CVE-2019-20922: Regular Expression Denial of Service in Handlebars

Background

Handlebars is a popular templating engine for JavaScript that allows developers to build dynamic HTML templates. It is widely used in web development frameworks and applications. The vulnerability identified as CVE-2019-20922 is a Regular Expression Denial of Service (ReDoS) issue in Handlebars. This vulnerability arises due to eager matching in the parser, which can lead to an endless loop when processing crafted templates. Exploiting this vulnerability can result in the exhaustion of system resources, potentially leading to a denial of service (DoS) condition.

Vulnerability Detail

The vulnerability in Handlebars arises from the eager matching behavior of the parser. When processing specially crafted templates, the parser can be forced into an infinite loop, consuming excessive CPU resources and causing a DoS condition. This vulnerability can be exploited by an attacker to exhaust system resources, impacting the availability of the affected application or system. The vulnerability has been assigned a CVSS score of 7.5, indicating a high severity level.

npm install handlebars@4.7.10

npm install handlebars@4.4.5

Workaround

If upgrading to the latest version of Handlebars is not immediately feasible, developers can implement input validation and sanitization techniques to mitigate the risk of exploitation. By carefully validating and sanitizing user input before passing it to the handlebars.compile function, the likelihood of triggering the ReDoS vulnerability can be reduced. Here is an example of input validation and sanitization using a regular expression to ensure the input does not contain any malicious patterns:

const input = req.body.template;
const sanitizedInput = input.replace(/[\^$*+?.()|{}\[\]]/g, '\\$&');
const compiledTemplate = handlebars.compile(sanitizedInput);

This code snippet escapes any special characters that could potentially trigger the ReDoS vulnerability, making the input safe for compilation.

Conclusion

The Regular Expression Denial of Service vulnerability in Handlebars (CVE-2019-20922) can have severe consequences, including the exhaustion of system resources and potential denial of service. It is crucial for developers to upgrade to the latest version of Handlebars (4.4.5 or later) to mitigate this vulnerability. Implementing input validation and sanitization techniques can provide an additional layer of defense. For further support and assistance in vulnerability remediation, please contact DevNack.com.

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