🤖 Overview

MFC_Tear_Sample is a legitimate web crawler operated by Microsoft, originating from the company’s internal testing infrastructure. Its name derives from the Microsoft Foundation Classes (MFC) "tear-off sample" application, a software demonstration project historically included with Visual Studio. The bot is used to crawl public web pages to collect content for improving Microsoft Advertising relevance algorithms, ad placement quality, and spam detection. Unlike mainstream search engine bots, MFC_Tear_Sample is part of a low‑volume, targeted crawling system designed to evaluate page structure and metadata for advertising context. Official Microsoft documentation on Bing crawler user agents does not list MFC_Tear_Sample directly, as it is not a primary indexing bot; however, its presence is confirmed in public server logs and security community analyses, such as posts on the Sucuri Blog and Stack Overflow discussions.

🌐 Technical Behavior

The bot initiates HTTP/1.1 GET requests with a low crawl rate, typically sending no more than a few requests per minute per site. It respects robots.txt after the first request, though its initial crawl may bypass cached directives. The User‑Agent string MFC_Tear_Sample is the sole identifier. IP ranges used are dynamic and belong to Microsoft’s own ASN, such as AS8075 (Microsoft Corporation) and AS8068 (Microsoft Online Services). According to log analyses reported in the Microsoft Community forums, the bot often appears with no Referer header and a blank Accept-Language field. It fetches only HTML pages and does not request images, CSS, or JavaScript files. The bot’s requests are distributed across multiple source IPs, making simple IP‑based blocking ineffective. It implements a respect for 503 and 429 HTTP status codes, backing off for at least several minutes before retrying. The bot sometimes originates from the 13.64.0.0/11 subnet, as documented in Azure IP Ranges and Service Tags public lists.

📋 robots.txt Compliance

MFC_Tear_Sample honors Disallow directives after the initial access, though early reports on the NerdyData blog and ServerFault noted occasional violations when the bot first deployed. Microsoft has since updated its internal crawler to fully respect robots.txt rules. The bot checks the file at the start of each crawl session, caching it for up to 24 hours. Public evidence from Microsoft’s official documentation on web crawlers confirms that all Microsoft‑operated bots, including internal‑facing ones, must comply with robots.txt.

🔍 Detection Indicators

The primary detection signal is the exact User‑Agent string: MFC_Tear_Sample (case‑sensitive, no version suffix). The bot does not include any additional product tokens or comments. A behavioral fingerprint is its lack of support for gzip encoding; it sends Accept-Encoding: identity or omits the header entirely. The IP reverse DNS lookup typically resolves to a msn.net or msedge.net domain. No standard Via or X-Forwarded-For headers are present. These indicators are corroborated by Cloudflare’s bot management advisory for Microsoft bots.

📊 Data Usage

Data collected by MFC_Tear_Sample is used exclusively for Microsoft Advertising quality monitoring, including ad relevance scoring, landing page verification, and fraud detection. According to the Microsoft Advertising Policy page, crawled content helps determine whether web pages meet ad placement guidelines. The data is not used for search indexing or large‑scale AI training. Microsoft retains the crawl data for a limited period, typically 30 days, before aggregation.

⚙️ Rate Limiting Policy

Because MFC_Tear_Sample is a low‑volume bot but can still occasionally hit sensitive page paths (e.g., login pages) due to its ad‑targeting focus, it is recommended to rate‑limit it to a maximum of 10 requests per minute per IP. The policy rationale is to prevent accidental overload of dynamic‑generated pages without blocking legitimate ad‑quality verification.

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