Executive Summary

The vulnerability in PyBlade versions 0.1.8-alpha and 0.1.9-alpha arises from a logic flaw in the _is_safe_ast() function within sandbox.py, which improperly validates Abstract Syntax Tree (AST) nodes during template rendering.

Specifically, the function only checks ast.Name nodes against a whitelist but ignores ast.Constant nodes, allowing attackers to bypass security checks and access dangerous Python magic methods like __class__.

This enables attackers to traverse Python's object model (e.g., __mro__, __subclasses__) and inject malicious code remotely via crafted templates.

In version 0.2.0-alpha, the vulnerability is even more severe because the evaluator.py file uses Python's eval() function directly on expressions without any AST validation, making arbitrary code execution trivial.

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Impact Analysis

This vulnerability allows remote attackers to perform arbitrary code execution on the server by injecting malicious templates.

Successful exploitation can lead to full system compromise, including unauthorized access to system commands and sensitive data.

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Detection Guidance

The vulnerability in PyBlade versions 0.1.8-alpha and 0.1.9-alpha arises from unsafe AST validation in the _is_safe_ast() function, allowing Server-Side Template Injection (SSTI) and Remote Code Execution (RCE). Detection involves identifying if your system is running these vulnerable versions and if templates are being rendered without proper AST validation.

To detect exploitation attempts, monitor logs for suspicious template expressions that attempt to access Python's object model, such as payloads containing '__class__', '__mro__', or '__subclasses__'.

• Check the PyBlade version installed on your system to confirm if it is vulnerable (v0.1.8-alpha or v0.1.9-alpha).
• Search application logs or template inputs for suspicious template expressions like: {{ ''.__class__.__mro__[1].__subclasses__() }} or similar patterns.
• Use commands such as grep or similar tools to scan source code or logs for unsafe eval usage or the presence of the vulnerable _is_safe_ast function in sandbox.py.
• Example command to find suspicious template usage in logs: grep -r "__class__" /path/to/logs/
• Example command to check PyBlade version: pip show pyblade

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Mitigation Strategies

The primary mitigation step is to upgrade PyBlade to the latest version that includes the fix introduced in commit 62c95c47 (dated 2026-02-24). This fix implements a SafeEvaluator class that properly validates all AST node types, blocks access to private attributes, and restricts method calls to a whitelist.

Until an upgrade is possible, avoid using vulnerable versions (0.1.8-alpha and 0.1.9-alpha) in production environments and restrict template inputs to trusted users only.

• Upgrade PyBlade to the fixed version that includes the SafeEvaluator implementation.
• Audit and sanitize all template inputs to prevent injection of malicious expressions.
• Monitor logs for exploitation attempts and block suspicious requests at the network or application firewall level.
• If possible, apply additional runtime restrictions such as limiting permissions of the application user to reduce impact of potential exploitation.

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Compliance Impact

The vulnerability in PyBlade allows remote attackers to perform arbitrary code execution on the server by injecting malicious templates, potentially leading to full system compromise.

Such a compromise could result in unauthorized access to sensitive data, which may impact compliance with data protection regulations like GDPR and HIPAA that require safeguarding personal and health information.

However, the provided information does not explicitly discuss the direct effects of this vulnerability on compliance with these standards or any regulatory implications.

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