⚠️ Overview

hstspreload-bot is an automated crawler operated by Google for the purpose of verifying domain eligibility for the HSTS (HTTP Strict Transport Security) preload list. This bot is not a malicious tool but a legitimate service; however, many web application security teams treat it as an unwanted scanner because it aggressively probes HTTPS configurations, and some organisations block it to reduce unnecessary traffic or false alarms. The bot is maintained by Google’s Chrome security team and runs from Google’s own IP ranges, with its User-Agent string documented in Google’s official crawler list.

🔧 Technical Capabilities

The bot performs a series of passive and active checks against a domain submitted for HSTS preload inclusion. It verifies that the server sends a valid Strict-Transport-Security header with a max-age of at least one year (31536000 seconds) and the includeSubDomains directive. Additionally, it confirms that the site redirects all HTTP traffic to HTTPS and that no mixed content warnings exist. The bot issues requests over both HTTP and HTTPS, inspects certificate validity and chain completeness, and checks for common misconfigurations such as missing HSTS headers on subdomains. It also validates that the domain resolves correctly in DNS and that no wildcard certificates expose broader attack surfaces. Unlike vulnerability scanners, it does not attempt exploitation, but its repeated, systematic probing can resemble reconnaissance behavior.

📜 History & Notable Incidents

First documented around 2015 when Google introduced the automated preload submission process via hstspreload.org, the bot replaced manual verification by Chrome engineers. In 2020, a misconfiguration caused the bot to retry submissions too frequently, leading to bandwidth spikes for some high-traffic sites. Google later throttled the crawler and added rate-limiting headers. There is no CVE associated with the bot itself, but its scans have occasionally triggered Web Application Firewall (WAF) rules designed to block automated scanners, resulting in site owners incorrectly flagging the bot as malicious.

🔍 Detection Indicators

The primary detection indicator is the User-Agent string: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/... Safari/537.36 appended with hstspreload-bot; for example, Mozilla/5.0 (compatible; hstspreload-bot; +https://hstspreload.org/). Behavioral fingerprints include a high frequency of requests to the root path (/) and well-known HSTS endpoints, originating from Google-owned IP ranges (e.g., 66.249.x.x, 216.239.x.x). Traffic typically arrives in short bursts from a single IP and does not follow typical human browsing patterns.

☠️ Risk & Impact

Although not inherently malicious, the bot can inadvertently expose misconfigured servers that leak internal information in redirect responses or error pages. Its consistent scanning may also degrade performance on resource-limited environments if rate limiting is not implemented. For web applications that treat all automated traffic as hostile, the bot’s presence can generate noise in security logs, potentially masking real attacks. The actual data exposure risk is negligible, but the operational impact includes wasted WAF capacity and false-positive alerts.

🛡️ Mitigation

While this bot is often blocked immediately on detection due to its resemblance to aggressive scanners, such blocking may prevent legitimate HSTS preload approval for domains that need it. A more nuanced approach is to rate-limit the bot rather than block entirely, whitelist Google’s IP ranges, or serve a dedicated response that satisfies HSTS checks without exposing sensitive content. Immediate blocking is recommended only if the site does not intend to be on the HSTS preload list.

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