What are the best methods to prevent spam email subscriptions and subscription bombing?

Key findings

• Subscription bombing: This is a form of attack where large numbers of fake or unwanted email addresses are subscribed to a list, often to overwhelm the target's inbox and potentially hide more malicious activity like account breaches. Sometimes the goal is to bury a legitimate notification about an account compromise.
• Impact on deliverability: A high volume of invalid or unwanted sign-ups can severely degrade sender reputation, leading to increased bounce rates, spam complaints, and ultimately, emails landing in the spam folder rather than the inbox.
• Plus addressing: Email addresses containing a '+' (e.g., your.email+tag@gmail.com) are a legitimate feature for users to manage and filter incoming mail. Automatically blocking emails with plus addressing can inadvertently exclude real subscribers.
• Mitigation layers: Effective prevention relies on a combination of techniques, including double opt-in (DOI), CAPTCHAs, honeypot fields, and rate limiting sign-ups from specific IP addresses. Each layer adds a defense against automated attacks.

Key considerations

• List cleaning: Regularly remove any duplicate or suspicious email addresses from your list. It is often recommended to remove even the original entry if it's part of a bombing attack, as the legitimate owner likely didn't subscribe.
• Double opt-in (DOI): Implementing DOI ensures that only confirmed subscribers are added to your list, significantly reducing the impact of bot sign-ups. However, if not combined with other measures, the DOI confirmation emails themselves could contribute to the bombing effect.
• Data validation: When checking for duplicate sign-ups, strip 'dots' and any characters after a '+' from Gmail addresses. This allows you to identify if the same core email address is attempting to subscribe multiple times using variations.
• Form protection: Utilize techniques like CAPTCHA and honeypot fields on your subscription forms to deter automated bots without hindering legitimate users. Explore further how to prevent nefarious email signups using these methods.
• Rate limiting: Implement a system to limit the number of sign-ups allowed from a single IP address within a specific timeframe. This helps to slow down or halt large-scale automated attacks. Understanding IP list bombing mitigation strategies is essential.

Key opinions

• Initial response: Marketers often start by filtering out suspicious addresses, such as those containing a '+' or duplicates, to prevent immediate deliverability damage from bot sign-ups.
• Double opt-in: Many marketers advocate for enabling double opt-in (DOI) on subscription forms as a primary defense, especially on platforms like Shopify, to ensure only confirmed users are added to the active list.
• Risk of over-filtering: There's concern that aggressively filtering legitimate features like plus addressing can block real subscribers, who use these to manage their inboxes. Some users actively employ plus addressing for tagging and organization.
• Form security: Implementing CAPTCHA and honeypot fields are widely recommended as foundational layers of security for web forms to deter automated bot submissions. Learn more about how to prevent spam bot signups on your website.

Key considerations

• Beyond initial fixes: While immediate filtering can help, a comprehensive strategy is needed to prevent recurring attacks. Marketers need to consider the long-term impact on their list health and sender reputation.
• Honeypot implementation: Honeypots are effective but often require development work or technical expertise to implement correctly on website forms. This is a key step in minimizing bot signups on email forms.
• Monitoring sign-up metadata: Checking IP addresses and user-agents associated with suspicious sign-ups can provide valuable clues for identifying and blocking bot activity.
• Comprehensive approach: Relying on a single defense mechanism (like DOI) might not be sufficient. A layered defense, including CAPTCHA, honeypots, and rate limiting, offers more robust protection against diverse bot attacks.

Key opinions

• Address normalization: Experts suggest stripping 'dots' and any characters after a '+' from Gmail addresses during new user validation to check for existing accounts. This helps prevent bots from signing up the same email multiple times using subtle variations. This is a critical step in identifying and removing bot-generated spam.
• Subscription bombing cause: Subscription bombing is identified as an attack, and experts recommend checking other signup metadata, such as IP addresses and user-agents, for patterns.
• Advanced form protection: CAPTCHA and honeypot fields are seen as essential, basic requirements for preventing bot sign-ups. Honeypots are particularly effective for blocking automated scripts without affecting human users.
• Multi-tenant detection: A more advanced measure suggested is detecting if the same email address quickly signs up across multiple tenants or brands, especially if they are on the same ESP. This could indicate coordinated bot activity or list brokering.
• DOI limitations: While double opt-in (DOI) is valuable, experts caution that if other measures aren't in place, the DOI confirmation emails themselves can contribute to a spam bomb, further burdening recipient inboxes.

Key considerations

• Stripping addresses carefully: Stripping '+' and '.' characters for duplicate detection is valid, but outright dumping or blocking emails containing '+' is a bad idea, as many legitimate users utilize this feature for tagging and organizing their mail.
• Comprehensive anti-fraud systems: Consider integrating anti-fraud systems at the signup stage. These systems can analyze various data points to identify and prevent fraudulent sign-ups. Backend validations are crucial for preventing spam.
• Rate limiting by IP: Implement rate limiting on signup forms to restrict the number of submissions from a single IP address within a given period, which can effectively slow down automated attacks.
• Honeypot technicality: While highly effective, honeypot fields typically require some coding or development work to implement correctly on website forms, making them a more advanced solution.
• Continuous monitoring: Even after implementing solutions, ongoing monitoring of sign-up activity and list health is necessary to detect new attack vectors and maintain good deliverability.

Key findings

• Definition of attack: Email bombing (or subscription bombing) is recognized as a denial-of-service attack designed to overwhelm an inbox with a flood of emails. This can be used to hide critical security notifications, such as account compromise alerts.
• Diversion tactic: Documentation often highlights that subscription bombing serves as a diversion. The primary aim is usually to distract the victim while a more serious cyberattack, like a bank account takeover, is in progress.
• Email filtering tools: Email service providers offer features like bulk mail filters, deny lists (blacklists), and allow lists to help manage incoming email. Deny lists block specific unwanted emails, while allow lists ensure trusted emails bypass filters.
• Email provider protection: Major providers like Microsoft are actively developing and deploying enhanced protections against email bombing attacks, indicating the seriousness and prevalence of this threat.

Key considerations

• Utilizing built-in features: Users should leverage the spam filtering and detection capabilities provided by their email service. These tools are often sophisticated and can automatically identify and quarantine suspicious emails.
• Regular rule updates: As new spam arrives, users should continuously update their email rules and filters to adapt to evolving attack patterns. This involves actively marking messages as spam to train the filtering system. Dartmouth College advises regularly updating rules to route new spam messages to junk folders. Read more about how to mitigate email bombing and manage incoming spam.
• User vigilance: Even with automated defenses, users should remain vigilant. Avoiding clicking suspicious links and monitoring financial accounts are crucial personal protection measures during such attacks.
• Multifactor authentication (MFA): Documentation often links email bombing to broader security threats. Implementing MFA on sensitive accounts (banking, brokerage) is strongly recommended as a fundamental defense against potential breaches that these attacks might conceal. Craig Ball emphasizes the value of deploying MFA to protect banking and brokerage accounts.