CVE-2025-49136: Listmonk's Open Window - How Sprig Functions Let Secrets Slip Out!

Robert Morgan

Robert Morgan

— 8 min read

Hey everyone, grab your security hats! Today, we're diving into CVE-2025-49136, a nifty vulnerability in Listmonk that could turn your email campaigns into an open book for your system's environment variables. If you're running a multi-user Listmonk setup, this one's especially for you. Let's unpack how a couple of seemingly harmless template functions could lead to some serious ouchies.

TL;DR / Executive Summary

CVE-2025-49136 affects Listmonk versions 4.0.0 through 5.0.1. It's a template injection vulnerability stemming from the env and expandenv functions in the Sprig library, which Listmonk uses for its campaign templates. These functions allow users, even those with minimal privileges like campaigns:get, to read sensitive environment variables from the host system directly through the campaign content editor. This could expose database credentials, API keys (like AWS_KEY), SMTP passwords, and other secrets. The impact is critical, potentially leading to full system compromise. The immediate fix is to upgrade to Listmonk v5.0.2 or later, which removes these dangerous functions.

Introduction: The Devil in the Details (and Defaults)

Picture this: you're using Listmonk, a fantastic open-source, self-hosted newsletter and mailing list manager. It's robust, it's flexible, and it uses Go's templating system, supercharged by the Sprig library, to let you customize your email campaigns. Sprig is like a Swiss Army knife for Go templates, packed with useful functions. But what happens when some of those tools are a bit too powerful for certain users?

That's where CVE-2025-49136 comes into play. In multi-user Listmonk instances, users who can edit campaign templates (even just to preview them) could, until recently, use specific Sprig functions to peek at the server's environment variables. Why does this matter? Environment variables often store the crown jewels: database connection strings, API keys, secret tokens – basically, all the stuff you really don't want falling into the wrong hands. It's like giving someone keys to your office to water a plant, but those keys also happen to open the company vault!

Technical Deep Dive: Unmasking the Environment Snoopers

Let's get our hands dirty and see what's really going on under the hood.

Vulnerability Details: env and expandenv - The Culprits

Listmonk utilizes the Sprig library to extend the capabilities of Go's native text/template package. Sprig offers a plethora of helper functions, and among them are env and expandenv.

  • {{ env "VAR_NAME" }}: This function directly retrieves the value of the environment variable named VAR_NAME.
  • {{ expandenv "string with $VAR_NAME" }}: This function takes a string and replaces any $VAR_NAME or ${VAR_NAME} style variables with their corresponding environment variable values.

By default, Sprig makes these functions available. Listmonk, in versions prior to 5.0.2, included the entire generic set of Sprig functions for its template processing, including these two.

Root Cause Analysis: The Overly Trusting Template Engine

The root cause is an Improper Control of Generation of Code ('Code Injection') or, more specifically, a Server-Side Template Injection (SSTI) variant. Listmonk allowed template code, editable by users with campaign modification permissions, to execute these powerful Sprig functions on the server-side.

Think of it like this: a web application lets you customize your profile page with some HTML. Benign, right? But what if you could embed server-side script tags that the server then executes? That's SSTI. Here, instead of arbitrary code execution, it's arbitrary environment variable reading.

The core issue wasn't that Sprig offered these functions, but that Listmonk didn't restrict their availability in contexts where low-privileged users could invoke them. In a single-admin setup, this might be less of a concern (though still not ideal). But in a multi-user environment, where different users have varying levels of trust and permissions, it's a gaping hole.

Attack Vectors: From Campaign Editor to Secret Spiller

The primary attack vector is an authenticated user with permissions to:

  1. Create or edit campaign content.
  2. Preview campaign content.

Even a user with just campaigns:get and campaigns:get_all privileges could exploit this. They don't need to send a campaign; just accessing the content editor and its preview function is enough.

