How severe is CVE-2026-57209?

No CVSS score has been assigned to CVE-2026-57209 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-57209?

CVE-2026-57209 affects the following packages: github.com/dadrus/heimdall (Go). Ecosystems affected: Go.

How do I fix CVE-2026-57209?

Update github.com/dadrus/heimdall to 0.17.17 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-57209 is resolved across your whole dependency graph.

How do I detect CVE-2026-57209 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/dadrus/heimdall. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-57209 if there is no patch (or I can't update yet)?

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 does O3 Security protect against CVE-2026-57209?

O3 pinpoints whether CVE-2026-57209 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-57209 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-57209 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was CVE-2026-57209 published, and has it been updated?

CVE-2026-57209 was published on June 18, 2026 and was last updated on July 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

CVE-2026-57209

CVE-2026-57209 is a security vulnerability in github.com/dadrus/heimdall. O3 Security confirms whether CVE-2026-57209 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

Heimdall: Forwarded Header Injection via Unsanitized Host Header in Proxy Mode

Also known asGO-2026-5125

Published

Jun 18, 2026

Updated

Jul 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Jul 20, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

1 pkg affected

🐹github.com/dadrus/heimdall

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Summary

When Heimdall operates in proxy mode, it constructs the Forwarded HTTP header after executing the matched rule pipeline by inserting the incoming request's Host header value directly into the header string without sanitizing commas or semicolons. This allows an attacker to inject additional parameters into the Forwarded header, potentially spoofing IP addresses for upstream services.

Vulnerable Code

File: proxy/request_context.go (line 201)

entry := "for=" + clientIP + ";host=" + in.Host + ";proto=" + proto

Go's net/http allows commas and semicolons in Host header values. No sanitization is applied before string concatenation.

PoC

# Inject a spoofed IP into the Forwarded header
curl -s -H "Host: evil.com,for=127.0.0.1" \
  "http://TARGET:PORT/protected-resource"

This produces the following Forwarded header sent to the upstream service:

Forwarded: for=1.2.3.4;host=evil.com, for=127.0.0.1;proto=http

Upstream services that parse the Forwarded header according to RFC 7239 will see two entries. If the service trusts the last or any for= value, the attacker successfully spoofs 127.0.0.1 as the client IP.

# More targeted attack: spoof to bypass IP allowlist
curl -s -H "Host: legit.com;for=10.0.0.1;proto=https,for=192.168.1.1" \
  "http://TARGET:PORT/admin-panel"

Impact

IP spoofing: Upstream services behind Heimdall may trust the injected for= value, believing the request originates from an internal/trusted IP
Access control bypass: Applications that restrict access based on IP address by themselves, without using the corresponding heimdall capabilities (e.g., admin panels, internal APIs), can be bypassed
Affects all proxy-mode deployments where upstream services parse the Forwarded header.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/dadrus/heimdall`	all versions	0.17.17

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/dadrus/heimdall. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/dadrus/heimdall to 0.17.17 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-57209 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 pinpoints whether CVE-2026-57209 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-57209. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### Summary When Heimdall operates in proxy mode, it constructs the `Forwarded` HTTP header after executing the matched rule pipeline by inserting the incoming request's `Host` header value directly into the header string without sanitizing commas or semicolons. This allows an attacker to inject additional parameters into the `Forwarded` header, potentially spoofing IP addresses for upstream services. ### Vulnerable Code **File: `proxy/request_context.go` (line 201)** ```go entry := "for=" + clientIP + ";host=" + in.Host + ";proto=" + proto ``` Go's `net/http` allows commas and semicolons

O3 Security · Impact-Aware SCA

Is CVE-2026-57209 in your dependencies?

O3 detects CVE-2026-57209 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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