Fix path-containment bypass in LocalFileHandler.can_show - #1116
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can_show() guarded the localfiles root with a bare
`fullpath.startswith(self.localfile_path)` check. Because that is a
string comparison rather than a path comparison, a sibling directory
whose absolute path shares the configured root as a string prefix
passes the guard. For example, with `--localfiles /data/notebooks`, a
request resolving to `/data/notebooks_backup/credentials.ipynb`
satisfies `startswith("/data/notebooks")` and is served, leaking files
outside the intended root (CWE-22).
Compare against the root joined with a trailing separator (and accept
the root itself) so only genuine descendants of the root are allowed.
Classical `../../../etc` traversal remains blocked as before.
Fixes jupyter#1115
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can_show() guarded the localfiles root with a bare
fullpath.startswith(self.localfile_path)check. Because that is a string comparison rather than a path comparison, a sibling directory whose absolute path shares the configured root as a string prefix passes the guard. For example, with--localfiles /data/notebooks, a request resolving to/data/notebooks_backup/credentials.ipynbsatisfiesstartswith("/data/notebooks")and is served, leaking files outside the intended root (CWE-22).Compare against the root joined with a trailing separator (and accept the root itself) so only genuine descendants of the root are allowed. Classical
../../../etctraversal remains blocked as before.Fixes #1115