Why is Google flagging my CDN or email content as malicious?
Michael Ko
Co-founder & CEO, Suped
Published 30 Jul 2025
Updated 19 Aug 2025
6 min read
Suddenly seeing a warning from Google that flags your CDN or email content as malicious or dangerous can be a significant setback. It impacts trust, deliverability, and user experience. Whether it’s a “Deceptive site ahead” warning on your website or an email banner saying “This message might be suspicious,” these alerts signal that Google's automated systems have detected something potentially harmful.
Such flags can stem from various sources, ranging from legitimate security threats like malware or phishing attempts to misconfigurations or even issues with shared infrastructure. The key is to act swiftly to identify the root cause and rectify it, restoring your online reputation and ensuring your content reaches its intended audience without alarming warnings.
It’s not uncommon for even reputable services to encounter these warnings. Sometimes, it feels like Google has become increasingly sensitive, and what might have passed unnoticed before is now triggering alerts. This article explores the common reasons behind these flags and outlines steps you can take to address them.
Why Google flags content as malicious
Google's primary goal with these warnings is to protect users from harmful content, whether that's malware, phishing schemes, or misleading information. Their automated systems constantly scan websites and analyze email patterns, leveraging a vast amount of data to identify suspicious activity. This proactive approach helps maintain a safer online environment, but it can also lead to legitimate content being flagged erroneously. You can read more about what Google considers dangerous or malicious content in their official guidelines.
The detection mechanisms are sophisticated and consider numerous factors. For websites, this includes the presence of malicious code, suspicious redirects, deceptive elements, or even links to other flagged sites. For email, it involves analyzing sender reputation, authentication records (like SPF, DKIM, DMARC), content patterns, and the nature of any embedded links or attachments.
A site might be labeled as dangerous or added to Google's blacklist (or blocklist) for many reasons, including a compromised domain or the presence of third-party scripts. This is especially true if there's any indication of phishing, where users are tricked into revealing sensitive information. Sometimes, it's not even your direct content, but something loaded from a third-party resource, like an ad network or an external script, that triggers the warning.
Common causes for CDN content being flagged
Content Delivery Networks (CDNs) are designed to speed up content delivery, but they can also become a vector for malicious activity if not properly secured or if compromised. If your CDN-hosted assets are flagged, it often points to one of several issues.
Compromised CDN storage
If your CDN storage bucket, like an AWS S3 bucket, is compromised, attackers might inject malicious scripts, redirects, or even phishing pages directly into your hosted content. This content is then served to your users via your trusted CDN, triggering Google’s warnings.
Shared IP reputation
CDNs use shared IP addresses. If another user on the same CDN IP address is hosting malicious content, that IP can end up on a Google blacklist (or blocklist). Your content, even if clean, might be caught in the crossfire due to this shared reputation. This is similar to how a shared IP can get blocklisted for email sending.
Third-party script injection
If your website loads third-party scripts (e.g., analytics, ads, widgets) via your CDN, and one of these scripts becomes compromised, it can inject malicious code. This is a supply chain attack where the legitimate CDN infrastructure is used to distribute malware.
Misconfigured CNAME records
Sometimes, an improperly configured CNAME record or a lack of a CNAME pointing to your organizational domain can contribute to Google's suspicion. If the domain doesn't align with your brand, it might appear suspicious, especially if the CDN's default domain is generic or has a poor reputation.
Even for well-known CDNs like Cloudflare, issues can arise. Google's systems might flag them if they detect unusual traffic patterns or if specific assets hosted on the CDN are suspicious, such as long or numeric tracking links which can sometimes be mistaken for malicious patterns.
Red flags in CDN content
IP addresses in links: Malicious actors often use IP addresses directly in URLs to hide their true destination. This can instantly trigger warnings.
Domain mismatches: When the visible link text differs significantly from the actual destination URL, it looks like a phishing attempt.
Suspicious redirects: If a CDN asset redirects users to an unexpected or known malicious site, it will be flagged.
Compromised third-party scripts: Scripts loaded from external sources that become infected can inject malicious content.
Why email content gets flagged as malicious
Email content being flagged as malicious (or spam) is a common headache for senders. Google (specifically Gmail) employs sophisticated filters to protect users from phishing, malware, and unwanted commercial messages. Even legitimate emails can sometimes trigger these warnings.
One of the most significant factors is sender reputation. If your domain or IP has a poor sending history, Google is more likely to view your emails with suspicion. This is particularly true if your domain has been associated with spam or phishing in the past. Emails from unindexed domains can also trigger dangerous flags.
Common email content triggers
Suspicious links: Links that appear deceptive, use IP addresses, or point to known malicious domains will be flagged.
