How to Ignore Invalid and Self-Signed SSL Certificates Using cURL

If you‘ve ever needed to quickly test an API or website, chances are you‘ve used cURL. cURL is a powerful command-line tool for making HTTP requests and transferring data. It‘s fast, flexible, and supports a wide range of protocols.

One common task you might need to do with cURL is interact with a server that uses HTTPS with a self-signed or invalid SSL certificate. In this guide, I‘ll walk you through how to ignore SSL certificate errors with cURL. I‘ll explain what SSL certificates are, the risks involved in ignoring certificate validation, and show you step-by-step how to use cURL to connect to sites with invalid or self-signed certificates.

By the end, you‘ll know how to use cURL to test services with invalid certificates. But I‘ll also make sure you understand the security implications and why you need to be extremely cautious. Let‘s jump in!

SSL Certificates and HTTPS Explained

First, let‘s make sure we understand what SSL certificates are and their role in securing HTTPS connections.

An SSL certificate (also called a TLS certificate) is a digital file that binds a cryptographic key to an organization‘s details. When installed on a server, it enables secure HTTPS connections between a web server and a browser.

SSL certificates are issued by certificate authorities (CAs). The CA acts as a trusted third party, validating the identity of the organization requesting the certificate. For an SSL certificate to be considered valid, it must be signed by a trusted CA.

Browsers and operating systems maintain lists of CAs that they trust. When a browser connects to a website over HTTPS, it checks that the site‘s SSL certificate is signed by one of these trusted CAs. This ensures that the website is authentic and that the connection is secure.

Self-Signed Certificates

It‘s also possible for an organization to sign its own SSL certificate, rather than obtaining one from a CA. This is called a "self-signed certificate".

Self-signed certificates are not considered trustworthy by browsers because the authenticity of the organization has not been validated by a third party. Anyone can create a self-signed certificate claiming to be any organization.

For this reason, self-signed certificates will generate browser warnings when used on public-facing websites. However, they are sometimes used for internal sites, testing, and development environments.

With that background in mind, let‘s look at how cURL handles SSL certificates by default and how to modify that behavior.

The Risks of Ignoring SSL Certificate Validation

Before I show you the cURL options for ignoring certificate errors, I want to emphasize that doing so is dangerous and should only be used as a last resort for testing purposes.

When you tell cURL to ignore SSL certificate validation, you are essentially disabling the checks that ensure you are communicating with an authentic, trusted server over a secure connection.

This opens you up to man-in-the-middle (MITM) attacks. In a MITM attack, a malicious party can intercept the communication between your computer and the server. They can eavesdrop on the data being transmitted, and even modify the data in transit.

Normally, the SSL validation process in cURL would reject a connection if the server‘s certificate didn‘t match or wasn‘t signed by a trusted CA. But if you disable this validation, an attacker could send back a fake SSL certificate and cURL would blindly accept it.

To put it another way – ignoring certificate errors means giving up the security protections of HTTPS. The communication might be encrypted, but you have no assurance of who you are actually communicating with.

So in general, ignoring certificate errors is something you should avoid. If you control the server in question, you should generate a proper CA-signed certificate, even for testing. Let‘s Encrypt and other services make this free and easy to do.

However, if you absolutely must test with an invalid certificate and are willing to accept the risks, here‘s how you do it in cURL.

How to Ignore SSL Certificate Errors in cURL

To ignore SSL certificate validation in cURL, you use the -k or –insecure option.

Here‘s an example of using -k to connect to a server with a self-signed certificate:

curl -k https://self-signed.example.com

The -k option tells cURL to skip the SSL certificate validation. It will not verify that the certificate is signed by a trusted CA or that the hostname matches. It will allow the connection to proceed regardless of any certificate errors.

You can also use the long form –insecure instead of -k:

curl --insecure https://self-signed.example.com 

Keep in mind that the -k or –insecure option disables certificate validation for the entire request. It‘s not possible to ignore only certain types of certificate errors.

