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Heimdall: IP Spoofing via Unvalidated Forwarding Headers

High severity GitHub Reviewed Published Jun 17, 2026 in dadrus/heimdall • Updated Jul 20, 2026

Package

gomod https://github.com/dadrus/heimdall (Go)

Affected versions

<= 0.17.16

Patched versions

0.17.17

Description

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

References

@dadrus dadrus published to dadrus/heimdall Jun 17, 2026
Published to the GitHub Advisory Database Jun 18, 2026
Reviewed Jun 18, 2026
Last updated Jul 20, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity High
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Weaknesses

Improper Input Validation

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Learn more on MITRE.

Improper Encoding or Escaping of Output

The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved. Learn more on MITRE.

Authentication Bypass by Spoofing

This attack-focused weakness is caused by incorrectly implemented authentication schemes that are subject to spoofing attacks. Learn more on MITRE.

CVE ID

CVE-2026-57210

GHSA ID

GHSA-38x9-25wx-7fg2

Source code

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