Skip to content

Kirby has Server-Side Template Injection (SSTI) via double template resolution in option rendering

High severity GitHub Reviewed Published Apr 23, 2026 in getkirby/kirby • Updated Apr 27, 2026

Package

composer getkirby/cms (Composer)

Affected versions

< 4.9.0
>= 5.0.0, < 5.4.0

Patched versions

4.9.0
5.4.0

Description

TL;DR

This vulnerability affects all Kirby sites that use option fields (checkboxes, color, multiselect, select, radio, tags or toggles) with options from a query or API whose values may not be fully trusted. It also affects direct uses of the OptionsApi or OptionsQuery classes of Kirby's Options package from plugin or site code. The attack requires either an attacker in the group of authenticated Panel users or user interaction of another authenticated user.

This vulnerability is of high severity for affected sites.

Users' Kirby sites are not affected if they are not using any of the mentioned fields or the Options package, if all options are defined statically in the blueprints or if all dynamically gathered options are to be trusted.

Introduction

Server-Side Template Injection vulnerabilities (SSTI) occur when user input is embedded in a template in an unsafe manner and results in remote code execution on the server.

Injected user input is wrongly treated as a template command instead of as a literal string of text. This allows attackers to query arbitrary information from the affected system or call arbitrary methods to perform actions.

In a Kirby site this can be used to access protected site information, alter site content or break site behavior.

Impact

Kirby provides field types (checkboxes, color, multiselect, select, radio, tags and toggles) that offer a fixed set of options from a configured list. This configured list can be statically defined in the blueprint or it can come from a Kirby query or (external) API source. Options coming from a query or API are treated as dynamic.

Static options can contain queries in the form {{ query }} or {< query >} that are then evaluated to a static value. Because the queries are defined in the blueprint, they can be trusted and cannot be controlled by attackers.

However, dynamic options can often not be trusted. This is why the "options from query" and "options from API" modes are intended to resolve the option values and text strings based on queries not defined within the data source but within the blueprint.

Unfortunately, the results of these trusted queries on untrusted source data are run through the query parser a second time in affected Kirby releases.

Because of the double-resolution of dynamic option values and text strings, attackers could place malicious query templates such as {{ users.first.password }} or {{ page.delete }} in the option sources such as page titles or external API data controlled by the attacker. These queries would then be executed when the field is loaded in the Panel. When the attacker directly accesses the respective Panel view, they could get access to information normally hidden from them. As the malicious query templates are loaded for all users, it could also lead to malicious write access when another user with a higher permission level accesses the manipulated Panel view.

Patches

The problem has been patched in Kirby 4.9.0 and Kirby 5.4.0. Please update to one of these or a later version to fix the vulnerability.

In all of the mentioned releases, Kirby has updated the Options logic to no longer double-resolve queries in option values coming from OptionsQuery or OptionsApi sources. Kirby now only resolves queries that are directly configured in the blueprints.

Credits

Kirby thanks to @offset for responsibly reporting the identified issue.

References

@bastianallgeier bastianallgeier published to getkirby/kirby Apr 23, 2026
Published to the GitHub Advisory Database Apr 23, 2026
Reviewed Apr 23, 2026
Published by the National Vulnerability Database Apr 24, 2026
Last updated Apr 27, 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 Low
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:L/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.
(11th percentile)

Weaknesses

Improper Neutralization of Special Elements Used in a Template Engine

The product uses a template engine to insert or process externally-influenced input, but it does not neutralize or incorrectly neutralizes special elements or syntax that can be interpreted as template expressions or other code directives when processed by the engine. Learn more on MITRE.

CVE ID

CVE-2026-34587

GHSA ID

GHSA-jcjw-58rv-c452

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.