How to prevent email listbombing and bot sign-up attacks?

Key findings

1. Evolving threats: The origin of bot attacks shifts, with recent increases noted from regions like Russia, replacing previous patterns from other countries (e.g., Mandarin-speaking regions).
2. Impact beyond forms: Attacks are not limited to direct sign-up forms. They can exploit integrations where user creation in one platform triggers emails from another, leading to a cascade of unwanted messages.
3. Reputation damage: High volumes of unconfirmed or suspicious sign-ups can severely impact your sender reputation, increasing the likelihood of your legitimate emails landing in the spam folder or being blocklisted.
4. Variety of targets: Bots often target smaller or less protected forms across various websites, not necessarily specific large companies, making it a widespread issue.

Key considerations

1. Proactive detection: Implement systems to detect and prevent bot sign-ups and subscription bombing before they trigger email sends. This includes monitoring for unusual patterns or suspicious IP addresses.
2. Form security: Secure all data entry points on your website, including sign-up forms, contact forms, and other user-generated content sections (e.g., wishlists), using appropriate bot prevention techniques. For more detailed advice, refer to our guide on how to protect email list sign-up forms from bots.
3. Email validation: Utilize robust email validation tools and practices at the point of sign-up to filter out invalid, disposable, or bot-generated addresses before they enter your list. This helps mitigate the risks associated with fraudulent sign-ups, as outlined by Mapp's blog on protecting your email list.
4. Continuous monitoring: Regularly review new sign-ups and email engagement metrics for suspicious activity, as bot tactics are constantly evolving.

Key opinions

1. Language patterns: Marketers observe shifts in bot attack origins, such as a recent surge in Russian language content, names, and mail.ru email addresses within listbombs, indicating a change in attacker tactics.
2. Indirect impact: Even if your own sign-up forms are secure, listbombing can still affect your email marketing. This occurs when integrations create users on your platform based on compromised forms elsewhere, triggering your automated welcome emails to fraudulent addresses.
3. Vulnerable features: Certain website features, like Magento's 'share wishlist,' which may lack built-in CAPTCHA, are often exploited by bots for listbombing purposes.
4. Scalability issues: While individual instances might seem small, the overall trend of bot attacks is on the rise, necessitating scalable and adaptive defense mechanisms.

Key considerations

1. Pattern analysis: Develop internal checks and rules by analyzing patterns in fraudulent sign-ups, such as specific email domains or naming conventions used by bots, to identify and block them.
2. Integration security: Ensure that any third-party platforms or integrations that create users in your system have robust bot prevention in place, as vulnerabilities there can directly impact your email deliverability. This is crucial for improving welcome series deliverability.
3. Double opt-in: Implementing double opt-in for all new subscribers is highly effective. It requires users to confirm their email address, preventing bots from adding unverified addresses to your list, as highlighted by Alterable's advice on protecting email marketing campaigns.
4. Continuous vigilance: Even with existing protections, spambots constantly evolve. Therefore, continuous vigilance and adaptation of security measures are essential to prevent them from hitting spam traps and impacting sender reputation.

Key opinions

1. User-agent string analysis: Experts recommend examining user-agent strings, as many bots utilize very specific or unusual strings that can be easily filtered to block malicious requests. This provides a granular method for identifying and preventing bot activity.
2. Holistic form security: Beyond basic CAPTCHAs, a comprehensive approach to form security is necessary. This involves evaluating whether forms are inherently unprotected or if attackers are consistently finding new vulnerabilities across different sites.
3. Reputation protection: Email bombing poses a serious threat to sender reputation, potentially leading to blocklisting (or blacklisting) by major providers like Spamhaus. Proactive measures are essential to safeguard deliverability.
4. Beyond direct forms: Attacks often target sites and brands not specifically because of their size, but because they have exploitable forms. The focus should be on securing all potential entry points, regardless of your company's profile.

Key considerations

1. Advanced filtering: Traditional email filters might not suffice against email bombing because the subscription emails often come from legitimate, albeit compromised, sources. Implementing advanced email filtering, possibly with rate limiting, is crucial for diagnosing and preventing emails from going to spam.
2. Behavioral analysis: Beyond simple checks, analyzing user behavior for suspicious patterns (e.g., rapid sign-ups from a single IP address, unusual naming conventions) can help identify and mitigate bot activity. Understanding spam trap hits can also provide insights into bot activity.
3. Threat intelligence: Staying informed about new vectors of abuse and emerging mail bombing techniques is vital. Resources like Sedara Security's guide on how to recognize and mitigate email bomb attacks offer valuable insights into current defense strategies.
4. Collaboration: In cases where attacks involve third-party integrations, collaboration with partners to secure their forms and prevent fraudulent sign-ups that impact your systems is crucial.

Key findings

1. CAPTCHA implementation: Placing CAPTCHA on sign-up forms is consistently recommended as a primary deterrent for bots, available in various types to suit different security needs.
2. Deny and allow lists: Utilizing deny lists to quarantine suspicious emails and allow lists for trusted senders is a common practice to manage incoming mail during bombing attacks.
3. Platform-specific protections: Some major email service providers and platforms (e.g., Klaviyo, Microsoft Defender for Office 365) have built-in systems to detect and prevent listbombing, such as IP management and default protection mechanisms.
4. Rate limiting: Setting limits on the volume of incoming emails or sign-up requests from a single source can prevent systems from being overwhelmed during a bombing attack.

Key considerations

1. Double opt-in as standard: Documentation frequently highlights double opt-in as one of the most effective preventative measures, requiring subscribers to confirm their email addresses before joining a list. This practice also helps in avoiding spam traps.
2. Email filtering limitations: It's noted that traditional email filters may struggle to block email bombing attacks because the subscription emails often originate from legitimate sources, necessitating more advanced solutions.
3. Auditing subscription lists: Periodically auditing subscription lists for all business email addresses can help in identifying and preventing list linking attacks. This involves removing suspicious or invalid entries, as discussed in strategies for blocking disposable email domains.
4. Continuous updates: Security features and protections are constantly updated. Staying informed about new releases, like Microsoft's default protections against email bombing, is key to maintaining effective defenses, as highlighted by Klaviyo's list bombing IP management system.