CVE-2026-119717 in pybars4
Summary
by MITRE • 10/30/2026
pybars4 through 0.9.13 contains a code injection vulnerability in pybars/_compiler.py that allows attackers to execute Python code via unescaped bracketed path segments in Handlebars templates. Attackers can supply a template with a crafted segment in a helper or block argument to break out of the generated resolve() string literal and execute code in the application process.
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Analysis
by VulDB Data Team • 10/30/2026
The vulnerability identified in pybars4, specifically affecting versions through 0.9.13, represents a critical server-side code injection flaw rooted in improper input validation within the template compilation engine. The core of this issue resides in the _compiler.py module, which is responsible for translating Handlebars templates into executable Python bytecode or strings. When processing these templates, the compiler generates internal helper functions and resolve methods to handle dynamic content resolution. However, the implementation fails to adequately sanitize or escape bracketed path segments that are passed as arguments to helpers or block statements within the template structure. This lack of rigorous escaping allows an attacker to inject malicious syntax directly into the generated code strings before they are executed by the Python interpreter.
From a technical perspective, this flaw constitutes a classic case of improper neutralization of special elements used in an OS command (CWE-78) or more specifically, injection via unescaped input leading to arbitrary code execution (CWE-94). The vulnerability exploits the way pybars constructs its internal resolve() string literals. By supplying a crafted segment within a helper argument, such as {{helper "unescaped_bracket_path"}}, an attacker can break out of the intended scope of the generated Python string literal. Once this boundary is breached, the injected payload becomes part of the executable code context, allowing for arbitrary Python command execution within the application process. This mechanism bypasses standard sandboxing measures that might otherwise restrict template rendering to safe operations like variable substitution or simple logic evaluation.
The operational impact of this vulnerability is severe, as it effectively grants an attacker full control over the underlying server environment where pybars4 is deployed. If a web application utilizes pybars for dynamic page generation and accepts user-supplied input in any part of the template data structure that flows into helper arguments or block parameters without sufficient validation, remote code execution becomes feasible. This can lead to complete system compromise, including unauthorized access to sensitive data, modification of application logic, installation of backdoors, or use of the compromised server as a pivot point for further network attacks. The severity is amplified by the fact that template engines are often central to web applications handling dynamic content, making them high-value targets for exploitation.
This vulnerability aligns with several entries in the MITRE ATT&CK framework, particularly those related to command and script interpretation (T1059) and injection techniques (T1190). It also reflects common weaknesses found in server-side template engines where dynamic code generation occurs without strict input sanitization protocols. The flaw underscores the risks associated with using interpreted languages for generating executable code from untrusted sources, a pattern that requires rigorous defensive coding practices to mitigate effectively.
To remediate this issue, organizations must upgrade pybars4 to version 0.9.14 or later, where the vulnerability has been addressed through improved input handling and escaping mechanisms in the compiler module. For applications unable to immediately update due to compatibility constraints, implementing strict allow-listing of allowed template variables and helper arguments is recommended. Additionally, applying web application firewall rules that detect patterns indicative of code injection attempts within HTTP requests containing Handlebars syntax can provide a layer of defense-in-depth. It is also crucial to ensure that any user-supplied data passed into templates is treated as untrusted input and never directly interpolated into structural elements like helper arguments without thorough validation against expected formats.