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 frequently cited $38 billion figure reflects an approximation of global damage—incorporating operational disruption and remediation efforts—rather than funds directly stolen by the threat actors. The sheer magnitude of this figure demonstrates how a self-spreading assault can inflict minor financial burdens across vast numbers of victims, even though the exact aggregate remains hard to substantiate.
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 often cited $37.1 billion projection merges expansive assumptions regarding infection rates, diminished productivity, and remediation efforts. Just as with MyDoom, this figure ought to be interpreted as a historical simulation of worldwide disruption rather than an audited invoice settled by a single entity.
3. Klez: approximately $19.8 billion
Klez started circulating back in 2001 and continued to cause issues throughout 2002. Various strains took advantage of flaws within email applications, capable of masking the true origin of a message, which consequently made compromised correspondence much harder for users to spot.
Its commonly cited cost of about $19.8 billion reflects the cumulative burden of a widespread outbreak: removing infections, restoring systems, and dealing with interrupted work. The estimate is widely repeated but lacks the precision of a modern company’s reported expenses.
4. Sasser: approximately $18 billion
Sasser emerged in 2004 and exploited a vulnerability in Microsoft Windows. Unlike the email worms mentioned earlier, it propagated across susceptible machines without requiring users to open attachments. System crashes, unexpected reboots, and widespread disruption to corporate and public sector operations were among its primary consequences.
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
ILOVEYOU worm emerged in 2000 as an email attachment featuring a seductive message. Upon being opened, the malware propagated across the victim’s address book while corrupting or overwriting specific files. Enterprises globally were forced to take their mail servers offline and sanitize compromised workstations.
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
NotPetya struck in 2017. It initially spread through compromised updates to widely used Ukrainian accounting software, then moved through corporate networks. Although it resembled ransomware, its destructive design left many victims unable to recover their data by paying.
The White House estimated worldwide losses at more than $10 billion. Global businesses suffered major damage: shipping company Maersk reported rebuilding large parts of its technology infrastructure, while other multinational firms faced disrupted production and distribution. Unlike many older worm estimates, NotPetya’s total is supported by substantial publicly reported losses from individual companies, although the full global cost remains an estimate.
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
In 2017, WannaCry ransomware spread rapidly by exploiting a Windows vulnerability. It encrypted files and demanded payment, but the broader economic harm came from interrupted operations and recovery efforts.
The United Kingdom’s National Health Service turned into one of its most prominent casualties, suffering widespread disruptions to appointments and services. Global losses are frequently estimated at approximately $4 billion, which reflects the overall impact rather than any ransom money delivered. This distinction is significant: while perpetrators gather relatively modest amounts, those targeted face massive expenses.
9. Change Healthcare: approximately $3.1 billion in reported 2024 impact
A 2024 ransomware assault targeting Change Healthcare paralyzed a significant portion of the medical billing and claims infrastructure across the United States. While medical providers grappled with cash flow problems and billing delays, organizations and patients alike confronted deep anxiety regarding compromised data.
Parent company UnitedHealth Group reported roughly $3.1 billion in 2024 cyberattack-related impacts. That figure is a company-reported financial measure, making it different from the modeled worldwide estimates above. It also does not capture every downstream cost borne by hospitals, medical practices, 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 only audited expenses, adjusting for inflation, or including all indirect costs—would produce a different top 10. The common thread is more useful than the precise order: cyberattacks become most expensive when they exploit a widely shared weakness or interrupt a service on which many others depend.
