CVE-2026-48807 Overview
CVE-2026-48807 is a sandbox bypass vulnerability in Twig, the template language for PHP maintained by Symfony. The flaw affects versions prior to 3.27.0. The sandbox __toString() checks fail to cover Traversable values passed to the join and replace filters, or operands evaluated by the in and not in operators. Contained Stringable objects are coerced to strings without consulting the sandbox policy. The issue is classified as [CWE-693: Protection Mechanism Failure]. Version 3.27.0 resolves the vulnerability.
Critical Impact
Authenticated attackers with template authoring privileges can bypass Twig sandbox restrictions and trigger __toString() methods on objects that the security policy would otherwise block.
Affected Products
- Symfony Twig versions prior to 3.27.0
- PHP applications embedding Twig with the sandbox extension enabled
- Content management and multi-tenant platforms exposing Twig templates to untrusted authors
Discovery Timeline
- 2026-07-14 - CVE-2026-48807 published to NVD
- 2026-07-17 - Last updated in NVD database
Technical Details for CVE-2026-48807
Vulnerability Analysis
Twig ships a sandbox extension that restricts which methods, properties, and functions untrusted templates may access. When a template triggers implicit string conversion on an object, the sandbox is expected to consult its policy before allowing the object's __toString() method to execute. This check is critical because __toString() implementations frequently contain application logic that leaks data or produces side effects.
The vulnerability stems from incomplete enforcement across specific Twig constructs. The join filter, replace filter, and the in and not in operators accept Traversable values. When these constructs iterate a Traversable containing Stringable objects, Twig coerces each element to a string without invoking the sandbox __toString() policy check. Attackers who can author or influence Twig templates can therefore reach __toString() on objects that the sandbox would normally reject.
Root Cause
The sandbox __toString() guard is applied to direct string coercions but not to the iteration paths inside join, replace, in, and not in. The security policy is bypassed for any Stringable element reached through a Traversable argument passed to these constructs. This is a protection mechanism failure rather than a memory safety or injection defect.
Attack Vector
Exploitation requires the ability to submit Twig template source that will be rendered inside a sandboxed context. This scenario is common in software-as-a-service platforms, email template editors, and CMS themes that accept user-supplied Twig markup. An attacker crafts a template that passes a controlled Traversable of Stringable objects to join, replace, in, or not in, causing the sandbox to skip the policy check and invoke the target __toString() method. The impact depends on what application-defined __toString() methods are reachable in the runtime object graph, and can include disclosure of confidential fields or triggering of unintended business logic.
No verified public proof-of-concept code is available for this issue. Refer to the GitHub Security Advisory GHSA-8x9c-rmqh-456c for authoritative technical details.
Detection Methods for CVE-2026-48807
Indicators of Compromise
- Sandboxed Twig templates containing join, replace, in, or not in operations against variables that resolve to Traversable objects rather than plain arrays or strings.
- Application logs recording unexpected invocations of __toString() methods on domain objects during template rendering.
- Template submissions from low-privilege authors that reference internal service or model objects exposed to the sandbox context.
Detection Strategies
- Inventory all Twig deployments and identify versions below 3.27.0 using dependency manifests such as composer.lock.
- Audit custom Twig extensions and template globals for objects that implement __toString() and expose sensitive state.
- Review template repositories for unexpected use of the join and replace filters or the in and not in operators against non-array inputs.
Monitoring Recommendations
- Enable Twig sandbox policy logging in staging and production to surface allow and deny decisions on method access.
- Instrument __toString() methods on sensitive classes with lightweight audit logging to detect anomalous callers.
- Forward web application logs and PHP error streams to a centralized analytics platform to correlate template rendering anomalies with authentication events.
How to Mitigate CVE-2026-48807
Immediate Actions Required
- Upgrade Twig to version 3.27.0 or later across all applications that rely on the sandbox extension.
- Restrict template authoring to trusted users until the upgrade is completed and validated.
- Review sandbox security policies and remove any objects from the template context that are not required for rendering.
Patch Information
The fix is included in Twig 3.27.0. Sandbox __toString() checks now cover Traversable values processed by the join and replace filters and by the in and not in operators. See the GitHub Release v3.27.0 notes and the GitHub Security Advisory GHSA-8x9c-rmqh-456c for release details.
Workarounds
- Reject or sanitize submitted templates that reference join, replace, in, or not in when a patched Twig version cannot be deployed immediately.
- Remove Traversable objects containing sensitive Stringable elements from the sandbox template context.
- Implement defensive __toString() methods on exposed classes that verify caller context before returning sensitive data.
# Upgrade Twig to the patched release using Composer
composer require "twig/twig:^3.27.0"
composer update twig/twig
# Verify the installed version
composer show twig/twig | grep versions
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