Inspecting SSL Certificates with cURL

In addition to ignoring certificate errors, cURL provides options to view the SSL certificate details for a given URL. This is useful for debugging HTTPS issues and for seeing the information sent by the server.

To view the SSL certificate details, you can use the -v or –verbose option. This enables verbose mode, which includes the certificate information in the output.

For example:

curl -v https://example.com

In the output, look for the lines beginning with an asterisk. These indicate SSL-related information. You‘ll see details about the SSL handshake, the certificate, cipher suite, and more.

Here‘s an example of what the SSL details might look like:

* SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
* ALPN, server accepted to use http/1.1
* Server certificate:
*  subject: C=US; ST=California; L=San Francisco; O=Slack Technologies, Inc.; CN=*.slack.com
*  start date: Nov 12 00:00:00 2022 GMT
*  expire date: Dec 20 23:59:59 2024 GMT
*  subjectAltName: host "slack.com" matched cert‘s "*.slack.com"
*  issuer: C=US; O=Amazon; OU=Server CA 1B; CN=Amazon
*  SSL certificate verify ok.

This shows that the connection to slack.com used TLS 1.2 with a strong cipher suite. The certificate was issued by Amazon to *.slack.com and is valid until December 2024. The hostname slack.com matched the certificate‘s subject alternative name.

If there are any issues with the certificate, they will be called out here. For example, an expired certificate:

* SSL certificate problem: certificate has expired

Or an untrusted self-signed certificate:

* SSL certificate problem: self signed certificate

Using cURL‘s verbose mode is an easy way to gather information about a site‘s SSL setup and troubleshoot any issues.

Testing an HTTPS Server with Self-Signed Certificates

Now let‘s walk through a practical example of using cURL to test an HTTPS server that is using a self-signed certificate.

Imagine you are a developer working on an internal web service. This service is not public-facing and does not have a CA-signed SSL certificate. Instead, you‘ve generated a self-signed certificate for testing purposes.

To start, try connecting to this server with cURL normally:

curl https://internal-service.company.net 

Because the server‘s certificate is self-signed and not trusted by your system, cURL will reject the connection with an error:

curl: (60) SSL certificate problem: self signed certificate
More details here: https://curl.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.

To proceed with testing, you‘ll need to ignore the self-signed certificate error using cURL‘s -k option:

curl -k https://internal-service.company.net

With -k, cURL will allow the connection to the server, ignoring the invalid SSL certificate. The HTTPS request will complete successfully, and you‘ll see the response from your test server.

Remember, using -k means the communication is encrypted but is not authenticated. You‘ve bypassed the important validation of the server‘s identity that SSL certificates provide.

In this testing scenario, that‘s an acceptable risk because you know you‘re connecting to your own internal server. But hopefully it‘s clear why you would never want to do this for communication across the public internet or for production traffic. Ignoring certificate errors opens you up to attack and defeats the purpose of using HTTPS in the first place!

Conclusion

In this guide, you learned how to use cURL‘s -k or –insecure option to ignore SSL certificate errors when connecting to HTTPS servers. This allows you to connect to servers that are using invalid, expired, or self-signed SSL certificates.

I walked through the basics of SSL certificates and how they enable secure, authenticated communication over HTTPS. You saw how to inspect certificate details with cURL‘s verbose mode.

Crucially, I also explained the risks of ignoring certificate validation. Doing so removes the protection against man-in-the-middle attacks that is the whole point of HTTPS.

For this reason, ignoring certificate errors should be done sparingly and only in controlled testing conditions. Never ignore certificate errors for production traffic or communication over untrusted networks.

When testing, it‘s still best to use a properly signed SSL certificate whenever possible. But when you must connect to a server with an invalid cert, using cURL with -k lets you do so.

I hope this guide has helped you understand how to use cURL to bypass SSL errors in a pinch. Just remember to be cautious and put security first!

If you‘d like to learn more about cURL and HTTPS, check out these resources:

• Everything curl – Online cURL book
• cURL Documentation – Official cURL docs
• Let‘s Encrypt – Free, automated SSL certificates
• SSL Labs – SSL server testing tools