VPN Brute Force Attacks: How They Work and How to Stop Them

By ThreatListPro Security Team · Published February 27, 2026 · Last verified: February 28, 2026

Every organization with an internet-facing VPN portal is a target. Automated botnets scan the entire IPv4 address space looking for GlobalProtect, SSL-VPN, AnyConnect, and other VPN login pages, then launch sustained brute force campaigns that can last for weeks. The attacks are relentless, indiscriminate, and growing exponentially.

This guide breaks down exactly how these attacks work, what damage they cause, and—most importantly—the fastest ways to stop them.

What Is a VPN Brute Force Attack?

A VPN brute force attack is an automated assault on a VPN authentication portal in which a bot rapidly submits thousands of username and password combinations, hoping to find a valid credential. There are three common variants:

• Classic brute force: The bot tries every possible password for a given username. This is slow and largely obsolete against modern systems with lockout policies, but it still occurs.
• Credential stuffing: The bot uses username/password pairs leaked from previous data breaches. Because users reuse passwords across services, this technique has a disturbingly high success rate—typically 0.1% to 2% of attempted credentials work.
• Password spraying: The bot tries a small number of common passwords (e.g., “Summer2025!”, “Welcome1”, “Company123”) against a large list of usernames. This is designed to stay just under account lockout thresholds, making it harder to detect.

Modern attacks combine all three techniques. A sophisticated campaign might start with password spraying to find easy wins, then shift to credential stuffing using breach data purchased on dark web marketplaces, and fall back to brute force only for high-value targets like admin accounts.

Why Attackers Target VPN Portals

VPN portals are among the highest-value targets on the internet, and for good reason. A single compromised VPN credential gives an attacker direct access to the internal corporate network, bypassing every perimeter defense the organization has built. Here is why VPN portals are disproportionately targeted:

• Internet-facing by design: Unlike internal applications hidden behind firewalls, VPN portals must be publicly accessible so remote employees can connect. This makes them trivially easy to discover through port scanning.
• High-value access: VPN access is typically full tunnel or split tunnel into the corporate LAN. A compromised account can reach file servers, databases, Active Directory, and other internal systems.
• Single-factor authentication: Despite years of warnings, a significant percentage of organizations still rely on username and password alone for VPN authentication. A 2025 survey by Rapid7 found that 38% of VPN portals exposed to the internet lacked MFA.
• Username enumeration: Many VPN portals return different error messages for “invalid username” versus “invalid password,” allowing attackers to confirm which usernames exist before launching password attacks.
• GlobalProtect is the #1 target: Palo Alto Networks GlobalProtect is the most widely deployed enterprise VPN portal. Its login page is easily identifiable via HTTP response headers, making it the first target for automated scanning tools.

The Real Damage: Beyond Security Risk

When most security teams think about brute force attacks, they focus on the risk of credential compromise. But the operational damage is often far worse, even when the attacker never guesses a correct password:

Account Lockouts at Scale

This is the number-one complaint from IT teams dealing with VPN brute force attacks. Most organizations enforce Active Directory account lockout policies—typically locking an account after 3 to 5 failed attempts within a 30-minute window. When a brute force bot tries passwords against hundreds of usernames, it triggers lockouts for every user whose username appears in the attack list.

Helpdesk Overload

Every locked account translates to a helpdesk call or ticket. During a sustained campaign, helpdesk teams spend hours per day simply unlocking accounts. This is time not spent on actual IT work, and it creates frustration for both support staff and affected users. Some organizations report that VPN brute force lockouts have become their single largest category of helpdesk tickets.

Log Noise and Alert Fatigue

A brute force campaign generating 10,000 failed login attempts per day creates massive volumes of authentication logs. SIEM systems trigger hundreds of alerts. Security analysts burn hours investigating what is ultimately automated bot traffic, developing alert fatigue that makes them less likely to notice a genuine compromise hidden in the noise.

Authentication Infrastructure Strain

Each failed VPN login attempt hits your RADIUS or LDAP server, your Active Directory domain controllers, and possibly your MFA provider. At sustained volumes of thousands of attempts per hour, this creates measurable load on authentication infrastructure, sometimes degrading performance for legitimate authentication across all services—not just VPN.

Solutions: How to Stop VPN Brute Force Attacks

There is no single silver bullet. A defense-in-depth approach combines multiple countermeasures. Here are the most effective solutions, ranked by how quickly you can deploy them:

1. IP Blocklist (5-Minute Setup)

The fastest solution by far. An IP blocklist configured as an External Dynamic List (EDL) on your firewall blocks known attacker IPs at the network layer, before traffic ever reaches your VPN portal. The attacker’s TCP connection is refused at the firewall, so your authentication server never processes the attempt.

ThreatListPro provides a curated blocklist of IP addresses actively engaged in VPN brute force attacks. You paste the URL into your firewall’s EDL configuration, bind it to a deny rule, and commit. Total deployment time: under 5 minutes. The blocklist updates weekly, so new attacker IPs are automatically blocked without any ongoing effort.

2. Rate Limiting (Hours to Configure)

Most next-generation firewalls can rate-limit connections to your VPN portal by source IP—for example, allowing no more than 10 connection attempts per minute per IP. This slows down brute force bots significantly but requires careful tuning. Set the threshold too low and you block legitimate users on shared networks; set it too high and the attacker still gets through.

3. Geo-Blocking (Hours to Configure)

If your users only connect from specific countries, you can block VPN access from all other regions using GeoIP policies on your firewall. This eliminates a large percentage of attack traffic but does not help against attackers using VPS infrastructure in your home country. It also creates issues for traveling employees.

4. Multi-Factor Authentication (Weeks to Months)

MFA is the gold standard for VPN security. Even if an attacker guesses a correct password, they cannot complete authentication without the second factor. However, MFA rollout is a significant project: you need to select a provider, integrate it with your VPN platform, enroll every user, handle exceptions, and provide support during the transition. For organizations with hundreds or thousands of users, this process typically takes 4 to 12 weeks.

5. Certificate-Based Authentication (Months)

The most robust solution: require a client certificate in addition to (or instead of) username/password. Without the certificate, the VPN portal refuses to even display a login page. This completely eliminates brute force attacks. However, deploying client certificates requires an internal PKI, an MDM solution to push certificates to devices, and a process for certificate lifecycle management. Expect a 3 to 6 month deployment.

Why IP Blocklists Provide the Best Immediate Protection

Every solution above has its place in a mature security program. But when your helpdesk is drowning in lockout tickets right now, you need a solution that works today. That is why IP blocklists are the critical first step:

• Deployment time: 5 minutes, not weeks or months
• User impact: Zero. No software to install, no behavior to change, no training required
• Stops lockouts immediately: Because attacks are blocked at the firewall, they never trigger authentication failures or account lockouts
• Complements every other solution: Blocklists work alongside MFA, rate limiting, and geo-blocking. There is no conflict.
• Reduces log noise: With attack traffic blocked at the perimeter, your SIEM and authentication logs become dramatically cleaner

An IP blocklist is not a replacement for MFA. You should absolutely deploy MFA as a long-term solution. But a blocklist gives you immediate relief while you plan and execute that larger project.