Rate limiting is one of the oldest and most widely used defenses in web security. It’s supposed to help protect login forms, password-reset flows, and other sensitive endpoints from abuse. But as the internet evolves, so do the tools attackers use—and some longstanding defenses no longer hold up the way they once did.
Recently, while testing the security of a “Forgot Password” flow, I uncovered a flaw that highlights just how outdated IP-based rate limiting has become. With nothing more than basic IP rotation techniques, I was able to bypass the application’s throttling entirely and send unlimited password-reset requests to any target email address.
This post walks through what I discovered, why it matters, and why developers need to rethink how they design defenses for critical authentication endpoints.
The Problem With IP-Based Throttling in 2025
Most applications today still follow a familiar pattern:
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A user enters their email into the “Forgot Password” form.
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The server sends a reset link.
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To prevent abuse, the server limits how many requests can be made from the same IP.
At first glance, this seems reasonable. But in practice, attackers have countless ways to cycle through IPs:
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VPNs and proxy services
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Shared hosting networks
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Cloud platforms like AWS, DigitalOcean, or Vultr
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IPv6 subnets that come with trillions of unique addresses
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Botnets of compromised devices around the world
What does this mean?
It means that IP-based rate limiting no longer meaningfully limits anything.
What I Observed During Testing
My test was simple:
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Submit multiple password-reset requests for the same email.
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Wait until the server returns a “too many attempts” or “rate limited” response.
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Change my IP.
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Try again.
Each time I switched IPs, the server completely reset the throttle. I was treated as a brand-new user—able to send more password-reset emails instantly.
It didn’t matter that I was requesting resets for the same email, in the same pattern, seconds apart. The only thing the server cared about was the client IP.
This meant that:
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An attacker could send unlimited reset emails.
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A user could be mail-bombed into submission.
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The underlying authentication endpoint could be brute-forced across thousands of IPs.
In short: the system’s main line of defense could be bypassed with a technique available to anyone with a VPN subscription.
Real-World Abuse Isn’t Hypothetical
This kind of vulnerability has precedent. One of the most infamous examples was the 2015 Apple “Celebgate” incident, where attackers brute-forced credentials against an Apple endpoint that relied heavily on IP-based throttling.
The takeaway from that event—and countless others since—is clear:
IP-based rate limiting is not a modern security control. It’s a speed bump at best.
Yet many applications still depend on it as if it were a reliable barrier.
Why It Matters: The Impact
This loophole opens the door to a range of abusive behaviors:
1. Mail Bombing
A malicious actor could bombard a victim’s inbox with hundreds or thousands of password-reset emails.
This can:
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overwhelm and frustrate the user
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hide legitimate security alerts
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degrade trust in the platform
2. Distributed Brute Force
Login or reset endpoints that rely solely on IP throttling are particularly vulnerable.
Attackers can distribute attempts across many IPs and avoid triggering any alarms.
3. User Experience Damage
If users regularly receive unsolicited reset emails, they’ll perceive the platform as insecure—even if their account hasn’t been compromised.
4. Potential Account Takeover
If combined with weak tokens, predictable reset flows, or other vulnerabilities, this issue could escalate to full account compromise.
Why This Is So Easy for Attackers
The reality is that rotating IP addresses is now trivial:
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Many cloud providers offer full /64 IPv6 ranges, giving attackers access to 18 quintillion addresses per VPS.
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Modern botnets contain millions of globally distributed devices.
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Proxy and VPN networks offer automated IP rotation as a standard feature.
A security control that assumes “one user = one IP” simply doesn’t reflect the state of the internet anymore.
How to Fix It: Modern Defenses That Actually Work
To meaningfully protect password-reset flows and similar endpoints, developers need to adopt multi-layered, identity-aware mitigation strategies.
Here are the most effective options:
1. CAPTCHA Challenges
A CAPTCHA (invisible or user-facing) prevents automated mass requests even across thousands of IPs.
2. Rate Limiting Based on the Target Email
Instead of only limiting per-IP, also limit how often each email can receive a reset.
3. Behavioral and Velocity Checks
Look beyond IPs:
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browser fingerprints
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request velocity
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unusual patterns or bursts
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device reputation
4. Honeypots or Invisible Fields
Bots often fill fields humans never touch—this is an easy way to detect automated abuse.
5. Abuse Detection Services
Modern WAFs (Web Application Firewalls) and anti-bot platforms use IP reputation, machine learning, and behavioral signals that far outperform raw IP throttling.
6. User Alerts
Notify users when multiple reset attempts occur. Visibility itself is a form of defense.
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