CVE-2026-119716 in pybars3
Summary
by MITRE • 10/30/2026
pybars3 through 0.9.7 contains a code injection vulnerability in pybars/_compiler.py that allows template authors to execute Python code via unescaped bracketed path segments. Attackers can supply Handlebars templates with crafted segments in helper or block arguments to break out of string literals and execute Python in the application process.
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Analysis
by VulDB Data Team • 10/30/2026
The vulnerability identified in pybars3 versions through 0.9.7 represents a critical server-side code injection flaw rooted in the template compilation logic within the pybars/_compiler.py module. This security issue arises from insufficient sanitization of input data when processing Handlebars templates, specifically targeting unescaped bracketed path segments used as helper or block arguments. In normal operation, these segments are intended to reference variables or properties within a given context object safely. However, due to the lack of proper escaping mechanisms for specific syntactic constructs, an attacker can craft malicious template content that breaks out of expected string literal boundaries. This breakage allows arbitrary Python code to be injected into the compilation process, effectively bypassing the sandboxed environment typically associated with template engines and granting execution capabilities within the application's runtime context.
From a technical perspective, this flaw is classified under CWE-94 as Improper Control of Generation of Code (Code Injection). The vulnerability exploits the way pybars3 parses and compiles Handlebars syntax into Python bytecode or executable structures. When an attacker supplies crafted segments in helper arguments, they manipulate the internal parser to interpret malicious payloads as valid code rather than data. This misinterpretation leads to the execution of arbitrary commands within the server process hosting the application. The severity is heightened by the fact that template authors are often trusted users or inputs derived from user-facing interfaces, meaning a remote attacker could potentially trigger this vulnerability if they have control over the templates rendered by the application.
The operational impact of this vulnerability is severe, as it leads to full Remote Code Execution (RCE) on the affected system. An adversary who successfully exploits this flaw can execute arbitrary Python code with the same privileges as the web server or backend service running pybars3. This capability allows for complete compromise of the underlying infrastructure, including data exfiltration, modification of application logic, installation of backdoors, and lateral movement within the network. The attack vector aligns with ATT&CK technique T1059 Command and Scripting Interpreter, specifically involving Python execution capabilities. Because the code runs in the context of the main application process, there are no additional privilege escalation steps required for an attacker to gain control over critical system resources or sensitive data stored by the application.
Mitigation strategies must focus on immediate patching and input validation. The primary remediation is to upgrade pybars3 to a version greater than 0.9.7 where this specific code injection vector has been addressed through improved parsing logic and stricter handling of bracketed path segments. In environments where upgrading is not immediately feasible, administrators should implement strict allow-lists for template inputs or disable the use of dynamic helper arguments that accept untrusted data. Additionally, deploying a Web Application Firewall with rules capable of detecting Handlebars-specific injection patterns may provide an additional layer of defense against exploitation attempts. Regular security audits and static code analysis tools configured to detect unsafe string concatenation in template engines can also help identify similar vulnerabilities before they are exploited in production environments.