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Kirby: External Initialization of the Panel on reverse proxy setups with the `Forwarded` header

Critical severity GitHub Reviewed Published Jun 17, 2026 in getkirby/kirby • Updated Jun 18, 2026

Package

composer getkirby/cms (Composer)

Affected versions

<= 4.9.3
>= 5.0.0-alpha.1, <= 5.4.3

Patched versions

4.9.4
5.4.4

Description

TL;DR

This vulnerability affects Kirby sites that have no configured user accounts and are running on publicly accessible servers behind a reverse proxy that sets the Forwarded: for=..., X-Client-IP, or X-Real-IP request header.

It was possible to install the Panel (= create the first admin user) in these setups even from remote IP addresses.

This vulnerability is of critical severity for affected sites.

Your site is not affected if any of the following apply:

  • An admin account has already been configured
  • The Panel and API are disabled
  • The site is not running behind a reverse proxy
  • The reverse proxy sets the X-Forwarded-For or Client-IP header instead of the affected ones.

Introduction

External Initialization is a type of vulnerability that allows attackers to initialize a system or configuration value without authentication.

This can give untrusted actors access to the system or let them control its behavior.

Affected components

The Kirby Panel and REST API are authenticated by local user accounts. If a Kirby installation does not yet have any users, it first needs to be installed. During the installation process, an initial admin user account is created.

To protect against external initialization attacks that would allow untrusted actors to create an admin user for the Kirby installation, Kirby already checked whether the current request came from a local IP address. This allows installing the Panel in local development setups. Installation on remote servers was only supposed to be possible when the panel.install configuration option was enabled.

The isLocal check takes all relevant request headers into account and treats a request as non-local as soon as any checked request header contains an external IP address.

Impact

In affected releases, the isLocal check for the installation logic did not properly take the Forwarded: for=... header into account. This header is set by modern reverse proxy servers. It also did not take into account the X-Client-IP or X-Real-IP headers, which are set by some custom reverse proxy setups.

This caused Kirby to falsely assume that an installation request was local and allowed creating an admin account even though the reverse proxy forwarded the request from an external IP address.

Reverse proxies setting the X-Forwarded-For or Client-IP headers were not affected. These headers were already properly checked for external IP addresses.

Patches

The problem has been patched in Kirby 4.9.4 and Kirby 5.4.4. Please update to one of these or a later version to fix the vulnerability.

In all of the mentioned releases, we fixed the isLocal check to also properly take Forwarded: for=..., X-Client-IP and X-Real-IP request headers into account.

Workarounds

Sites on older Kirby versions (Kirby 3 starting at 3.7.0) can be protected with one of the following workarounds:

  • Perform the Panel installation yourself by creating an initial admin account. As soon as one or more accounts are present, the vulnerable installation code is no longer active.
  • If you don't need the Panel, disable the REST API with the 'api' => false option in config.php.

Credits

Thanks to Peter Levashov (@petersevera) for responsibly reporting the identified issue.

References

@bastianallgeier bastianallgeier published to getkirby/kirby Jun 17, 2026
Published to the GitHub Advisory Database Jun 18, 2026
Reviewed Jun 18, 2026
Last updated Jun 18, 2026

Severity

Critical

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 High
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
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:H/VI:H/VA:N/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(42nd percentile)

Weaknesses

External Initialization of Trusted Variables or Data Stores

The product initializes critical internal variables or data stores using inputs that can be modified by untrusted actors. Learn more on MITRE.

CVE ID

CVE-2026-54003

GHSA ID

GHSA-whxw-24jc-cwmv

Source code

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