5 min read

What is an enumeration attack?

Farah Amod Mar 26, 2025 7:20:00 PM

Cybersecurity

An enumeration attack is a method used by hackers to confirm and verify records stored on a web server using brute-force techniques. These attacks commonly target web forms that interact with a server’s database, such as login pages and password reset forms. Attackers attempt to systematically test for valid usernames, email addresses, or other credentials, exploiting subtle server responses that reveal whether an account exists.

How enumeration attacks work

Enumeration attacks allow cybercriminals to determine whether an email address or username is linked to an account on a website. These attacks often target login pages, password reset forms, and account creation fields, where a system’s response such as an error message or a slight delay in processing can reveal whether an account exists.

A study published in ACM Digital Library examined enumeration vulnerabilities across 63 popular online services. Researchers tested three attack methods: login attempts, password reset requests, and account creation forms. They found that 93.7% of these services were susceptible, often exposing account existence through inconsistencies in error messages or response behaviors.

Attackers typically automate enumeration attempts using bots or scripts that rapidly input different email addresses or usernames. Some attacks also exploit timing differences in server responses or valid accounts may trigger slightly longer processing times compared to non-existent ones, inadvertently confirming the presence of an account. A separate study in IEEE Transactions on Information Forensics and Security showed how even minor variations in error messages or response times can give attackers the information they need to compile lists of valid accounts.

Beyond identifying accounts, enumeration attacks create opportunities for phishing. The ACM study surveyed 318 participants and held focus groups to assess public awareness. Results showed that many users were unaware of these threats and unknowingly made themselves more vulnerable by reusing the same email across multiple platforms. The research also found that users were twice as likely to fall for phishing emails from services they actually use, proving how enumeration attacks can be a precursor to more dangerous cyber threats.

The researchers shared their findings with national data protection authorities and vulnerable service providers, leading to updates in developer security guidelines. However, at the time of reporting, only one out of 59 contacted services had fully addressed the issue. The study called for stronger countermeasures, such as standardized error messages, improved login security, and support for anonymous or alias-based email registrations.

Read more: What is a phishing attack?

Understanding enumeration attacks in healthcare

Healthcare systems are frequent targets for cyberattacks, and enumeration attacks are one of the first steps attackers use to gain access. In healthcare, these attacks exploit login pages, patient portals, and other authentication points to confirm which usernames, emails, or account numbers exist in a system.

For example, if a hospital’s password reset page responds differently when an email exists versus when it doesn’t, an attacker can systematically test large lists of emails to confirm which ones belong to real users. Once they have that information, they can launch phishing scams, attempt password guessing, or use stolen credentials from other data breaches to break in.

The best defense is limiting the information attackers can gather. Healthcare organizations should use generic error messages that don’t confirm whether an account exists, apply rate limiting to block repeated attempts and require multi-factor authentication. Since the Health Insurance Portability and Accountability Act (HIPAA) requires strong safeguards for patient data, preventing enumeration attacks should be part of every healthcare provider’s security strategy.

Types of enumeration attacks

According to Forbes, hackers and penetration testers use several types of enumeration attacks to gather details about a target network. SNMP enumeration extracts information from network devices like routers and switches. NetBIOS enumeration helps identify shared resources and services within a local network. LDAP enumeration can reveal “usernames, groups, and system configurations” in Active Directory, which could give an attacker a clear view of how a network is set up. NTP enumeration targets network time servers to uncover connected devices. SMTP enumeration is used to verify email addresses on a mail server, making it easier to launch phishing or brute-force attacks. DNS enumeration helps attackers “map out a network” by discovering subdomains and related services. Other methods, such as IPsec, VoIP, RPC, and Unix/Linux User enumeration, also help gather intelligence. Since enumeration is often the first step in an attack, understanding these techniques can help prevent unauthorized access.

Real-world examples of enumeration attacks

In 2013, a security researcher identified a vulnerability in Facebook's password recovery mechanism. The standard process involved users entering their email address, receiving a six-digit recovery code via SMS, and submitting this code through an API endpoint. While Facebook had implemented rate limiting to prevent brute-force attacks on the main API endpoint, the researcher discovered that alternative subdomains (beta.facebook.com and mbasic.beta.facebook.com) lacked these protections. The oversight allowed attackers to brute-force the recovery code, effectively resetting the passwords of targeted accounts and gaining unauthorized access. The researcher reported this issue to Facebook, which subsequently addressed the vulnerability and awarded a $15,000 bounty for the discovery.

