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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

7.0 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N · score source

EPSS

• CVE-2026-57209: 0.49% estimated exploitation activity in the next 30 days; percentile 42.08 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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/requestcontext.go (line 201) Go's net/http allows commas and semicolons in Host header values. No sanitization is applied before string concatenation. PoC This produces the following Forwarded header sent to the upstream service: 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. Impact - IP spoofing: Upstream services behind Heimdall may trust the injected for= value, believing the request originates from an…

Published by the source

18 Jun 2026, 14:24 UTC

Last modified by the source

20 Jul 2026, 21:28 UTC

Source last checked

26 Oct 2026, 15:25 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-4jgr-pg2m-m988

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# Patch Next advisory brief: CVE-2026-57209

Advisory: Heimdall: Forwarded Header Injection via Unsanitized Host Header in Proxy Mode
IDs: CVE-2026-57209, GHSA-4jgr-pg2m-m988
Record: https://patchnext.com/advisories/cve-2026-57209 (revision 1, source checked 26 Oct 2026, 15:25 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/dadrus/heimdall 0.17.17. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/dadrus/heimdall: affected <= 0.17.16; fixed in 0.17.17

## Suggested inspection (read-only)
- Go github.com/dadrus/heimdall: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/dadrus/heimdall.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-4jgr-pg2m-m988 (GitHub-reviewed advisory GHSA-4jgr-pg2m-m988 lists affected ranges: Go github.com/dadrus/heimdall >=0 <=0.17.16)

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