` | npm | From 20.0.0-next.0 | 20.3.21 |
| `@angular/ssr` | npm | From 21.0.0-next.0 | 21.2.3 |
| `@angular/ssr` | npm | From 21.0.0-next.0 | 21.2.3 |
| `@angular/ssr` | npm | From 22.0.0-next.0 | 22.0.0-next.2 |
| `@angular/ssr` | npm | From 22.0.0-next.0 | 22.0.0-next.2 |

Products: Angular Angular Cli.

## How do I fix CVE-2026-33397?

Upgrade @angular/ssr to a fixed version for your release line: 20.3.21, 21.2.3 or 22.0.0-next.2.

## Is CVE-2026-33397 being exploited?

There is no known exploitation of CVE-2026-33397 in the wild and no public exploit code.

## Other vulnerabilities in angular-cli

- [CVE-2026-32635](https://insights.sbomify.com/CVE-2026-32635/): Angular has XSS in i18n attribute bindings (high 8.6)
- [CVE-2026-44437](https://insights.sbomify.com/CVE-2026-44437/): Angular SSR: Open Redirect and Request Steering via Encoded X-Forwarded-Prefix (medium 6.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33397 and fix it if it is affected.

What it is: CVE-2026-33397 is a medium-severity vulnerability in angular-cli, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-33397 in the wild and no public exploit code.

Affected packages and fixed versions:
- @angular/ssr (npm): affected from 20.0.0-next.0; fixed in 20.3.21
- @angular/ssr (npm): affected from 20.0.0-next.0; fixed in 20.3.21
- @angular/ssr (npm): affected from 21.0.0-next.0; fixed in 21.2.3
- @angular/ssr (npm): affected from 21.0.0-next.0; fixed in 21.2.3
- @angular/ssr (npm): affected from 22.0.0-next.0; fixed in 22.0.0-next.2
- @angular/ssr (npm): affected from 22.0.0-next.0; fixed in 22.0.0-next.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33397/ (Markdown: https://insights.sbomify.com/CVE-2026-33397/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33397
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33397
- OSV: https://osv.dev/vulnerability/CVE-2026-33397

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33397/
Summaries are generated and may contain mistakes; the official description above is the record.