Summary
Gmail shows a 'Suspicious Link' notification for HTTPS websites due to a combination of factors, going beyond just basic encryption. These include: Google Safe Browsing flags for malware, phishing, or social engineering; SSL certificate misconfigurations (mixed content, outdated protocols); Subresource Integrity (SRI) failures; inconsistent URL canonicalization; strict or misconfigured Content Security Policies (CSP); hosting unfavorable or malicious content; the use of shared link redirectors; link cloaking; poor sender (IP and domain) reputation; the use of link shortening services; excessive or obfuscated tracking parameters; and multiple URL redirections. Gmail's algorithm considers numerous signals to protect users from potential threats even when HTTPS is present.
Key findings
- Google Safe Browsing: Websites flagged by Google Safe Browsing for malicious activity (malware, phishing, social engineering) will trigger the Gmail warning.
- SSL Configuration Issues: Misconfigured SSL certificates, such as mixed content or outdated protocols, can lead to the 'Suspicious Link' notification.
- SRI Failures: If Subresource Integrity checks fail (linked resource hash doesn't match), the warning can be triggered.
- URL Canonicalization: Inconsistent URL canonicalization (e.g., capitalization, trailing slashes) can be a factor.
- CSP Issues: A strict or misconfigured Content Security Policy (CSP) can lead to false positives, triggering the warning.
- Malicious Hosting: Hosting unfavorable or malicious content, even on the same shared host, can cause the warning.
- Link Cloaking: Link cloaking (disguising the destination URL) is a common tactic used in phishing and triggers the warning.
- Sender Reputation: A poor sender reputation, based on IP and domain, is a significant factor in triggering the 'Suspicious Link' warning.
- URL Shorteners: The use of URL shortening services is frequently associated with spam and can trigger the warning.
- Tracking Parameters: Excessive or obfuscated tracking parameters in the URL can trigger spam filters and the notification.
- Redirection Chains: Multiple URL redirections are often associated with malicious activity and can trigger the warning.
- CNAME issues: Certificate errors on CNAME records used for click tracking can trigger warnings.
Key considerations
- Google Safe Browsing: Ensure your website is not flagged by Google Safe Browsing for any malicious activity.
- SSL Configuration: Properly configure and maintain your SSL certificate, avoiding mixed content and using up-to-date protocols.
- SRI Implementation: Implement Subresource Integrity (SRI) for any externally hosted resources.
- URL Consistency: Maintain consistent URL canonicalization across your website.
- CSP Configuration: Carefully configure your Content Security Policy (CSP) to avoid blocking legitimate resources.
- Content Scrutiny: Regularly scrutinize your website content and links for any potentially unfavorable or malicious elements.
- Transparent Linking: Avoid link cloaking and ensure the displayed URL accurately reflects the destination.
- Reputation Management: Monitor and manage your sender reputation to prevent blacklisting.
- Direct Linking: Avoid using URL shortening services whenever possible; use direct links instead.
- Parameter Control: Limit the use of excessive tracking parameters in your URLs.
- Minimize Redirects: Minimize the number of redirects in your URLs.
- CNAME Configuration: Ensure SSL certificates are correctly configured on CNAME records used for click tracking.
What email marketers say
11 marketer opinions
Gmail displays a 'Suspicious Link' notification for HTTPS websites due to a variety of factors beyond just basic SSL encryption. These include issues with the certificate itself (invalid, self-signed, or misconfigured), problems with the linking domain or sending IP's reputation (due to past association with spam or phishing), the use of URL shortening services, excessive tracking parameters, multiple redirects, link cloaking, or the presence of mismatches between the displayed and actual link destinations. Gmail's algorithm considers various signals to protect users from potential threats even when HTTPS is present.
Key opinions
- Certificate Issues: Invalid, self-signed, or misconfigured SSL certificates can trigger warnings, even on HTTPS sites.
- Reputation Matters: Poor domain or sending IP reputation due to spam or phishing history can lead to warnings.
- Link Redirection Services: Using URL shortening services can raise red flags due to their association with malicious activities.
- Tracking Parameters: Excessively long or obfuscated tracking parameters can trigger spam filters.
- Multiple Redirects: Chains of URL redirections are often associated with malicious activities and can trigger warnings.
- Link Cloaking: Disguising the true URL destination (link cloaking) is a red flag for Gmail.
- CNAME issues: Certificate errors on CNAME records used for click tracking can trigger warnings.
Key considerations
- Monitor Reputation: Regularly monitor your domain and sending IP reputation to ensure they are not blacklisted.
- Valid Certificates: Ensure SSL certificates are correctly configured and up-to-date with no errors.
- Avoid Shortened URLs: Avoid using URL shortening services in email campaigns; use the full, direct URL when possible.
