cURL Error 35: SSL/TLS Connection Fix Guide (2026)

What is cURL Error 35?

cURL Error 35 is an SSL/TLS connection error that occurs when the cURL library fails to establish a secure connection with the remote server. The full error message typically reads: “cURL error 35: SSL connect error” or “error:1407742E:SSL routines:SSL23_GET_SERVER_HELLO:tlsv1 alert protocol version”. This error is common in PHP applications, API integrations, WordPress sites, and command-line scripts when attempting HTTPS requests.

Quick Answer: How to Fix cURL Error 35

Update your SSL/TLS libraries, ensure your system supports TLS 1.2 or higher, verify the remote server’s SSL certificate is valid, and check that your cURL version is up to date. For immediate fixes, try forcing TLS 1.2 or updating your CA certificate bundle.

What Causes cURL Error 35?

Understanding the root causes is essential for effective troubleshooting:

Outdated SSL/TLS Libraries: Older OpenSSL versions do not support modern TLS protocols
TLS Version Mismatch: Client and server cannot agree on a compatible TLS version
Expired or Invalid SSL Certificates: The remote server’s certificate has expired or is self-signed
Firewall or Proxy Interference: Network security tools blocking SSL handshakes
Outdated cURL Version: Older cURL builds lack support for newer encryption standards
Missing CA Certificates: The system lacks the root certificates needed to verify SSL
Server-Side SSL Misconfiguration: The remote server has improper SSL settings

How to Fix cURL Error 35: Step-by-Step Solutions

Method 1: Update cURL and OpenSSL

Outdated libraries are the most common cause of Error 35. Update them:

Operating System	Update Command
Ubuntu/Debian	`sudo apt-get update && sudo apt-get install curl openssl libssl-dev`
CentOS/RHEL	`sudo yum update curl openssl openssl-devel`
macOS (Homebrew)	`brew upgrade curl openssl`
Windows	Download latest cURL from curl.se

Method 2: Force TLS 1.2 or Higher

Many servers now require TLS 1.2 minimum. Force the correct version:

Command Line: curl --tlsv1.2 https://1000sciencefairprojects.com/tech/
PHP cURL: curl_setopt($ch, CURLOPT_SSLVERSION, CURL_SSLVERSION_TLSv1_2);
Python requests: Use requests.get(url, ssl_version=ssl.PROTOCOL_TLSv1_2)

View SSL Certificate Guide

Method 3: Update CA Certificate Bundle

Missing or outdated CA certificates cause verification failures:

Download latest CA bundle: From curl.se/ca/cacert.pem
PHP: Update curl.cainfo and openssl.cafile in php.ini
cURL command line: Use --cacert /path/to/cacert.pem
Verify path: Check php -i | grep cafile for correct location

Method 4: Check SSL Certificate Validity

Verify the remote server’s certificate is not expired:

Test with OpenSSL: openssl s_client -connect 1000sciencefairprojects.com/tech/ -servername 1000sciencefairprojects.com/tech/
Check expiration: Look for “Verify return code: 0 (ok)”
Use SSL Labs: Test at SSL Labs
Verify certificate chain: Ensure all intermediate certificates are present

Method 5: Disable SSL Verification (Temporary Debug Only)

Warning: Only use this for testing, never in production:

Command Line: curl -k https://1000sciencefairprojects.com/tech/ or curl --insecure
PHP: curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
Always re-enable verification after identifying the issue

Method 6: Check Firewall and Proxy Settings

Network security tools can interfere with SSL handshakes:

Temporarily disable firewall to test if it is the cause
Configure proxy settings: curl -x proxy:port https://1000sciencefairprojects.com/tech/
Check corporate proxies: Some intercept and re-encrypt SSL traffic
Whitelist the domain in your security software

Method 7: Verify Server-Side SSL Configuration

If you control the server, check these settings:

Enable TLS 1.2 and 1.3: Disable SSLv3 and TLS 1.0/1.1
Update server certificate: Renew if expired
Install intermediate certificates: Complete the certificate chain
Check cipher suites: Ensure compatibility with client libraries

cURL Error 35: Platform-Specific Fixes

Platform	Specific Solution
WordPress	Update WordPress core, plugins, and themes. Check SSL plugin settings. Verify hosting provider supports TLS 1.2+
PHP Applications	Update PHP to 7.4+ or 8.x. Set curl.cainfo in php.ini. Use stream_context_set_default for SSL options
Linux Servers	Update OpenSSL with package manager. Rebuild cURL against new OpenSSL. Check /etc/ssl/certs/ directory
Windows WAMP/XAMPP	Download cacert.pem and place in php/extras/ssl/. Update php.ini to point to this file
Docker Containers	Update base image. Install ca-certificates package. Mount host CA bundle if needed

How to Prevent cURL Error 35

Keep cURL and OpenSSL updated to latest stable versions
Monitor certificate expiration dates and set renewal reminders
Use TLS 1.2 or higher for all connections
Regularly update CA certificate bundles
Test SSL configuration with SSL Labs monthly
Document proxy and firewall rules for your network

Common cURL Error 35 Variations

Error Message	Meaning
`SSL connect error`	General SSL handshake failure
`tlsv1 alert protocol version`	TLS version mismatch – server requires newer TLS
`certificate verify failed`	CA certificate missing or invalid
`sslv3 alert handshake failure`	Cipher suite mismatch between client and server

TL;DR – Quick Fix Summary

cURL Error 35 is an SSL/TLS connection error fixed by:

Updating cURL and OpenSSL to latest versions
Forcing TLS 1.2 or higher in your requests
Updating CA certificate bundle
Checking SSL certificate validity on the remote server
Verifying firewall/proxy is not blocking SSL handshakes

Start with Method 1 (update libraries) and Method 2 (force TLS 1.2) for fastest resolution.

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