why is Yahoo dumping http://www.ip-adress.com/whois/zenfs.com cookies on our pc?

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"IF" you had taken the time to actually look at the report you posted, you would see
http://www.markmonitor.com
Here's what they do for Yahoo and many others

MarkMonitor® offers the only comprehensive suite of solutions for establishing and protecting brands online. Already in use by more than half the Fortune 100, these hosted software or managed-services solutions each focus on a key area of online brand management and protection, providing prioritized, actionable brand protection intelligence.

We recommend a holistic approach to online brand protection. Designed to enable companies to protect revenue, reputation and customer trust online, all MarkMonitor brand protection solutions leverage unmatched access to data, plus unparalleled capabilities for prevention, detection and response. While all are integrated, each is fully self-contained, enabling enterprises to add new capabilities as business needs dictate.

MarkMonitor Brand Protectionâ„¢

Protect brand equity, web traffic, marketing investments, revenue and reputation from paid search scams, unauthorized channels, counterfeit and grey market sales, false association, cybersquatting and other online threats.
MarkMonitor AntiPiracyâ„¢

Protect revenues by shutting down the illegal distribution and promotion of pirated digital content across the widest-array of Internet channels—from peer-to-peer (P2P) networks, user generated content sites, video streaming sites and usenet services to search engines, social media and other websites.
MarkMonitor AntiFraudâ„¢

Secure reputation and customer trust by preventing and rapidly detecting phishing and malware attacks, then swiftly monitoring and taking down confirmed fraudulent sites.
Domain Management

Establish online brand presence, then leverage complete domain visibility to proactively identify and seize opportunities, prevent brand abuse and efficiently manage and secure domain assets.
Managed Services

Leverage the expertise of highly experienced brand protection professionals to analyze and execute your enterprise's brand protection strategies. Services include Brand Threat Reporting, Enforcement Lifecycle Management, complete Outsourced Services, and a range of custom-configured Advisory Services.
Domain Advisory Services

MarkMonitor Domain Advisory Services help global companies develop and refine corporate domain strategies, as well as support related initiatives including portfolio consolidation, product launches and M&A activities.
Top-Level Domain Advisory Services

MarkMonitor Top-Level Domain Advisory Services assist corporations throughout the TLD process—from developing the right strategy to defending brands during TLD expansion.

I would suggest from now on, do a bit of research before accusing anyone of spying.
NOTE: I have NO such cookies here