Twig: `template_from_string()` escapes a SourcePolicy-driven sandbox via synthesized template name
Date: 2026-05
Affected: composer/twig/twig
Attack vector: other
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-46634
CWE: CWE-693

### Description When the sandbox is enabled selectively via `SourcePolicyInterface` (and not globally), a sandboxed template that is allowed to call `template_from_string` and `include` can render an arbitrary inner template with no security policy enforcement. `Environment::createTemplate()` compiles the inner string under a synthesized name (`__string_template__<hash>`), so a name/path-based `SourcePolicy` returns `false` for it, and the inner template's `checkSecurity()` becomes a no-op. From a template the integrator believes is sandboxed, an attacker can use any tag/filter/function (including `constant()` to read secrets, or `|map("system")` to execute shell commands). ### Resolution This is a configuration trap rather than a code bug: there is no legitimate use case for exposing `template_from_string` to untrusted template authors, and propagating the parent sandbox state through `template_from_string` would require invasive changes to `SourcePolicyInterface` semantics with their own risks. Starting with Twig 3.26.0, the documentation and the PHPDoc of `StringLoaderExtension::templateFromString()` explicitly warn against allowing `template_from_string` in a sandboxed environment (i.e. listing it in a `SecurityPolicy` allowed-functions list). Integrators using a `SourcePolicyInterface` MUST NOT allow `template_from_string` in their allowed functions; the safest option is not to register `StringLoaderExtension` at all when a sandbox is in use. ### Credits Twig woul


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Architectural layers (MAESTRO):