isoshelf downloads files from the internet and puts them in a folder you boot from, so it is worth knowing how it tries not to hurt you, and how to tell me when it does.

Use GitHub's private reporting: Security → Report a vulnerability on this repository. That reaches me without the report being public first.

Please don't open a public issue for anything that could be used against someone before it's fixed. Anything else — a crash, a wrong version, a broken download — is an ordinary issue.

I'm one person doing this in my spare time. I'll reply as soon as I can, and I'd rather hear about something small than not hear about it.

These are the rules the code is written against. A report that one of them is broken is a security report, not a feature request.

• A checksum mismatch always blocks the file. A download that doesn't match the checksum the project published is deleted, not placed.
• Checksums come from the project's own HTTPS site, never a mirror. Image bytes may come from a mirror, because those bytes are checked against a checksum fetched from the origin. A checksum file that redirects to another host is refused.
• Nothing is replaced before the new file is verified. The order is download, verify, rename into place, and only then deal with the old file — and only if you chose replacing.
• A download nobody can verify never replaces anything by itself. Nor does one that only an MD5 or SHA-1 vouches for: those still catch a damaged download, but they can't show a file is the one the project made.
• A file is only called checked when it was. Each file's records say where it came from and, when its hash matched a checksum the project publishes, which checksum and when. Nothing else marks a file checked — not a copy from another isoshelf, not the name it has, not a guess.
• isoshelf only writes inside the folder you pick (plus its own settings folder, and its own program's folder when you update it), and only deletes image files there. Removing one asks you first, file by file, and always offers to archive it instead. An update does with the old file what Settings says — replace, archive or keep both — rather than asking again, and never touches a file you pinned.
• On your own computer, the web UI answers your computer only. It listens on 127.0.0.1, needs a random token issued at startup, checks the Host header so a hostile website can't reach it by DNS rebinding, requires a custom header and a same-origin Origin for anything that changes state, and never inserts text from your drive as HTML.
• On a server, nothing but a bare "ok" health check answers until you sign in. Run in a container or on a network address, isoshelf asks for a username and password to be chosen the first time it's opened, and until then whoever opens it first is the one who chooses; it says so in its log. Set ISOSHELF_USERNAME and ISOSHELF_PASSWORD before the first start to close that window. The log also has a link with a random secret on the end, which stops working as soon as a password exists. The same checks on changes apply. It is one login, not user accounts: keep it on a network you trust.
• The catalog it downloads is validated before use. A catalog that doesn't parse, has an unknown key, a pattern that won't compile, or a schema from the future is refused, and the previous one is kept.
• isoshelf installs a new version of itself only if the release carries this project's signature. Each release's SHA256SUMS is signed with the project's Ed25519 key, whose public half is built into isoshelf, and every program in the update must match its line there. A missing or wrong signature blocks the update exactly as a checksum mismatch blocks an image. Nothing is downloaded until you press Update now; image downloads finish first; the old program is set aside, not deleted, until the new one has started, and put back if it doesn't. In a container isoshelf never replaces itself — pull the new image instead.
• Sharing images with another isoshelf is off unless you turn it on. When it's on, whoever can sign in can copy the images in the folder. An isoshelf copying from another one still checks every file against the project's own published checksum, so a wrong copy is thrown away, never placed.
• No telemetry. isoshelf talks to the sites in the catalog, to GitHub for release information and its own list of images, to another isoshelf on your network if you point it at one, and nowhere else.

• That an image is what its project says it is. isoshelf checks that the bytes match the checksum the project published. If a project's own site is compromised, a matching checksum proves nothing. Checking images' own signatures is planned and not done yet; isoshelf's updates of itself are signed (see above).
• That an image is safe to boot. It's an operating system image; isoshelf only manages the file.
• Anything about a catalog you or someone else wrote by hand. Entries in your own catalog file are used as written.

Only the newest release is supported: security fixes go into a new release, not into older ones. isoshelf tells you when a newer version is out, and since v0.6.0 installs it itself when you press Update now.