The Best Internal Network Penetration Testing Companies in the UK (2026)

The best internal network penetration testing companies in the UK for 2026 are listed below. Each one holds a CREST penetration testing accreditation and publishes a public page covering internal network testing, and each is compared on published pricing, retests and report turnaround so you can shortlist quickly.

1. OnSecurity

OnSecurity is a UK penetration testing company with a dedicated internal infrastructure penetration testing page describing ethical hacking of the internal Windows network infrastructure, including attempts at lateral movement and protection of privileged accounts such as domain administrators. That page states free retests for resolved issues within a flexible window and findings available instantly through its platform. CREST-accredited for penetration testing. Verified: CREST Marketplace listing.

2. EJN Labs

EJN Labs is a CREST-accredited UK penetration testing consultancy with a dedicated internal network penetration testing service that models an attacker already inside the perimeter, such as a stolen laptop, a rogue insider or a phished employee. Testing covers LLMNR and NBT-NS poisoning, NTLM relay, BloodHound-led Active Directory attack paths, Kerberos attacks, Active Directory Certificate Services abuse, lateral movement and segmentation testing. It is one of two companies on this list that publish pricing. Internal network tests start from £1,200 on the pricing page. Small covers up to about 150 hosts and one Active Directory domain for £1,200 to £3,600 over 1 to 3 days, Growth covers 150 to 400 hosts for £3,600 to £9,600 over 3 to 8 days, and Enterprise covers 400 or more hosts for £9,600 to £14,400 over 8 to 12 days. Scoping assumes around 50 live hosts per tester-day. Every engagement is a fixed price for a defined scope, quoted within 24 hours of the scoping call. The business holds ISO 27001, ISO 9001, Cyber Essentials and Cyber Essentials Plus. Verified: CREST Marketplace listing for EJN Labs.

Retesting of fixed findings is free until every finding is closed. By EJN Labs’ own figures, free retests save clients up to 35% against paying for retesting separately, and free rescheduling means a slipped release date costs nothing. Findings are posted to a client portal as they are confirmed. The report arrives on the last day of testing, the shortest stated turnaround on this list, with a CVSS score and prioritised remediation guidance for each finding, followed by a walkthrough call. There are no callout, cancellation or out-of-hours fees.

Testing is delivered by UK-based, CREST-certified penetration testers, priced at a fair-market £1,100 to £1,400 per day. Production networks can be tested where required: checks are safe-by-default, denial of service is never run against production, and any disruptive step waits for your sign off. Out of hours or overnight slots are available.

OnSecurity’s dedicated internal infrastructure page, free retests for resolved issues and instant access to findings keep it first, as the company whose public material is most centred on this service. On the criteria this page can verify from public pages, EJN Labs has the most complete published answer, with a £1,200 starting price, free retests until every finding is closed and a report on the last day of testing, which is why it sits second.

3. Evalian

Evalian is a UK data protection and security consultancy with an infrastructure penetration testing page covering external and internal tests, and stating that internal testing shows how far a single point of compromise could spread. That page states a free retest of remediations, and its penetration testing page states a report turnaround after completing the test. CREST-accredited for penetration testing, application security testing, vulnerability assessment, incident response and security operations centre. Verified: CREST Marketplace listing.

4. Bridewell

Bridewell is a UK cyber security and managed services company whose infrastructure penetration testing page describes assessing the security of an organisation’s network from an internal and external perspective. CREST-accredited for penetration testing, vulnerability assessment, incident response, security operations centre and threat led penetration testing. Verified: CREST Marketplace listing.

5. JUMPSEC

JUMPSEC is a UK offensive security company whose network infrastructure testing page states that network penetration testing can be performed both internally and externally, and says scoping needs the number of internal and external IPs, subnets and locations. CREST-accredited for penetration testing, vulnerability assessment, incident response, security operations centre and threat led penetration testing. Verified: CREST Marketplace listing.

6. Sentrium

Sentrium is an independent UK penetration testing consultancy whose network and infrastructure penetration testing page sets out an internal network assessment, in which testers move laterally and review Active Directory permissions. Its penetration testing page states retesting of high and critical findings for a stated period after the test. CREST-accredited for penetration testing and cyber audit. Verified: CREST Marketplace listing.

7. Precursor Security

Precursor Security is a UK offensive security consultancy with a dedicated internal network penetration testing page covering Active Directory, lateral movement, privilege escalation and network segmentation. That page states a starting price for a single domain and an expected duration, with retest included. Its penetration testing page states a report turnaround after test completion. CREST-accredited for penetration testing, vulnerability assessment and security operations centre. Verified: CREST Marketplace listing.

