1. Proton VPN
90.0Open-source clients, 5 published audits. The best-documented security stack of the four, and the only one you can test properly without paying.
Against it: Refunds are prorated against usage, not returned in full.
Visit Proton VPN1 of 4 qualify · Updated August 13, 2026
VPNs with open-source apps
Which VPN clients can I read the source of?
Proton VPN is the only one of the four we cover that publishes client source across its platforms. NordVPN, PureVPN and FastestVPN all ship closed-source applications, so their leak handling and kill-switch behaviour can only be checked by an auditor they hire, not by anyone who wants to look.
A closed-source client asks you to trust that it does what the marketing page says. An open-source one lets a researcher check. This page is short because the answer is short: one provider of four publishes its client source, and that is worth stating plainly rather than dressing up as a list.
The rule for this page. Only providers publishing open-source clients. Ordered by transparency score.
| # | Provider | Deciding fact | Transparency | Overall |
|---|---|---|---|---|
| 1 | Proton VPN | Open-source clients, 5 published audits | 9/10 | 90.0 |
Top of this list
Proton VPN
Open-source clients, 5 published audits. The best-documented security stack of the four, and the only one you can test properly without paying.
Visit Proton VPNWe earn a commission if you subscribe through this link. It does not change Proton VPN’s score.
Why these, in this order
What did not qualify, and why
Excluded providers are named rather than omitted. If one of them is the VPN you already pay for, this tells you exactly which requirement it misses.
| Provider | Why it is not on this list |
|---|---|
| NordVPN | Client applications are closed source and cannot be independently inspected. |
| PureVPN | Client applications are closed source and cannot be independently inspected. |
| FastestVPN | Client applications are closed source and cannot be independently inspected. |
Questions
- Which VPN clients can I read the source of?
- Proton VPN is the only one of the four we cover that publishes client source across its platforms. NordVPN, PureVPN and FastestVPN all ship closed-source applications, so their leak handling and kill-switch behaviour can only be checked by an auditor they hire, not by anyone who wants to look.
- How many providers qualify for this list?
- 1 of the 4 providers we cover. NordVPN and PureVPN and FastestVPN do not, and the specific requirement they miss is stated rather than quietly dropped.
- What does this ranking deliberately ignore?
- This shortlist ignores whether the published code has been audited — that is a separate page. Everything it does weigh is stated in the rule above the table, so you can tell whether the filter matches your own requirement.
- How is the order on this page decided?
- Only providers publishing open-source clients. Ordered by transparency score. The overall score shown alongside is the weighted total from our published rubric, which is a separate calculation covering all seven criteria.
Before you pick from this list
A shortlist answers one requirement. These cover the decisions that come before it, and the things a VPN will not do for you whichever one you choose.