The answer depends on what counts as a cost. A global computer worm can generate billions of dollars in estimated cleanup and lost productivity across many organizations. A breach at one company produces a narrower, often better-documented bill. The ranking below uses commonly cited nominal dollar estimates to compare both kinds of attack. It is a guide to their reported economic scale, not an audited league table: estimates for older worms are especially uncertain, and the figures have not been adjusted for inflation.
1. MyDoom: approximately $38 billion
First identified back in 2004, MyDoom propagated via email attachments, infecting systems with extraordinary velocity. Furthermore, it facilitated distributed denial-of-service assaults, wherein numerous compromised devices flood a target with overwhelming traffic.
The often-cited $38 billion figure represents an estimate of worldwide damage, including disruption and recovery work, rather than money stolen by the attackers. Its scale illustrates how a self-propagating attack can impose small costs on enormous numbers of victims, though the precise total is difficult to verify.
2. Sobig: approximately $37.1 billion
The Sobig family of email worms circulated in 2003. Its best-known variant, Sobig.F, spread through infected messages and helped turn compromised computers into a distribution network for further malicious activity.
The frequently quoted $37.1 billion estimate combines broad assumptions about infection, lost productivity, and cleanup. As with MyDoom, it should be read as a historical model of global impact, not as a documented bill paid by one organization.
3. Klez: approximately $19.8 billion
Klez began spreading in 2001 and remained troublesome in 2002. Variants exploited weaknesses in email software and could disguise a message’s apparent sender, making infected mail harder for recipients to recognize.
Its frequently cited price tag of roughly $19.8 billion illustrates the aggregate impact of a widespread contagion: eradicating infections, repairing networks, and managing halted operations. Although this projection is frequently shared, it lacks the exactness found in contemporary corporate financial disclosures.
4. Sasser: approximately $18 billion
Sasser emerged in 2004 and exploited a vulnerability in Microsoft Windows. Unlike the email worms above, it could spread between vulnerable computers without someone opening an attachment. Its effects included crashes, restarts, and operational disruption at businesses and public services.
An estimated global economic toll of roughly $18 billion is frequently linked to the outbreak. Even in cases where damages defied precise quantification, the takeaway remained unmistakable: unmitigated vulnerabilities could enable malware to spread more rapidly than enterprises were able to react.
5. ILOVEYOU: approximately $15 billion
The ILOVEYOU worm appeared in 2000 as an email attachment accompanied by an enticing message. When opened, it spread through the victim’s contacts and damaged or replaced certain files. Organizations around the world had to disconnect email systems and clean infected computers.
Estimates of its damage vary substantially; $15 billion is a commonly cited upper-end figure, not a settled accounting result. The attack remains a defining example of how a simple social-engineering lure can amplify a technical threat.
6. NotPetya: approximately $10 billion
In 2017, NotPetya struck. It initially propagated via tainted updates within popular Ukrainian accounting packages, subsequently sweeping across enterprise networks. Even though it mirrored ransomware, its catastrophic architecture meant numerous targets could not restore their information through extortion payments.
The White House calculated worldwide damages at upwards of $10 billion. Major disruptions hit international enterprises: shipping giant Maersk disclosed that massive segments of its tech framework required rebuilding, whereas various multinational corporations encountered halted manufacturing and logistics. Unlike numerous legacy worm projections, the aggregate impact of NotPetya is backed by considerable publicly disclosed deficits from specific businesses, even though the total worldwide price tag remains an approximation.
7. Conficker: approximately $9 billion
Discovered in 2008, Conficker exploited a Windows vulnerability and built a large network of infected computers. It also used techniques that made removal and containment difficult. Its persistence forced organizations to spend time identifying vulnerable machines, applying updates, and cleaning infections.
A figure near $9 billion is often repeated for its worldwide impact, but the basis for that precise amount is not transparent. Conficker belongs on lists of economically significant attacks; its exact placement in a cost ranking is much less certain.
8. WannaCry: approximately $4 billion
During 2017, the WannaCry ransomware propagated swiftly through the exploitation of a security flaw in Windows. Although it encrypted data and demanded ransoms, the greater financial damage stemmed from halted business operations and subsequent remediation costs.
The United Kingdom’s National Health Service became one of its most visible victims, with disrupted appointments and services. Around $4 billion is a frequently cited estimate of global losses, not a measure of ransom paid. The contrast matters: attackers can collect relatively little while victims bear enormous costs.
9. Change Healthcare: approximately $3.1 billion in reported 2024 impact
A 2024 ransomware attack on Change Healthcare disrupted a major part of the United States health payment and claims system. Providers struggled with billing and cash flow, while patients and organizations faced uncertainty about exposed information.
Parent organization UnitedHealth Group disclosed approximately $3.1 billion in financial consequences linked to the 2024 cyberattack. As a metric disclosed directly by the enterprise, this amount differs from the projected global estimates mentioned previously. Furthermore, it fails to account for every secondary expense absorbed by clinics, healthcare facilities, and patients.
10. Code Red: approximately $2.6 billion
Code Red spread in 2001 by exploiting a vulnerability in Microsoft web server software. It infected vulnerable servers, defaced some websites, and generated traffic intended to overwhelm a target.
Its commonly cited $2.6 billion cost reflects estimates of lost productivity and the work required to protect and repair systems. Like the other early-worm figures, it is best treated as an indication of scale rather than an exact total.
How to read the ranking
These attacks did not all cause loss in the same way. Early worms reached vast numbers of machines, so their headline figures rely heavily on assumptions about time spent cleaning and restoring them. NotPetya combined worldwide spread with severe, documented corporate disruption. Change Healthcare shows how an attack on one critical intermediary can create large costs beyond the company itself.
For that reason, a different method—ranking exclusively audited expenses, adjusting for inflation, or factoring in all indirect costs—would generate a distinct top 10. The underlying theme proves more valuable than the exact sequence: cyber incidents turn out to be costliest whenever they leverage a broadly shared vulnerability or disrupt a service upon which numerous others rely.
