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filesystem: decide whether to refuse NTFS alternate-data-stream paths (file:stream) on Windows #5375
Description
Problem
On Windows, NTFS lets a path name an alternate data stream with a colon after the file name (file.txt:stream). The filesystem server's path validation (isPathWithinAllowedDirectories in path-validation.ts) accepts such a path whenever the file part lies inside an allowed directory, so the tools can read and write alternate streams there, and a stream written that way is invisible to ordinary directory listings and to most tools a user would check with.
A first look suggests the containment holds: a stream on a file inside an allowed directory is stored in that file, and a stream suffix on the allowed directory itself (C:\allowed:stream) fails the containment check. So the open question is whether the server should refuse alternate-stream paths anyway, as defense in depth and to keep hidden data out of reach of the tools, or keep accepting them.
Expected behavior
A decision, either way. If the server should refuse them: on Windows, reject a path holding a colon anywhere but the drive-letter prefix, with a clear error, cover it with tests, and note it in the filesystem README.
Reference prototype
#4523 by @andreiminca rejects paths with a colon beyond the drive-letter prefix on Windows, in isPathWithinAllowedDirectories. It was opened before this repository moved to issues-only contributions, and is closed in favor of this issue; it serves as the reference prototype for the fix, and its author will be credited when the fix ships.
Activity
cliffhall commentedon Oct 28, 2026
Triage: Priority Low (total 4)
Board: Servers V2 (#43), Status
Incoming, awaiting maintainer review (no milestone yet).youlemei2022-web commentedon Oct 29, 2026
My vote: refuse them, and treat "containment holds" as the wrong test.
The containment argument answers "does the write land inside the allowed directory?" but the security property a filesystem server actually provides is closer to "the user can see and review what changed". An alternate stream breaks exactly that: it does not appear in
dir, it does not show up in a diff, many sync and backup tools ignore it, and a reviewer looking at the file afterwards sees an unchanged file with extra bytes attached.Two more specifics worth putting in the issue before the decision:
file.txt:hiddencan hold arbitrary size without changing the visible file, so a caller that can write ADS can hide payloads inside an allowed directory while every listing and hash of the visible file stays identical.:Zone.Identifieris the common one (mark-of-the-web). Everything else -:evil,:$DATAvariants,:stream:$DATA- is either pointless or actively evasive. That argues for "reject writes to any stream, allow listing/reading:Zone.Identifierif you need it", rather than blanket acceptance.If you go with rejection, the check should be slightly wider than "colon beyond the drive prefix", because the same class of bypass has a few neighbours on Windows:
C:\dir\file.txt:stream- ADS (the issue at hand)C:\dir\file.txt::$DATA- default stream spelled explicitly\\?\C:\dir\...- long-path prefix that skips normalisationC:\dir\CON,C:\dir\NUL- device names that resolve elsewhereC:\dir\file.txt./ trailing space - stripped by Win32, so two paths name one fileTests worth having: drive-relative (
C:file.txt), UNC (\\server\share\file.txt:stream), and a stream attached to a directory (C:\allowed:stream, which you already note fails containment - worth pinning so it stays that way).Disclosure: I maintain MOOGH, a Windows desktop agent that hands these same filesystem tools to a model, so this is a daily concern rather than a theoretical one. No link - happy to turn the list above into test cases in a PR if that helps you decide.
cliffhall commentedon Nov 5, 2026
Decision: refuse alternate-data-stream paths on Windows. A colon anywhere but the drive-letter prefix is now refused with a clear error by every tool; POSIX is unaffected. Implemented in #5450, following the #4523 prototype. The suggested device-name (
CON,NUL) and trailing dot/space checks are out of this issue's scope and are tracked in #5444.feat(filesystem): refuse Windows device names and trailing dots/spaces