Heimdall: IP Spoofing via Unvalidated Forwarding Headers

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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 the trusted_proxies option is configured, heimdall extracts client IP addresses from the Forwarded (for= parameter) and X-Forwarded-For headers and exposes them as Request.ClientIPAddresses to the rule pipeline. However, extracted values are not validated to be syntactically valid IP addresses. Arbitrary strings, malformed IP literals, and RFC 7239 unknown values and obfuscated identifiers are accepted without further checks. In addition, the Forwarded header parser splits on , and ; without accounting for RFC 7239 quoted strings, which can cause a single quoted value to be parsed as multiple entries, with fragments — including trailing quote characters — treated as independent addresses. This issue has been patched in version 0.17.17.

Source published

23 Oct 2026 12:28:09 UTC

Source modified

23 Oct 2026 12:28:09 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 the trusted_proxies option is configured, heimdall extracts client IP addresses from the Forwarded (for= parameter) and X-Forwarded-For headers and exposes them as Request.ClientIPAddresses to the rule pipeline. However, extracted values are not validated to be syntactically valid IP addresses. Arbitrary strings, malformed IP literals, and RFC 7239 unknown values and obfuscated identifiers are accepted without further checks. In addition, the Forwarded header parser splits on , and ; without accounting for RFC 7239 quoted strings, which can cause a single quoted value to be parsed as multiple entries, with fragments — including trailing quote characters — treated as independent addresses. 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 14:19:16 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 the `trusted_proxies` option is configured, heimdall extracts client IP addresses from the `Forwarded` (`for=` parameter) and `X-Forwarded-For` headers and exposes them as `Request.ClientIPAddresses` to the rule pipeline. However, extracted values are not validated to be syntactically valid IP addresses. Arbitrary strings, malformed IP literals, and RFC 7239 `unknown` values and obfuscated identifiers are accepted without further checks. In addition, the `Forwarded` header parser splits on `,` and `;` without accounting for RFC 7239 quoted strings, which can cause a single quoted value to be parsed as multiple entries, with fragments — including trailing quote characters — treated as independent addresses. ### Impact `Request.ClientIPAddresses` is available to all pipeline mechanisms. Its contents can therefore influence rule evaluation in deployments where rules reference this property — for example, in a `CEL` authorizer that checks whether a request originates from a trusted IP range using the `networks()` function, or in a `Remote` authorizer that forwards the client IP as part of its payload to an external authorization system. Whether and how `Request.ClientIPAddresses` is used is entirely determined by the rule configuration. Additionally, in proxy mode, `Request.ClientIPAddresses` is used directly to construct the `X-Forwarded-For` and `Forwarded` headers forwarded to upstream services. Injected or malformed values are therefore propagated to upstream services unchanged. ### Attack Scenarios All scenarios require that `trusted_proxies` is configured. If this option is not set, heimdall ignores forwarding headers entirely, and this vulnerability is not exploitable. Scenarios A and C (see below) additionally require that rules reference `Request.ClientIPAddresses` in their pipeline. #### Scenario A – Manipulation of rule evaluation An attacker who can influence forwarding headers — either by connecting directly to heimdall or through a proxy that does not sanitize these headers — can inject arbitrary values into `Request.ClientIPAddresses`. In deployments where a rule references this property (e.g. to restrict access to specific IP ranges), this may allow an attacker to bypass the intended access control logic. #### Scenario B – IP spoofing against upstream services (proxy mode) In proxy mode, injected or malformed values in `Request.ClientIPAddresses` are written unchanged into the `X-Forwarded-For` header sent to upstream services. Upstream services that trust this header may therefore receive and act on attacker-controlled IP values. #### Scenario C – Malformed entries via quoted-string misparse A `Forwarded` header containing a quoted value with embedded delimiters (`,` or `;`) is misparsed, producing unintended additional entries in `Request.ClientIPAddresses`, including malformed fragments with trailing quote characters. ### Workarounds * Ensure at the network level that only trusted proxies can communicate directly with heimdall. * Ensure that the proxy forwarding the requests to heimdall sanitizes or overrides (not merely appends to) `Forwarded` or `X-Forwarded-For` headers before forwarding them. * Avoid relying on `Request.ClientIPAddresses` for security-sensitive decisions until patched

Source published

18 Jun 2026 14:24:37 UTC

Source modified

11 Aug 2026 20:51:05 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

When the trusted_proxies option is configured, heimdall extracts client IP addresses from the Forwarded and X-Forwarded-For headers and exposes them as Request.ClientIPAddresses to the rule pipeline. However, extracted values are not validated to be syntactically valid IP addresses. Arbitrary strings, malformed IP literals, and RFC 7239 unknown values and obfuscated identifiers are accepted without further checks. In addition, the Forwarded header parser splits on commas and semicolons without accounting for RFC 7239 quoted strings, which can cause a single quoted value to be parsed as multiple entries, with fragments treated as independent addresses. Request.ClientIPAddresses is available to all pipeline mechanisms. Its contents can therefore influence rule evaluation in deployments where rules reference this property. Additionally, in proxy mode, Request.ClientIPAddresses is used directly to construct the X-Forwarded-For and Forwarded headers forwarded to upstream services. Injected or malformed values are therefore propagated to upstream services unchanged. An attacker who can influence forwarding headers can inject arbitrary values into Request.ClientIPAddresses. In deployments where a rule references this property, this may allow an attacker to bypass the intended access control logic. In proxy mode, upstream services that trust this header may receive and act on attacker-controlled IP values.

Source published

11 Aug 2026 20:19:48 UTC

Source modified

11 Aug 2026 20:33:01 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.