How to fix fatal: Authentication failed for 'https://github.com/user/repo.git/' in Git

GitHub and many other Git hosting services no longer accept passwords for HTTPS authentication. You need to create a Personal Access Token (PAT):

For GitHub:
1. Go to [github.com/settings/tokens](https://github.com/settings/tokens)
2. Click Generate new token > Generate new token (classic)
3. Give it a descriptive name (e.g., "Development laptop")
4. Set an expiration date (or "No expiration" for convenience, though less secure)
5. Select scopes:
- repo - Full control of private repositories (required for push/pull)
- workflow - If you need to update GitHub Actions workflows
6. Click Generate token and copy it immediately (you won't see it again!)

Use the token as your password:

git push origin main
# When prompted for password, paste your Personal Access Token

For GitLab: Settings > Access Tokens > Add new token

For Bitbucket: Personal Settings > App passwords > Create app password

Old passwords or tokens may be cached in your operating system's credential manager. Clear them to force Git to prompt for new credentials:

On Windows (Credential Manager):

# Open Credential Manager
control /name Microsoft.CredentialManager

# Or via command line - list credentials
cmdkey /list

# Delete GitHub credentials
cmdkey /delete:git:https://github.com

Alternatively:
1. Open Control Panel > Credential Manager > Windows Credentials
2. Find entries starting with git:https://github.com
3. Click the entry and select Remove

On macOS (Keychain):

# Delete Git credentials from Keychain
git credential-osxkeychain erase
host=github.com
protocol=https
# Press Enter twice

# Or use Keychain Access app to find and delete "github.com" entries

On Linux (credential helper):

# Check what credential helper is configured
git config --global credential.helper

# If using 'store' (plain text file)
rm ~/.git-credentials

# If using 'cache'
git credential-cache exit

# If using GNOME Keyring or libsecret
# Open Seahorse (Passwords and Keys) and remove GitHub entries

After clearing, try your Git operation again - you'll be prompted for credentials.

After generating a new token, configure Git to remember it so you don't have to enter it repeatedly:

Using Git Credential Manager (Recommended):

# Install Git Credential Manager (if not already installed)
# On Windows: comes with Git for Windows
# On macOS: brew install git-credential-manager
# On Linux: see https://github.com/git-ecosystem/git-credential-manager

# Configure Git to use it
git config --global credential.helper manager

Simple credential caching:

# Cache credentials in memory for 1 hour (3600 seconds)
git config --global credential.helper 'cache --timeout=3600'

# Or store permanently in a plain text file (less secure)
git config --global credential.helper store

macOS Keychain:

git config --global credential.helper osxkeychain

Now when you enter your username and Personal Access Token, they'll be saved for future use.

SSH authentication is often more reliable and doesn't require tokens. Here's how to set it up:

Step 1: Check for existing SSH keys:

ls -la ~/.ssh
# Look for id_ed25519.pub or id_rsa.pub

Step 2: Generate a new SSH key (if needed):

ssh-keygen -t ed25519 -C "[email protected]"
# Press Enter to accept default location
# Enter a passphrase (optional but recommended)

Step 3: Add the SSH key to your SSH agent:

# Start the SSH agent
eval "$(ssh-agent -s)"

# Add your key
ssh-add ~/.ssh/id_ed25519

Step 4: Add the public key to GitHub:

# Copy your public key
cat ~/.ssh/id_ed25519.pub
# Copy the output

1. Go to [github.com/settings/keys](https://github.com/settings/keys)
2. Click New SSH key
3. Paste your public key and save

Step 5: Change your remote URL to SSH:

# Check current remote
git remote -v

# Change from HTTPS to SSH
git remote set-url origin [email protected]:username/repo.git

# Test the connection
ssh -T [email protected]

For CI/CD pipelines or automated scripts, you can embed credentials in the URL (use secrets management in production):

# Set remote with token embedded (for automation only)
git remote set-url origin https://USERNAME:[email protected]/username/repo.git

# Clone with token
git clone https://USERNAME:[email protected]/username/repo.git

Important security considerations:
- Never commit URLs with tokens to version control
- Use CI/CD secret management (GitHub Actions secrets, GitLab CI variables)
- Tokens in URLs may appear in logs - be careful

Better approach for CI/CD:

# Use environment variables
git config --global credential.helper '!f() { echo "password=$GIT_TOKEN"; }; f'

# Or configure inline
git -c "credential.helper=!f() { echo password=$GIT_TOKEN; }; f" push origin main

Authentication can fail if you're using the wrong username. Verify you're using the correct one:

For GitHub:
- Use your username, not your email address
- Your username is shown at [github.com/settings/profile](https://github.com/settings/profile)
- Usernames are case-insensitive but stored as lowercase

# Check your configured username
git config --global user.name

# The username for authentication should match your GitHub username
# NOT your display name or email

Check repository access:

# Ensure you have access to the repository
# Try opening the repository URL in a browser while logged in

For organization repositories:
- Ensure your account has been granted access to the organization
- Check if SSO is required: [github.com/settings/organizations](https://github.com/settings/organizations)
- You may need to authorize your token for SSO

Older versions of Git may have bugs with authentication or lack support for modern credential managers:

Check your Git version:

git --version

Update Git:

On Windows:

# Using Git itself
git update-git-for-windows

# Or download from https://git-scm.com/download/win

On macOS:

# Using Homebrew
brew upgrade git

# Or download from https://git-scm.com/download/mac

On Ubuntu/Debian:

sudo add-apt-repository ppa:git-core/ppa
sudo apt update
sudo apt install git

On Fedora/RHEL:

sudo dnf update git

After updating, try your Git operation again.