The attacker crafts a template snippet like:

Access Key: {{ env "AWS_ACCESS_KEY_ID" }}
Secret Key: {{ env "AWS_SECRET_ACCESS_KEY" }}
DB User: {{ env "LISTMONK_db__user" }}
DB Pass: {{ env "LISTMONK_db__password" }}
Some API Token: {{ env "MY_APP_API_TOKEN" }}

When Listmonk processes this template for a preview, it dutifully replaces these placeholders with the actual environment variable values from the server, displaying them to the attacker.

Business Impact: More Than Just Spilled Milk

The business impact can be severe:

  • Data Breach: Exposure of database credentials can lead to theft or manipulation of subscriber lists and campaign data.
  • Financial Loss: Leaked API keys for cloud services (AWS, Azure, GCP) or payment gateways can be abused, racking up huge bills.
  • System Compromise: If environment variables contain SSH keys, admin credentials for other services, or secrets that grant further access, an attacker could pivot and gain deeper control over the host system or other connected services.
  • Reputational Damage: A publicized breach erodes user trust.
  • Compliance Violations: Depending on the data exposed (e.g., if PII is somehow in an env var, though unlikely for this specific vector), this could lead to GDPR or CCPA headaches.

It's not just about one variable; it's about the chain reaction it can start.

Proof of Concept: Seeing is Believing

Let's walk through how an attacker, "Mal," with limited campaign permissions, could exploit this. (This PoC is based on the details from the GitHub advisory).

  1. User Setup: An administrator creates a user "Mal" and grants them only campaigns:get and campaigns:get_all privileges.
    User Permissions Setup (Image from GHSA-jc7g-x28f-3v3h)

  2. Malicious Input: Mal logs in, navigates to any existing campaign, and goes to the "Content" section. In the email body editor (which processes Go templates), Mal inputs the following:

    My AWS Key is: {{ env "AWS_KEY" }}
My Database User is: {{ env "LISTMONK_db__user" }}
My Database Password is: {{ env "LISTMONK_db__password" }}

    Malicious Template Input (Image from GHSA-jc7g-x28f-3v3h)

  3. The Reveal: Mal clicks the "Preview" button. The server processes the template, and voilà!
    Preview Revealing Secrets (Image from GHSA-jc7g-x28f-3v3h)

    The preview now displays the actual values of AWS_KEY, LISTMONK_db__user, and LISTMONK_db__password from the server's environment. Game over for those secrets.
    (The advisory even shows an example where AWS_KEY was set to confirm the leak: AWS_KEY Confirmation (Image from GHSA-jc7g-x28f-3v3h))

This is a simplified example. An attacker could try to enumerate many common environment variable names.

Mitigation and Remediation: Slamming the Window Shut

Okay, scary stuff. How do we fix it and prevent it?

Immediate Fixes: Patch, Patch, Patch!

  • Upgrade Listmonk: The absolute best and most direct fix is to upgrade your Listmonk instance to version 5.0.2 or later. The Listmonk team addressed this by explicitly removing the env and expandenv functions from the set of available Sprig functions. You can find the latest releases here: https://github.com/knadh/listmonk/releases

Patch Analysis: What Changed?

The fix, committed in d27d2c32cf3a, is beautifully simple and effective. It targets the initialization of template functions in cmd/init.go and internal/manager/manager.go.

Previously, the code looked something like this (simplified):

// Old code in cmd/init.go and internal/manager/manager.go
import (
    "text/template"
    "github.com/Masterminds/sprig/v3"
    "golang.org/x/exp/maps"
)

func initTplFuncs(...) template.FuncMap {
    funcs := make(template.FuncMap)
    // ... other custom funcs ...

    // Copy ALL Sprig functions
    maps.Copy(funcs, sprig.GenericFuncMap()) // Uh oh!

    return funcs
}

The patch changes this to:

// Patched code in cmd/init.go and internal/manager/manager.go
import (
    "text/template"
    "github.com/Masterminds/sprig/v3"
    "golang.org/x/exp/maps"
)

func initTplFuncs(...) template.FuncMap {
    funcs := make(template.FuncMap)
    // ... other custom funcs ...