Domain mismatch in links: If the visible domain in a link doesn't match the actual linked domain (e.g., due to click tracking), Google might flag it. For legitimate email service providers (ESPs), this can cause their tracking links to be blocked.
Poor email authentication: Lacking or misconfigured SPF, DKIM, or DMARC records can severely impact your sender reputation and lead to messages being flagged or sent to spam.
High bounce rates: Regularly sending to invalid email addresses signals poor list hygiene and can hurt your reputation.
Content patterns: Using common spam keywords, excessive capitalization, or suspicious image-to-text ratios can trigger filters.
Additionally, if you're experiencing inconsistent warnings, or if Google itself is flagging emails from its own services as malicious, this suggests that the underlying issue might be a system-wide detection sensitivity rather than a direct problem with your content, although it's still crucial to ensure your setup is flawless.
Steps to diagnose and resolve the issue
When Google flags your content, a systematic approach is essential for diagnosis and resolution. Start by checking Google's own tools and then delve into your infrastructure and email practices.
Check Google Search Console: For websites, this is your first stop. Look for Security Issues reports that detail any detected malware or hacked content. You'll often find specific URLs that are problematic.
Utilize Google Postmaster Tools: If email is being flagged, Postmaster Tools (GMT) provides insights into your domain's reputation, spam rate, and DMARC failures. This can help identify why your emails are marked as suspicious or sent to spam.
Scan your website and CDN content: Use a reliable malware scanner to thoroughly check your website files, database, and any content hosted on your CDN. This includes images, scripts, and other assets.
Resolving CDN and email issues
For CDN issues, ensure your CDN storage buckets are secure with proper access controls. If you use shared CDN IPs, consider custom domains or dedicated IPs if reputation issues persist. For email, verify your email authentication protocols are correctly set up and passing.
After resolving the detected issues, request a review through Google Search Console for websites. For email, monitor your Postmaster Tools data and ensure consistent, healthy sending practices. Be patient, as reputation rebuilding can take time. Consistent adherence to best practices, including regular blocklist monitoring, will prevent future issues.
Views from the trenches
Best practices
Regularly audit your CDN-hosted content for any unexpected files or scripts. Implement strict access controls for your storage buckets.
Ensure all your email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned for your sending domains.
Keep an eye on Google Search Console for any security warnings related to your website. Respond to these alerts promptly.
Common pitfalls
Using default CDN domains or generic S3 bucket URLs without a custom CNAME can raise red flags for Google's systems.
Long, numeric, or overly complex tracking links in emails can sometimes be mistaken for malicious patterns by spam filters.
Ignoring bounce rates or sending emails to unverified lists can quickly degrade your sender reputation, leading to blocklisting.
Expert tips
If your CDN is shared, investigate if other users on the same IP are causing reputation issues and consider a dedicated IP if available.
For email, ensure images are hosted on your primary sending domain, which should pass SPF and DKIM authentication.
When encountering 'deceptive site ahead' warnings, check for compromised third-party scripts or redirects on your site.
Marketer view
Marketer from Email Geeks says they had issues with their AWS S3 bucket being flagged by Google. They speculated that not having a CNAME setup with their organizational domain might have contributed to the problem, suggesting custom domains could help.
2019-06-14 - Email Geeks
Marketer view
Marketer from Email Geeks says Google seemed to get 'trigger happy' with their warnings around mid-2019, indicating a possible increased sensitivity in their detection algorithms.
2019-06-14 - Email Geeks
Maintaining a clean online presence
Preventing Google from flagging your CDN or email content as malicious requires ongoing vigilance and adherence to best practices. This isn't a one-time fix but a continuous effort to maintain a trustworthy online presence. Regularly reviewing your website security and email deliverability metrics is crucial.
For your website and CDN, ensure all software, plugins, and themes are updated to prevent vulnerabilities. Implement a Web Application Firewall (WAF) and regularly scan for malware. Be cautious with third-party scripts and ensure they come from reputable sources. Consider using a custom domain for your CDN to build a dedicated reputation.
For email, commit to excellent list hygiene, regularly removing invalid or inactive addresses. Continuously monitor your sender reputation and DMARC reports to catch any authentication failures or suspicious activity early. By adopting these proactive measures, you can significantly reduce the risk of your content being flagged and ensure your messages reach the inbox reliably.
AWS S3 bucket, is compromised, attackers might inject malicious scripts, redirects, or even phishing pages directly into your hosted content. This content is then served to your users via your trusted CDN, triggering Google’s warnings.
Cloudflare, issues can arise. Google's systems might flag them if they detect unusual traffic patterns or if specific assets hosted on the CDN are suspicious, such as long or numeric tracking links which can sometimes be mistaken for malicious patterns.
