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Miscreants from online underground economies regularly exploit website vulnerabilities and inject fraudulent content into victim web pages to promote illicit goods and services. Scalable detection of such promotional website defacements remains an open problem despite their prevalence in Black Hat Search Engine Optimization (SEO) campaigns. Adversaries often manage to inject content in a stealthy manner by obfuscating the description of illicit products and/or the presence of defacements to make them undetectable. In this paper, we design and implement DMoS—a Defacement Monitoring System which protects websites from promotional defacements at scale. Our design is based on two key observations: Firstly, for effective advertising, the obfuscated jargons of illicit goods or services need to be easily understood by their target customers (i.e., sharing similar shape or pronunciation). Secondly, to promote the underground business, the defacements are crafted to boost search engine ranking of the defaced web pages while trying to stay stealthy from the maintainers and legitimate users of the compromised websites. Leveraging these insights, we first follow the human convention and design a jargon normalization algorithm to map obfuscated jargons to their original forms. We then develop a tag embedding mechanism, which enables DMoS to focus more on those not-so-visually-obvious, yet site-ranking influential HTML tags (i.e., title, meta). Consequently, DMoS can reliably detect illicit content hidden in compromised web pages. In particular, we have deployed DMoS as a cloud-based monitoring service for a five-month trial run. It has analyzed more than 38 million web pages across 7000+ commercial Chinese websites and found defacements in 11% of these websites. It achieves a recall over 99% with a precision about 89%. While the original design of DMoS focuses on the detection of Chinese promotional defacements, we have extended the system and demonstrated its applicability for English website defacement detection via proof-of-concept experiments.