    // Get all Sprig functions
    sprigFuncs := sprig.GenericFuncMap()

    // Explicitly delete the dangerous ones!
    delete(sprigFuncs, "env")
    delete(sprigFuncs, "expandenv")

    // Now copy the sanitized map
    maps.Copy(funcs, sprigFuncs) // Much safer!

    return funcs
}

By explicitly deleting env and expandenv from the sprigFuncs map before copying them into Listmonk's active function map, these functions are no longer available to templates, effectively neutralizing the vulnerability. Simple, clean, and effective.

Long-Term Solutions & Best Practices:

  • Principle of Least Privilege for Functions: When integrating libraries that provide a broad set of functionalities (like Sprig), adopt an allow-list approach for features like template functions, rather than a block-list or just enabling everything by default. Only enable what's truly necessary.
  • Sandboxing Template Environments: For features that allow user-supplied templates, consider stricter sandboxing if possible, or at least context-aware filtering of available functions based on user roles.
  • Secrets Management: Avoid storing highly sensitive secrets directly in environment variables if possible. Use dedicated secrets management tools (e.g., HashiCorp Vault, AWS Secrets Manager) that provide better access control and auditing. This is more of a defense-in-depth measure.
  • Regular Dependency Audits: Keep your dependencies up-to-date and be aware of the features they introduce or enable by default.

Verification Steps:

  1. Upgrade to Listmonk v5.0.2 or later.
  2. Attempt the Proof of Concept again. The template preview should now either error out when trying to use {{ env "..." }} or simply render it as literal text without interpolation, indicating the function is no longer active.
  3. Check your Listmonk logs for any errors related to unknown template functions if you try the PoC.

Timeline of CVE-2025-49136

  • Discovery Date: Not explicitly public, but likely shortly before vendor notification.
  • Vendor Notification (GitHub): The issue was reported to the Listmonk maintainers via GitHub.
  • Patch Development: The Listmonk team developed and committed the fix (d27d2c32cf3a).
  • Patch Availability: Listmonk v5.0.2 containing the fix was released. (The advisory points to this version).
  • Public Disclosure (GitHub Advisory GHSA-jc7g-x28f-3v3h): June 8/9, 2025 (as per CVE details).

Lessons Learned: Convenience vs. Security

This CVE is a classic reminder of a few key cybersecurity tenets:

  1. Defaults Can Be Dangerous: Libraries often enable many features by default for convenience. It's crucial to review these defaults and disable anything not strictly needed, especially if it has security implications. Sprig itself is not "insecure"; it provides tools. How those tools are wielded matters.
  2. Privilege Segregation Extends to Features: Just as users have different privilege levels, the features they can access within an application should also be appropriately scoped. A low-privilege user editing a template shouldn't have the same functional power as a system administrator.
  3. Input is Untrustworthy, Even From Authenticated Users: While authentication is one layer, authorization and input validation/sanitization are others. What an authenticated user can do with their input needs careful control.

One Key Takeaway:
The power of modern libraries and frameworks is immense, but so is the responsibility to understand and manage the capabilities they grant. Always ask: "What's the most restrictive way I can use this feature/library while still achieving my goal?"

Behind the Scenes: The Quiet Fix

While the full story of the discovery isn't detailed in the public advisory, vulnerabilities like CVE-2025-49136 are often found by security researchers probing applications, or sometimes even by astute developers within the project itself who realize the potential implications of a feature. The GitHub Security Advisory system (GHSA) provides a good channel for responsible disclosure, allowing maintainers to fix issues before they become widely exploited. In this case, it seems the process worked well: report, patch, disclose. Kudos to the Listmonk team for the swift and precise fix!

References and Further Reading

This vulnerability serves as a great case study. It wasn't an exotic zero-day exploit requiring arcane knowledge, but rather an oversight in how powerful, standard library features were exposed.

So, the question for you today is: Are there any "overly helpful" features in the tools you use or develop that might be inadvertently leaving a door ajar?

Stay safe out there, and keep those environment variables under wraps!

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