8. Sencode

Sencode is a UK penetration testing company whose network penetration testing page covers internal and external networks and follows PTES, OWASP and NIST. The page states a complimentary retest with a stated retest window, fixed-scope pricing after a short scoping call, and a typical report turnaround after testing is complete. CREST-accredited for penetration testing. Verified: CREST Marketplace listing.

9. SecureTeam

SecureTeam is a UK security company with an internal network penetration testing page describing testing of workstations and servers for ways to steal passwords or pivot into an Active Directory domain. CREST-accredited for penetration testing. Verified: CREST Marketplace listing.

How we selected these companies

Every company on this list passed the same two entry checks: a live listing in the CREST Marketplace supplier directory confirming penetration testing accreditation, and a live public page for internal network penetration testing, either a dedicated page or an infrastructure page whose text names internal testing. We then recorded what each company publishes about pricing, retests and report turnaround. All pages were checked on 25 October 2026. Entries from three onwards are in no particular order.

• CREST accreditation, verified. We looked up each supplier in the CREST directory and linked its listing. No listing, no entry: brands absorbed into a parent company appear under the parent.
• An internal network page. A home page that lists network testing in a menu was not enough. Each entry needed a public page whose text covers internal testing. Well-known testing companies without one are not on this list, although they appear on our wider list of the best CREST-accredited penetration testing companies in the UK.
• Published pricing. We recorded whether a company publishes its own prices for this service on a public page. General statements about what the market charges were not counted as a company’s own pricing.
• Retests and report turnaround. An internal test often has to be followed by proof that attack paths were closed, by an audit or insurance deadline. Where a company states its retest terms or report turnaround we noted it; where we could not find it, we recorded not found rather than inventing an answer.

How the companies compare

All nine companies hold verified CREST penetration testing accreditation, show a United Kingdom headquarters on their listing, and have a public page covering internal network testing. The practical differences are in what they publish.

Accreditation is taken from each company’s CREST Marketplace listing. Pricing, retest and report details are taken from each company’s own public pages, linked in its profile above, as they stood on 25 October 2026. Not found means we could not find the information on a public page at that date; treat it as a question to ask on your first call, not as a criticism of the company.

Do you have to have an internal network penetration test?

No rule names an internal network penetration test as mandatory for every organisation. The duties that lead teams to commission one are narrower or broader than that. If your network holds a payment card environment, PCI DSS 4.0.1 requirement 11.4.2 requires internal penetration testing at least once every 12 months and after significant change, and requirement 11.4.3 requires the same for external testing. Where segmentation isolates the cardholder data environment, requirements 11.4.5 and 11.4.6 require segmentation testing. That applies to the cardholder data environment, not to every network.

UK GDPR Article 32(1)(d) requires a process for regularly testing, assessing and evaluating the effectiveness of your security measures. It does not name penetration testing as the method, but testing an internal network that holds personal data is one recognised way to evidence that process. ISO 27001 does not require a penetration test either: Annex A 8.8 on management of technical vulnerabilities is apply-or-justify, and the ISO 27002 guidance for it lists periodic, documented penetration tests as one approach. The NCSC’s penetration testing guidance describes a penetration test as a method of gaining assurance in your vulnerability assessment and management processes.

Cyber Essentials Plus is different again. The NCSC Cyber Essentials overview describes it as the same protections with more rigorous, independent technical testing, but that is an assessor-led technical verification of the five controls, not a penetration test, and neither Cyber Essentials tier requires one. In practice the request for an internal test usually comes from outside the team: an auditor, a cyber insurer or an enterprise customer. For scope and cost, see our guide to network penetration testing cost and the pricing page.

What a buyer should ask an internal network penetration testing company

1. Which legal entity holds your CREST accreditation? Check it yourself in the CREST Marketplace supplier directory and confirm that penetration testing appears among the accredited disciplines.
2. Is the testing assumed-breach and manual? Ask how testers move from a standard user to domain admin, and whether Active Directory attack paths, credential relay and certificate services are covered, because a scanner alone does not find them.
3. How is scope counted? Ask whether the price is set by hosts, subnets, sites or domains, and what happens if the estate is larger than expected.
4. Is a retest included, and for how long? Auditors and insurers ask for evidence that attack paths were closed, so a retest that is capped or charged separately changes the real price.
5. When do we get the report, and do we see findings before it? If an audit or renewal deadline is driving the test, the gap between the end of testing and the report matters.
6. How do you protect our live network? Ask how disruptive checks are handled, whether denial of service is excluded, and whether out of hours testing is available.

Get a quote

Read the published day rate of £1,100 to £1,400 and the internal network starting price of £1,200 on the pricing page, then request a fixed quote for a defined scope with no obligation. Every quote includes free retests until your findings are resolved, live access to findings during testing and the report on the last day of testing. Get a penetration testing quote, or read more about internal network penetration testing from EJN Labs.