Another notable example occurred with PayPal, where security researchers found that the platform’s password reset function would confirm whether an email address was registered. The vulnerability was later addressed by displaying a generic message regardless of whether the email existed in their database. Similar attacks have been observed on e-commerce platforms and banking websites, where attackers systematically check for valid accounts and use this information to commit fraud or identity theft.

Go deeper: What is a brute force attack?

Why enumeration attacks are effective

Enumeration attacks are effective because they exploit predictable web application behaviors. Many websites provide different responses based on whether a username or email address is valid, which attackers can systematically analyze. Even if explicit error messages are avoided, timing differences in response times can provide subtle clues that hackers can use.

Another reason these attacks succeed is that many users reuse their email addresses across multiple online services. Once an attacker confirms an email address or username on one platform, they can use it to launch attacks on other sites. Additionally, poorly configured web applications that do not implement security best practices make it easier for attackers to execute enumeration attacks without facing obstacles.

How to prevent enumeration attacks

According to TechTarget, web application security best practices can help mitigate enumeration attacks:

Use generic error messages: Instead of explicitly stating that an email or username is invalid, a message should simply indicate that the provided credentials are incorrect without confirming their existence.
Standardize response times: Web servers should avoid varying response times based on the validity of user inputs. Attackers often rely on subtle delays to infer valid credentials.
Limit login attempts: Implementing rate limiting and CAPTCHA challenges can reduce the effectiveness of brute-force enumeration attempts.
Monitor and log suspicious activity: Security teams should track repeated failed login attempts from the same IP address, which may indicate an enumeration attempt.
Implement multi-factor authentication (MFA): Requiring an additional layer of security makes it harder for attackers to exploit enumerated credentials.
Mask user account details in password reset requests: When users request a password reset, websites should send an email with reset instructions without indicating whether the email is associated with an account.

Read also: A guide to cybersecurity policies

In the news

Visa has introduced a new generative AI-powered tool to combat enumeration attacks, a serious source of fraud in card-not-present (CNP) transactions. Enumeration attacks cost the industry an estimated $1.1 billion annually according to Fintech Times.

To address this threat, Visa has enhanced its Visa Account Attack Intelligence (VAAI) system with the VAAI Score, a real-time risk assessment tool. The tool assigns a risk score to each transaction, helping issuers detect and prevent enumeration attacks before they can cause further damage. Initially available to U.S. issuers, the VAAI Score tries to reduce fraud and operational costs while improving the overall cardholder experience.

Paul Fabara, Visa’s chief risk and client services officer, discussed the need for real-time fraud detection, noting that 33% of enumerated accounts experience fraudulent activity within just five days of exposure. Using generative AI, the VAAI Score enhances Visa’s fraud detection capabilities, reducing false positives by 85% compared to traditional models. Improved accuracy allows issuers to make more informed decisions about blocking suspicious transactions without unnecessarily disrupting legitimate cardholder activity.

Visa has long prioritized security, investing over $10 billion in fraud prevention technology over the past five years. In FY23 alone, Visa blocked $40 billion in fraudulent transactions, proving its commitment to safeguarding the payment ecosystem. With the introduction of the VAAI Score, Visa continues to strengthen its defenses against cybercriminals, ensuring safer transactions for businesses and consumers alike.

FAQs

How is an enumeration attack different from a brute-force attack?

Enumeration attacks focus on confirming valid usernames or email addresses, while brute-force attacks attempt to guess passwords or authentication tokens to gain direct access to accounts.

Why do websites sometimes reveal whether an email or username exists?

Some websites provide specific error messages for user convenience, but this can unintentionally help attackers confirm valid accounts, making them vulnerable to enumeration attacks.

Can enumeration attacks be used for phishing scams?

Yes, attackers use enumeration to compile lists of valid accounts, which they can then target with phishing emails designed to steal credentials or personal information.