A significant security vulnerability has been identified in ProjectDiscovery’s Nuclei, an open-source vulnerability scanner popular among cybersecurity professionals. This flaw, tracked as CVE-2024-43405, has a CVSS score of 7.4, indicating its critical nature, and affects all versions of Nuclei above 3.0.0.

The vulnerability arises from the way Nuclei’s signature verification process interacts with newline characters during template processing. Specifically, discrepancies in how signatures are validated versus how the YAML parser interprets these characters allow attackers to insert malicious content into a template without invalidating its benign signature.

Nuclei utilizes a series of YAML files as templates to perform scans across applications, infrastructure, and cloud platforms, detecting security weaknesses. The scanning engine processes these templates to send specific requests, thus identifying vulnerabilities.

Discovered by cloud security firm Wiz, this flaw poses serious risks as it can potentially let attackers exploit the template signature verification process, which is meant to ensure the integrity of the templates from the Nuclei official repository. By overriding this verification, attackers can create maliciously crafted templates able to execute arbitrary code or exfiltrate sensitive data.

The root of this issue lies in the dual use of regular expressions (regex) for signature validation alongside the YAML parser, resulting in a parsing conflict. Attackers could exploit this by injecting a "r" character designed to escape through the regex checks while being interpreted as a line break by the YAML parser. Such exploitation could permit the creation of additional, unverified "# digest:" lines within the template that remain valid for execution.

ProjectDiscovery patched this issue on September 4, 2024, releasing version 3.3.2 to address the vulnerability. The latest version now stands at 3.3.7.

Experts caution that the vulnerability underscores a critical point of failure in the verification mechanism. Organizations running unvalidated or community-sourced templates risk falling victim to these malicious manipulations, with the potential for severe consequences, including arbitrary command execution and system compromise.

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