How to Ignore SSL Certificate Errors with cURL: A Step-by-Step Guide

While performing HTTP requests using cURL, sometimes there is an issue with verifying SSL certificates. This can happen when a self-signed server certificate is used, or if it is expired. In such circumstances, cURL by default denies the connection to safeguard the user from possible risks. However, this might obstruct the process during API testing, working on closed networks, or using local services. That’s why we need to find out in what circumstances cURL ignores SSL and how to implement it.

This article will detail how to ignore SSL certificates in cURL, the specific commands that need to be executed, and possible concerns related to security.

What Are cURL and SSL?

cURL (Client URL) is a software project that provides a command line and library for transferring data using various protocols. This tool is very useful for automating requests, API testing, or information retrieval from various web resources because it works with a multitude of network types.

SSL (Secure Sockets Layer) is a form of encryption technology that secures the transmission of data over the Internet. This certificate is a document that proves a certain website is authentic and establishes a secure connection between the user and the server. Self-signed, expired, or invalid server certificates lead to blocks while operating cURL.

To escape such challenges, it is better to cURL disable SSL, thus, performing cURL requests will have a higher success rate. This method of testing is very useful, but can be dangerous security-wise.

cURL, curl, or Curl: What's the Difference?

The official name for the tool is cURL, the first letter of the word is lowercase and the rest are uppercase. In the documents, articles, and codes, you will find several other forms:

• curl (all lowercase) – This is how most developers tend to write it in codes and command lines.
• Curl (capital C) – This is sometimes used in unofficial documents but is incorrect.

Regardless of spelling, cURL is always cURL. It is a cross-platform and command line tool for transferring data with URL syntax. Most people in web development, system administration, and DevOps use it to make HTTP requests, test APIs, and perform server automation interactions.

Why Disable SSL Verification?

Normally, the data transfer utility validates certificates appearing on the user’s screen in order to thwart fake sites and attacks. There are scenarios, however, when the cURL ignore certificate check. Let’s look at when those instances arise:

• Self-issued certificates being used with test or local servers. While testing APIs or applications, developers often create self-signed certificates that are not endorsed by official Certification Authorities (CA).
• Employee access to internal corporate services. An organization’s Internal servers usually have certificates that aren’t trusted. Where automated access to such resources is mandatory, turning off certificate checking is an optimal temporary solution.
• Certain sites have a server certificate that is either expired or invalid. If it is crucial to pull information from such a resource, validation can be turned off temporarily without the user encountering the specific certificate error. In this case cURL ignores SSL errors and everything works fine.
• Disabling checking helps avoid failure in scripts that require automation of network requests. When a script consistently sends requests to a server that has an incorrect certificate, checking needs to be disabled.

Despite checking being a vital factor, turning it all the way off can pose problems and risks. We will discuss those next.

Security Risks of Disabling SSL

When cURL ignores certificates it means that the connection can be exploited through various security attacks, and this is particularly important when sensitive information including passwords, tokens, or even payment data is involved.

Main threats:

• Man In The Middle Attack (MITM) – a technique where the information between the user and the server is intercepted and the data is either altered or substituted. It poses an even greater risk when there is no one to verify because it becomes way too easy to implement malicious attacks. For example, a login page can be replaced with a fake one so that the attacker can easily harvest credentials.
• cURL is unable to verify the server certificate, invalidating it. Thus, there is no way to ensure that the wanted node is the one being connected to, and allows substitution of sites by posing actors as trusted resources.
• If during testing, the verification gets disabled and that code is carried to the production environment, then there can be an increase in the chance of leakage or even brute force attacks that compromise confidential information.

If it is required cURL ignore SSL errors, then it is paramount to view self-signed certificates:

• Bypass verification only in a test environment, and avoid it in production;
• Do not transmit confidential information through an unencrypted connection;
• Add the self-signed certificate to trusted manually;
• Monitor the code, excluding configuration errors, to ensure verification remains in the production code.

In the next block, we will analyze in detail the processes that are carried out in the command line for disabling certificate verification.

How to Bypass SSL Errors with cURL

In order to make cURL ignore SSL, you must include specific parameters that suspend authentication verification. As noted previously, such practices can only be used within a properly secured environment.

The most effective method to mitigate an invalid site certificate error is to manually configure a temporary verification spoof. This can be done with the following parameters:

• -k or --insecure—for use connecting to a site where the server’s certificate turns out to be invalid;
• --cacert—to define the location where the user certificate file is stored.

Here is how you can make a cURL call example that ignores certificate verification:

curl -k https://example.com

This will allow the cURL call to go through even with an expired server certificate.

If you would like to send data (such as in a POST request):

curl -k -X POST -H "Content-Type: application/x-www-form-urlencoded" -d "param=value" https://example.com/api

For utmost security, you would directly select the directory where the file is located.

curl --cacert /path/to/cert.pem https://example.com

Disabling SSL Verification in Your Script

If the request is meant to be automated through a command line and the user’s presence should not be needed, then it is beneficial to disable certificate checks in such scenarios.

Here is an example in Bash, how cURL ignore SSL:

#!/bin/bash
URL="https://example.com"
DATA=$(curl -k $URL)
echo "Server response: $DATA"

With Python, you can use the requests module with verify set to False:

import requests
url = "https://example.com"
response = requests.get(url, verify=False)
print(response.text)

When running this command, Python will give you a warning regarding an unsafe connection. Additionally, you will need to make sure you write code to disable warnings that arise in the event that the site has an invalid server certificate:

import requests import urllib3 

urllib3.disable_warnings(urllib3.exceptions.InsecureRequestWarning) 

url = "https://example.com"
response = requests.get(url, verify=False)
print(response.text)

And now for PHP with cURL:

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://example.com");
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$output = curl_exec($ch);
curl_close($ch);
echo $output;

The table below lists the most common methods of ignoring SSL checking, their usage, and possible risks.

Conclusion

Malfunctioning server certificates create more issues than just when they are operated with cURL. Certificates assure safety for users during data transfer. Thus, disabling verification is the last resort when one finds an error debugging cURL. Based on the cause of the error, there are other, less extreme approaches, such as document updating, adding the to trusted region, or just swapping it with the correct one.

Without SSL checking, the connection would be unhealthy and easily abused by other users. This approach is only acceptable under controlled environments such as when one is doing a test, or when the service is external. Under normal circumstances when an invalid server certificate is presented, it is crucial that this clause is not set in the working code as data masking and other security holes would be presented.