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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

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

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

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.

What is the EPSS score for CVE-2026-57209?

CVE-2026-57209 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 27th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-57209?

CVE-2026-57209 is classified as Improper Input Validation (CWE-20), CWE-74 (CWE-74). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

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

CVE-2026-57209 was published on October 23, 2026 and was last updated on October 25, 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 — heimdall

CVE-2026-57209 is a Improper Input Validation vulnerability in github.com/dadrus/heimdall. A fix is available for github.com/dadrus/heimdall — see the affected versions and patch details below.

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

Also known asGHSA-4jgr-pg2m-m988GO-2026-5125

Published

Oct 23, 2026

Updated

Oct 25, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 27, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs27th percentile — riskier than 27% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from 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`go get github.com/dadrus/heimdall@v0.17.17`

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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

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