mirror of
https://github.com/theupdateframework/python-tuf
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If you do the following steps: 1. call Updater.refresh() and load, verify and cache all metadata files 2. modify timestamp snapshot meta information: (One or more of hashes or length for snapshot changes here) 3. call Updater.refresh() again 4. root and timestamp will be updated to their latest versions 5. local snapshot will be loaded, but hashes/length will be different than the ones in timestamp.snapshot_meta and that will prevent loading 6. remote snapshot is loaded and verification starts then when executing step 6 the rollback checks will not be done because the old snapshot was not loaded on step 5. In order to resolve this issue, we are introducing the idea of trusted and untrusted snapshot. Trusted snapshot is the locally available cached version. This version has been verified at least once meaning hashes and length were already checked against timestamp.snapshot_meta hashes and length. That's why we can allow loading a trusted snapshot version even if there is a mismatch between the current timestamp.snapshot_meta hashes/length and hashes/length inside the trusted snapshot. Untrusted snapshot is the one downloaded from the web. It hasn't been verified before and that's why we mandate that timestamp.snapshot_meta hashes and length should match the hashes and legth calculated on this untrusted version of snapshot. As the TrustedMetadataSet doesn't have information which snapshot is trusted or not, so possibly the best solution is to add a new argument "trusted" to update_snapshot. Even though this is ugly as the rest of the update functions doesn't have such an argument, it seems the best solution as it seems to work in all cases: - when loading a local snapshot, we know the data has at some point been trusted (signatures have been checked): it doesn't need to match hashes now - if there is no local snapshot and we're updating from remote, the remote data must match meta hashes in timestamp - if there is a local snapshot and we're updating from remote, the remote data must match meta hashes in timestamp Lastly, I want to point out that hash checks for metadata files are not essential to TUF security guarantees: they are just an additional layer of security that allows us to avoid even parsing json that could be malicious - we already know the malicious metadata would be stopped at metadata verification after the parsing. Signed-off-by: Martin Vrachev <mvrachev@vmware.com> |
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| .. | ||
| api | ||
| client | ||
| ngclient | ||
| scripts | ||
| __init__.py | ||
| ATTACKS.md | ||
| developer_tool.py | ||
| download.py | ||
| exceptions.py | ||
| formats.py | ||
| keydb.py | ||
| log.py | ||
| mirrors.py | ||
| README-developer-tools.md | ||
| README.md | ||
| repository_lib.py | ||
| repository_tool.py | ||
| requests_fetcher.py | ||
| roledb.py | ||
| settings.py | ||
| sig.py | ||
| unittest_toolbox.py | ||