Critical Nuclei Vulnerability (CVE-2024-43405) Exposes Systems to Signature Bypass and Code Execution

A newly disclosed high-severity security flaw in ProjectDiscovery's Nuclei, a widely used open-source vulnerability scanner, poses a significant risk by allowing signature bypass and potential remote code execution (RCE).

CVE-2024-43405: A Breakdown of the Nuclei Security Flaw

The vulnerability, tracked as CVE-2024-43405, has been assigned a CVSS score of 7.4 (out of 10), indicating a substantial risk. It affects all Nuclei versions later than 3.0.0 and originates from a flaw in the signature verification process, which can be exploited to inject malicious content into templates.

🔴 Key Issue: A discrepancy between Nuclei’s YAML parser and its regex-based signature verification creates a loophole that attackers can exploit by injecting a carriage return character ("\r"), effectively bypassing integrity checks while maintaining a valid signature.

🔍 According to researchers at Wiz, this enables attackers to:

• Inject malicious code into templates while retaining a seemingly valid signature.
• Exploit regular expression mismatches to bypass signature verification.
• Execute arbitrary commands on affected systems.

How the Vulnerability Works

Nuclei relies on YAML-based templates to scan applications, networks, cloud infrastructure, and other systems for security vulnerabilities. These templates include digital signatures to ensure authenticity. However, the flaw arises from the way multiple "# digest:" lines are processed:

• Regex-based signature verification treats \r as part of a single line.
• YAML parsers, however, interpret \r as a line break, allowing attackers to introduce additional # digest: lines that bypass validation.
• This discrepancy leads to successful signature bypass, allowing unauthorized execution of malicious templates.

Exploitation Risks and Attack Scenarios

🔺 Potential attack scenarios include:
✔️ Arbitrary Code Execution – Attackers can craft malicious templates to execute system commands.
✔️ Data Exfiltration – Sensitive information stored on the host system may be compromised.
✔️ Privilege Escalation – The flaw can be leveraged to gain higher access privileges.

🚨 Wiz security researcher Guy Goldenberg warns:
"Since this signature verification is currently the only validation method for Nuclei templates, it represents a single point of failure. Organizations running untrusted or community-contributed templates are particularly vulnerable."

Patch & Mitigation: Upgrade to Nuclei 3.3.7

ProjectDiscovery has addressed CVE-2024-43405 in Nuclei version 3.3.2 (released on September 4, 2024), with further improvements in version 3.3.7. Users should immediately update to the latest version to mitigate potential attacks.

Security Best Practices

✅ Upgrade to Nuclei 3.3.7 or later to eliminate the vulnerability.
✅ Validate all Nuclei templates before execution, especially from untrusted sources.
✅ Implement security controls to prevent unauthorized template execution.
✅ Monitor network activity for suspicious scans or template modifications.

🔐 Final Thoughts: This vulnerability highlights the risks of parser discrepancies in security tools and the importance of strict validation mechanisms. Organizations leveraging Nuclei must apply patches promptly and enforce template security measures to prevent exploitation.

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