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 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 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 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 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 approximately $18 billion worldwide cost is often attributed to the outbreak. The lesson was clear even where losses could not be measured precisely: unpatched systems could allow an infection to travel faster than organizations could respond.
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
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 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.
While a figure approaching $9 billion is frequently cited regarding its global footprint, the foundation behind that specific sum lacks transparency. Conficker unquestionably warrants inclusion among major economically impactful breaches, yet its exact position within cost hierarchies remains considerably harder to pin down.
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 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 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 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.
