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A software flaw in Companies House’s WebFiling service potentially let authenticated users view or change selected information belonging to other companies. The flaw affected a service connected to a register of millions of firms, but that is not evidence that millions of records were stolen: Companies House says bulk extraction was not possible and later identified only a very small number of instances of unauthorised access or attempted changes.
What happened at Companies House?
Companies House disclosed a security defect in WebFiling on 13 March 2026. It said the defect was introduced during a major system update on 11 October 2025 and could be exploited by a registered, logged-in WebFiling user following a specific sequence of actions. Companies House described it as a software defect, not a malicious external attack. Its incident guidance sets out the technical and practical details.
WebFiling is the authenticated service companies use to submit information to Companies House. It is separate from the public-facing register, where many company details and filings are deliberately searchable. The security concern was that a user logged in to WebFiling might reach information or filing functions associated with a different company.
Companies House took WebFiling offline at about 1:30 p.m. on 13 March. It returned at 9:00 a.m. on 16 March after remediation and independent testing. The agency said it notified the Information Commissioner’s Office (ICO) and the National Cyber Security Centre (NCSC), and emailed registered company addresses between 17 and 19 March.
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What information could have been exposed?
Companies House said the flaw could potentially have made selected information not normally published on the public register visible to another authenticated user. The categories included directors’ dates of birth and residential addresses, company email addresses, and certain details in private company dashboards. The flaw may also have allowed attempts to submit unauthorised filings or changes, such as accounts or director changes.
That possibility matters even though the incident was not described as an external attack: unauthorised access to private details or filing controls is still a security incident. It does not mean every company’s full record was available, or that all the information potentially reachable was actually viewed.
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What Companies House said was not affected
- Passwords were not compromised, and Companies House said a password reset was not required solely because of this incident.
- Passport information and other identity-verification data were not accessed.
- Existing filed documents, including accounts and confirmation statements, could not be altered through this issue.
- People who had successfully applied to protect their personal details under the relevant Companies Act process were not affected by this issue.
- Bulk extraction was not possible, according to Companies House; records could be accessed only one company at a time.
Were millions of firms actually affected?
No public evidence establishes that millions of companies were accessed, that millions of directors’ details were downloaded, or that millions of filings were changed. The headline figure is best understood as potential scope, not a count of confirmed victims.
| Figure | What it means |
|---|---|
| 5.48 million companies | Companies House reported this register size as of 31 March 2026 in its 2025–26 annual report. |
| Millions potentially in scope | The flaw concerned a service connected to the register; potential reach is not proof that every company was viewed or changed. |
| A very small number of instances | Updated Companies House guidance says its investigation identified a very small number of instances of unauthorised access or attempted changes. |
The government’s position changed as the investigation developed. On 19 March, the Department for Business and Trade told Parliament there were then no confirmed cases of personal data accessed without permission and no confirmed evidence of changed records. Later, Companies House guidance referred to a very small number of instances. The two statements describe different stages of the investigation, not the same final count.
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How the flaw was reportedly triggered
Secondary reporting and a letter from the House of Commons Business and Trade Committee described a reported browser “back” button behaviour. The Independent reported that repeated use of the back button could reproduce the issue; the official Companies House account does not publish a full exploit sequence. It is therefore more accurate to treat that as reported technical detail than as a complete, official explanation. There is no need to try to reproduce it: report concerns through the official channel instead.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What companies and directors should check
Companies House recommended checking company information and filing history. A company may warrant a check even if it did not itself use WebFiling during the period, because the issue involved one authenticated user potentially reaching another company’s information.
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- Review the public record. Use Find and update company information to check the registered office address, officers and directors, persons with significant control, filing history, and any recent changes or unfamiliar filings.
- Check the WebFiling dashboard. Log in to the company’s WebFiling account and review the private company details for unexpected changes.
- Report anything suspicious. Email [email protected] with “WebFiling issue” in the subject line. Include the company number, dates and times, relevant filing references, a description of the concern, and screenshots or notifications where available.
- Keep an eye on unusual contact. The incident does not prove that a later scam or phishing message came from exposed Companies House data. Treat unexpected requests for money, credentials or personal information cautiously and investigate their source separately.
Directors should treat a possible exposure of a home address as a privacy and personal-safety concern, but not assume their address was accessed. People facing a serious risk of violence, intimidation or harassment may be eligible to apply to protect certain information under Companies Act mechanisms. Companies House says people whose details were already protected were outside the scope of this issue; a protection application should not be assumed to reverse any past exposure or remove copies held elsewhere.
If a filing deadline fell during the outage
Companies House said a company that missed a deadline because WebFiling was unavailable from 13 to 16 March 2026 should file as soon as possible and use the online service to appeal any late-filing penalty. Keep screenshots and timestamps of attempted filings to support an appeal.
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What is known about the investigation—and what remains unclear?
Companies House’s 2025–26 annual report says the evidence indicated the overall impact was “very low” and that the ICO had closed its case. That is Companies House’s account of the regulator’s position; it should not be read as proof that the defect was harmless, that no personal data was exposed, or that every company has been individually cleared.
Public materials do not give a precise total for companies whose private information was viewed, affected directors, successful or attempted unauthorised filings, or user accounts that exploited the flaw. They also do not provide a complete technical post-mortem or the total cost of investigation and remediation. The annual report’s overall-impact assessment and the guidance’s “very small number” description are the most specific later public statements in the available official material.
The incident highlights a particular challenge for a public corporate register: information intended for public inspection must be separated reliably from private account and filing workflows. “Not an external cyberattack” does not make a cross-company access defect insignificant; equally, the existence of a vulnerability is not evidence of mass data theft.
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