CVE-2026-33397 — @angular/ssr
Fix: angular/angular-cli#32771CVE-2026-33397 is a Open Redirect vulnerability in @angular/ssr. A fix is available for @angular/ssr — see the affected versions and patch details below.
Angular SSR Vulnerable to Protocol-Relative URL Injection via Single Backslash Bypass
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33397.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@angular/ssrnpmDescription
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
Locationheader 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 large-scale phishing and SEO hijacking:
- Scale: A single request can poison a high-traffic route, impacting all users until the cache expires.
- SEO Poisoning: Search engine crawlers may follow and index these malicious redirects, causing the legitimate site to be delisted or associated with malicious domains.
- Trust: Because the initial URL belongs to the trusted domain, users and security tools are less likely to flag the redirect as malicious.
Patches
- 22.0.0-next.2
- 21.2.3
- 20.3.21
Workarounds
Until the patch is applied, developers should sanitize the X-Forwarded-Prefix header in their server.ts before the Angular engine processes the request:
app.use((req, res, next) => {
const prefix = req.headers['x-forwarded-prefix'];
if (typeof prefix === 'string') {
// Sanitize by removing all leading forward and backward slashes
req.headers['x-forwarded-prefix'] = prefix.trim().replace(/^[/\\]+/, '/');
}
next();
});
References
- Fix: https://github.com/angular/angular-cli/pull/32771
- Original CVE: CVE-2026-27738
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @angular/ssr | ≥ 22.0.0-next.0&&< 22.0.0-next.2 | 22.0.0-next.2npm install @angular/ssr@22.0.0-next.2 |
| 📦npm | @angular/ssr | ≥ 21.0.0-next.0&&< 21.2.3 | 21.2.3npm install @angular/ssr@21.2.3 |
| 📦npm | @angular/ssr | ≥ 20.0.0-next.0&&< 20.3.21 | 20.3.21npm install @angular/ssr@20.3.21 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @angular/ssr, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @angular/ssr to 22.0.0-next.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33397 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-33397 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-33397. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-33397 in your dependencies?
O3 Security finds CVE-2026-33397 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.