- Limit Tracking Parameters: Minimize the use of excessive tracking parameters in URLs.
- Reduce Redirects: Reduce the number of redirects in URLs to avoid triggering spam filters.
- Transparency: Avoid link cloaking, ensuring the displayed URL matches the destination.
- CNAME Configuration: Ensure SSL certificates are correctly configured on CNAME records.
Marketer view
Email marketer from Litmus Blog shares that excessively long or obfuscated tracking parameters added to URLs can sometimes trigger Gmail’s spam filters and lead to warnings, even for HTTPS sites, as these are often used to mask the true destination.
17 Dec 2021 - Litmus Blog
Marketer view
Email marketer from Google Support Forum explains that Gmail displays a 'Suspicious Link' warning when the system detects characteristics commonly used in phishing or other malicious attacks. This includes mismatches between the displayed link and the actual destination, or unusual URL structures.
11 Dec 2021 - Google Support Forum
What the experts say
4 expert opinions
Gmail's 'Suspicious Link' notification for HTTPS websites can be triggered by several factors. Even with HTTPS, the presence of malicious content on the same hosting, link cloaking (where the displayed URL differs from the actual destination), and a poor sender reputation (IP and domain) can lead to these warnings. Google's systems consider the broader context of the linked content and sender behavior when determining if a link is suspicious, going beyond just whether the site uses HTTPS.
Key opinions
- Malicious Content: Hosting or linking to malicious content, even on an HTTPS site, can trigger warnings.
- Shared Hosting Risk: Shared hosting environments can lead to warnings if other sites on the same host are flagged as malicious.
- Link Cloaking: Link cloaking, disguising the destination URL, is a common tactic used by spammers and phishers and is flagged by Gmail.
- Sender Reputation: A poor sender reputation, related to the sending IP and domain, can cause links to be flagged even if they lead to HTTPS sites.
Key considerations
- Content Monitoring: Regularly monitor your website and linked content for any malicious elements or security vulnerabilities.
- Hosting Environment: Be aware of the risks associated with shared hosting environments and the potential impact of other sites on your reputation.
- Transparency: Avoid link cloaking and ensure the displayed URL accurately reflects the destination.
- Reputation Management: Actively manage and protect your sender reputation by following email best practices and monitoring for blacklisting.
Expert view
Expert from Email Geeks suggests that the 'Suspicious Link' notification might appear if the website hosts other content that Google deems unfavorable.
16 Aug 2021 - Email Geeks
Expert view
Expert from Spam Resource explains that the sender's IP and domain reputation strongly influences whether links are flagged as suspicious. Even if the linked site is secure with HTTPS, a poor sender reputation can trigger warnings.
20 Dec 2024 - Spam Resource
What the documentation says
5 technical articles
Gmail's 'Suspicious Link' notification for HTTPS websites arises due to a combination of security measures and configurations. Google Safe Browsing flags sites distributing malware, engaging in phishing, or using social engineering tactics. Technical issues, such as misconfigured SSL certificates (mixed content, outdated protocols), Subresource Integrity (SRI) failures, inconsistent URL canonicalization, and strict or misconfigured Content Security Policies (CSP), can also trigger warnings, even on HTTPS sites.
Key findings
- Google Safe Browsing: Websites flagged by Google Safe Browsing for malicious activities trigger Gmail warnings.
- SSL Misconfiguration: Misconfigured SSL certificates (mixed content, outdated protocols) cause warnings.
- SRI Failures: Subresource Integrity (SRI) failures, where resource hashes don't match, trigger warnings.
- URL Canonicalization: Inconsistent URL canonicalization can be interpreted as suspicious.
- CSP Violations: Strict or misconfigured Content Security Policies (CSP) can lead to false positives.
Key considerations
- Safe Browsing Compliance: Ensure your website complies with Google Safe Browsing guidelines to avoid being flagged.
- SSL Configuration: Properly configure SSL certificates and avoid mixed content issues.
- Implement SRI: Implement Subresource Integrity (SRI) for linked resources.
- URL Consistency: Maintain consistent URL canonicalization practices.
- CSP Configuration: Carefully configure Content Security Policies (CSP) to avoid false positives.
Technical article
Documentation from SSL Labs explains that misconfigured SSL certificates, such as mixed content (HTTPS page loading HTTP resources) or outdated protocols, can lead browsers to display warnings, even if the site uses HTTPS.
1 Nov 2022 - SSL Labs Documentation
Technical article
Documentation from W3C states that a strict Content Security Policy (CSP) can cause warnings if linked resources violate the policy rules. While CSP enhances security, misconfiguration can lead to false positives and warnings in Gmail.
21 Oct 2023 - W3.org
