Penetration Testing for Accountancy Firms: What It Covers and When You Need It

Penetration testing for accountancy firms is a controlled, authorised attack on your systems, run by a specialist who tries to reach client data the way a criminal would. It shows you which weaknesses are real, how far an intruder could get, and what to fix first. If your practice holds payroll, tax and bank details for hundreds of clients, it is one of the few checks that proves your defences work rather than assuming they do.

This guide explains what such a test covers, when it makes sense, what it costs in time and effort, and how to get value from it.

Why do accountancy firms get targeted by attackers?

Accountants sit on a rich pile of data. Client bank details, tax references, payroll records and company filings are all useful to fraudsters. A single compromised mailbox can be enough to redirect a payment or file a fake return.

Deadlines make it worse. Firms work under pressure around year end, self-assessment and VAT dates, so staff open attachments quickly and approve requests without a second look. Attackers know the calendar as well as you do.

The government’s Cyber Security Breaches Survey 2025 found that 43% of UK businesses reported a breach or attack in the previous 12 months. Among those affected, 85% had faced phishing. That is the route most likely to hit your staff first.

What is a penetration test, and what does it not do?

A pen test is a simulated attack with permission. The tester uses the same tools and tricks as a real intruder, then reports what worked. In the context of penetration testing for accountancy firms, the NCSC describes it as a way to validate your security, and warns that a test can only confirm your systems are not vulnerable to known issues on the day of the test.

That last point matters. A test is a snapshot, not a certificate for the year. New flaws appear every week, and your own changes create new gaps. Treat the result as strong evidence for a moment in time, then keep patching and monitoring.

It also differs from a vulnerability scan. A scan lists known flaws automatically, and our note on how long each stage of a test takes shows where the human work comes in. A pen test has a person chain those flaws together, try to use them, and judge what an attacker could actually reach.

What should penetration testing for accountancy firms cover?

Scope should follow where your client data lives and how people get to it. For most practices that means the following areas.

• External network: your public-facing systems (see external network testing), such as remote access, VPN, web portals and mail gateways. These are the doors an outsider tries first.
• Microsoft 365 or Google Workspace: mailbox rules, multi-factor authentication gaps, risky app permissions and shared drives.
• Internal network: what an intruder could do after landing on one laptop, including reaching file servers that hold client folders.
• Client portals and web apps: the login pages and upload areas your clients use to send documents.
• Cloud accounting and practice software: how it connects to your identity systems and who can export data.
• People: phishing or phone-based tests that show whether a convincing request gets past your team.

You do not need everything at once. A sensible first round of penetration testing for accountancy firms often pairs the external network with your cloud email, because that is where most attacks begin.

Is penetration testing for accountancy firms a legal requirement?

No law says every practice must commission one. There is, though, a duty to protect client data. The ICO says UK GDPR requires “appropriate technical and organisational measures” and expects organisations to have a process for regularly testing and evaluating how well those measures work. The right frequency depends on your size, systems and the sensitivity of the data.

In practice, other pressures often decide it. Cyber insurers ask about testing on renewal forms. Larger clients send supplier questionnaires. Some firms also work towards Cyber Essentials Plus or ISO 27001, where independent technical testing is part of the picture.

How often should a practice run a pen test?

Once a year is a common rhythm for penetration testing for accountancy firms, plus a fresh test after a major change. Moving to a new practice management system, opening a client portal, migrating email or merging with another firm all justify a new look. Smaller firms with simple setups may reasonably test less often, but should still test after big changes.

What happens during a test?

1. Scoping. You agree the targets, dates and rules, and name a contact for emergencies.
2. Testing. The tester probes your systems. Well-run tests avoid disrupting your work, and busy filing weeks can be avoided.
3. Reporting. You receive findings with a risk rating, evidence and fix advice, written so a non-specialist can act on it.
4. Fixing and retesting. You remediate, and the tester confirms the fixes hold.

Ask for a debrief call. Walking through the top findings with the tester is where most of the value appears.

What does a test cost, and what drives the price?

The price of penetration testing for accountancy firms depends on scope and effort, not on the size of your logo. The number of public IP addresses, internal hosts, web applications and users all change the days a tester needs. A small practice with one office and Microsoft 365 will need far less time than a multi-site group with its own client portal.

Our guide to UK penetration test costs explains why. Be wary of very cheap quotes. They often mean an automated scan with a cover page. If you want a fair comparison, ask each provider for the same scope in writing and compare the days of expert time, not just the total.

What mistakes do firms make?

• Testing only the office network while client data sits in the cloud.
• Skipping the retest, so nobody knows whether the fixes worked.
• Filing the report without giving each finding an owner and a date.
• Treating one clean result as permanent protection.

Most of these come down to planning. Decide before the test who will act on the results, because a report that sits in an inbox protects nobody.

How do you choose a tester?

Look for named, qualified people rather than a tool run through a template. Ask who will actually do the work, what accreditations they hold, and to see a sample report. The NCSC notes that test quality depends heavily on the expertise of the people doing it. Check that the report speaks plainly and that retesting is included or clearly priced.

Our penetration testing service is run by named testers. Aardwolf Security runs scoped penetration testing for accountancy firms and other professional services practices. If you want to talk through what a sensible first test looks like for your practice, get in touch with the team.

Frequently asked questions

Will a test disrupt our work?

It should not. Testers agree timing in advance, avoid destructive actions and stop if something looks fragile. You can also keep filing deadlines out of the window.

Is our data safe during a test?

Testers work under a signed authorisation and confidentiality terms. Ask how evidence is stored and when it is deleted after the engagement.

Is Cyber Essentials enough without penetration testing for accountancy firms?

Cyber Essentials checks that basic controls are in place. A pen test asks a different question: can someone still get in? Many firms find the two work well together.

What will we get at the end?

A report with prioritised findings, plain-English explanations and fix guidance. Decisions about risk stay with you, as the NCSC points out.

What if the test finds nothing serious?

That is a good result, and it is evidence you can show clients and insurers. Keep the report and retest after your next big change.