Unleashing the Power of Curl for Reconnaissance

In the realm of cybersecurity and ethical hacking, information is everything. Gathering intelligence about potential targets is a crucial step in understanding their digital presence and potential vulnerabilities. For this purpose, Curl, a command-line tool, emerges as a potent ally for reconnaissance. In this short article, we will explore how Curl can be harnessed for reconnaissance and OSINT (Open-Source Intelligence) gathering.

What is Curl?

Curl, short for "Client URL," is a versatile command-line tool used to transfer data to or from a server. Initially developed to work with URLs, Curl has evolved to support various protocols, including HTTP, HTTPS, FTP, FTPS, SCP, SFTP, LDAP, and more. This swiss army knife for data transfers makes it an indispensable asset for web developers and, in the cybersecurity world, for reconnaissance purposes.

Curl for Reconnaissance

1. HTTP Requests and Responses

At its core, Curl allows you to make HTTP requests to web servers, revealing the responses and hidden information in the headers and content. By using Curl, ethical hackers can view HTTP response headers, detect server information, and discover potential vulnerabilities that might be exposed.



For example, to view the HTTP response headers of a target website, use the following command:

 curl -I https://example.com

Hypothetical Results after Running `curl -I https://example.com`

 HTTP/2 200 
server: nginx
date: Mon, 15 May 2023 10:30:15 GMT
content-type: text/html; charset=utf-8
content-length: 21548
last-modified: Fri, 12 May 2023 14:25:07 GMT
etag: "5cd44fb3-53cc"
accept-ranges: bytes
strict-transport-security: max-age=31536000
x-content-type-options: nosniff
x-frame-options: SAMEORIGIN
x-xss-protection: 1; mode=block
referrer-policy: strict-origin-when-cross-origin
content-security-policy: default-src 'self' https://cdn.example.com; script-src 'self' 'unsafe-inline' 'unsafe-eval' https://cdn.example.com; style-src 'self' 'unsafe-inline' https://cdn.example.com; img-src 'self' https://cdn.example.com data:; font-src 'self' https://cdn.example.com; connect-src 'self' https://api.example.com; form-action 'self';
expires: Mon, 22 May 2023 10:30:15 GMT
cache-control: max-age=604800
x-proxy-cache: MISS
x-cache-status: MISS
x-powered-by: PHP/7.4.9

In this hypothetical scenario, running `curl -I https://example.com` produced the above HTTP response headers for the website "https://example.com."

Here's an interpretation of the key response headers:

• `HTTP/2 200`: The website responded with a "200 OK" status code, indicating a successful request.
•  `server: nginx`: The web server used by the website is Nginx.
•  `content-type: text/html; charset=utf-8`: The content type of the response is "text/html," and the character encoding is "UTF-8."
•  `content-length: 21548`: The size of the response content is 21548 bytes.
•  `last-modified: Fri, 12 May 2023 14:25:07 GMT`: The last modification date of the requested resource.
• `etag: "5cd44fb3-53cc"`: The entity tag, used for cache validation.
•  `expires: Mon, 22 May 2023 10:30:15 GMT`: The date and time when the response content should be considered stale.
•  `cache-control: max-age=604800`: The maximum time (in seconds) for which the response content can be cached.
•  `strict-transport-security: max-age=31536000`: The website enforces strict transport security, requiring the use of HTTPS.
•  `x-content-type-options: nosniff`: Prevents MIME type sniffing.
•  `x-frame-options: SAMEORIGIN`: Protects against clickjacking attacks by allowing the page to be framed only by pages from the same origin.
•  `x-xss-protection: 1; mode=block`: Enables Cross-Site Scripting (XSS) protection in the browser.
•  `referrer-policy: strict-origin-when-cross-origin`: Specifies the referrer policy for fetch requests.
•  `content-security-policy`: The Content Security Policy (CSP) that defines which resources can be loaded on the page.
• `x-proxy-cache: MISS`: Indicates that the response was not served from the proxy cache.
• `x-cache-status: MISS`: Indicates that the response was not found in the cache.
•  `x-powered-by: PHP/7.4.9`: The website is powered by PHP version 7.4.9.

2. SSL Certificate Details

Curl can also provide valuable insights into SSL certificates of a target domain. SSL certificate details, such as expiration date, issuer, and encryption algorithms, can give an indication of the website's security posture.



To retrieve SSL certificate details, use the following command:

curl -v https://example.com

3. Spidering Websites

Curl is excellent for spidering websites, meaning it can follow links on a page and retrieve related URLs. This functionality can help ethical hackers discover hidden pages, directories, or even outdated URLs that might lead to sensitive information.


To spider a website with Curl, use the following command:

curl -L https://example.com

4. Fetching Resources

Curl can download files, scripts, and other resources from websites, providing valuable intelligence on potential attack vectors or vulnerabilities.

To download a file from a website with Curl, use the following command:

curl -O https://example.com/path/to/file.txt

5. API Testing

Many websites offer APIs for their services. Curl can be used to interact with these APIs, exploring the endpoints and data they provide. This can reveal potential weaknesses or security flaws in the API implementation.



In the realm of cybersecurity, reconnaissance lays the groundwork for successful ethical hacking and vulnerability assessments. Curl, with its versatile and powerful capabilities, is a potent tool for OSINT gathering and reconnaissance purposes. From inspecting HTTP responses and SSL certificates to spidering websites and interacting with APIs, Curl equips cybersecurity professionals with valuable information that forms the basis for informed decision-making and robust security strategies. However, as with any ethical hacking tool, it is crucial to use Curl responsibly and with the proper authorization to uphold the principles of ethical hacking and protect the privacy and security of individuals and organizations alike.