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Twig: Sandbox filter, tag and function allow-list bypass when sandbox state changes between renders for a cached `Template`

High severity GitHub Reviewed Published May 27, 2026 in twigphp/Twig • Updated Jul 1, 2026

Package

composer twig/twig (Composer)

Affected versions

<= 3.26.0

Patched versions

3.27.0

Description

Description

The per-template filter, tag and function allow-list check is compiled into the checkSecurity() method of each Template subclass and was invoked once from the constructor, gated by SandboxExtension::isSandboxed($source). Template instances are then cached on the Environment in $loadedTemplates, so the verdict computed at construction time was sticky for the rest of the process.

Any later change of sandbox state on the same Environment left that cached verdict in place: toggling SandboxExtension::enableSandbox()/disableSandbox(), swapping the policy via setSecurityPolicy(), a SourcePolicyInterface decision flip, or simply having a parent, macro or included template pre-instantiated outside the sandbox before a sandboxed render reached it. In all of these cases, the filters, tags and functions used by the affected template kept running with the original (typically empty) check, bypassing the SecurityPolicy allow-list.

Method, property and __toString allow-lists are not affected: they are enforced at every call site at runtime through SandboxExtension::checkMethodAllowed(), checkPropertyAllowed() and ensureToStringAllowed(), which re-read the current state on every call.

Long-lived workers (FrankenPHP, RoadRunner, Symfony Messenger consumers, FPM with hot autoloading) that share a single Environment between sandboxed and non-sandboxed renders are the most exposed: a single non-sandboxed render of a shared layout pre-warms its Template instance, after which any later sandboxed render that extends, uses, includes or imports from that layout silently skips the filter/tag/function allow-list for the pre-warmed instance.

Resolution

The allow-list check is no longer run from the constructor. Template gains a public ensureSecurityChecked() method that calls the compiled checkSecurity() only when SandboxExtension::isSandboxed($source) returns true for the current source, and it is invoked at every entry point that can reach a Template instance whose security has not yet been verified against the current state: Template::yield(), Template::yieldBlock() (on the resolved block template, which covers extends, use, traits and parent blocks), Template::getParent() (which evaluates user code when the parent name is dynamic) and Template::getTemplateForMacro() (on the resolved macro template).

The explicit checkSecurity() calls previously emitted by IncludeNode and CoreExtension::include() are removed: the included template's own yield() now re-runs the check against the current sandbox state. The compiled checkSecurity() body is a cheap walk over compile-time-static arrays, so the per-render cost is negligible. Old cached compiled PHP files keep working unchanged: the constructor-time call they still contain is idempotent.

Credits

Twig would like to thank Fabien Potencier for reporting and fixing the issue.

References

@fabpot fabpot published to twigphp/Twig May 27, 2026
Published to the GitHub Advisory Database Jul 1, 2026
Reviewed Jul 1, 2026
Last updated Jul 1, 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 None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
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:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Protection Mechanism Failure

The product does not use or incorrectly uses a protection mechanism that provides sufficient defense against directed attacks against the product. Learn more on MITRE.

Incorrect Authorization

The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. Learn more on MITRE.

CVE ID

CVE-2026-49981

GHSA ID

GHSA-529h-vh3j-85hq

Source code

Credits

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