add contain_symlinks option to prevent symlink escape attacks#361
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abhinavagarwal07 merged 1 commit intoMay 29, 2026
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A malicious SFTP server can return symlink targets that the local kernel VFS resolves outside the mount root, enabling local file reads or writes through ordinary operations like cp following a symlink. Add a contain_symlinks option (default on) that rejects absolute symlink targets and any target containing a `..` component, returning EPERM. Users who need legacy pass-through for trusted servers can opt out with -o no_contain_symlinks. The check is purely lexical and deliberately strict: in an adversarial filesystem the server controls intermediate path components, so any non-`..` component could be a symlink anywhere, making lexical depth tracking unreliable. Rejecting absolute and any `..` is the simplest rule that is provably complete against the threat model. transform_symlinks composes poorly with containment because transformed results often contain `..`; a warning is emitted when both are enabled. Tests cover default-on containment (readlink + open/stat traversal), opt-out behavior, transform_symlinks interaction (both arms), and option precedence.
h4sh5
approved these changes
May 29, 2026
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A malicious SFTP server can return symlink targets that SSHFS passes to the kernel unchanged. The kernel resolves those targets on the client’s local filesystem, allowing local file read/write through ordinary operations such as cp.
Add -o contain_symlinks, enabled by default, to reject symlink targets that are absolute or contain .. path components. Blocked symlinks return EPERM. The check is intentionally strict: rejecting any .. component closes the reported absolute-path, relative-traversal, and intermediate-symlink-assisted variants without relying on unsafe lexical normalization.
Users who need raw symlink pass-through for trusted servers can opt out with -o no_contain_symlinks.