@postalsys/ee-client is the EmailEngine client library for browsers and Node.js. In the browser it renders untrusted email content (subjects, sender and recipient addresses, attachment names, folder names, and message bodies) into the DOM, so its primary security surface is cross-site scripting (XSS) in the message browser UI. It also handles EmailEngine access tokens. We welcome responsible reports of vulnerabilities that present a credible security risk to users. Please distinguish security vulnerabilities from ordinary bugs, unusual edge cases, and hardening suggestions.

Security fixes are released only against the latest major version. We do not backport patches to older majors, so upgrading to the current release line is the supported way to receive security updates.

If you are on an older major, please upgrade. See the release notes at https://github.com/postalsys/ee-client/releases before updating.

A security report should identify a realistic way for an attacker to cross a security boundary in a supported version of @postalsys/ee-client. Unexpected behavior, a failing test, or a technically correct proof of concept is not sufficient on its own to establish security impact.

In particular, explain:

• Attacker capabilities: Who is the attacker, and what inputs, systems, or responses can they control without already having the privileges or secrets they are trying to gain?
• Attack path: How does attacker-controlled input reach the affected code through a supported public API or a realistic application integration?
• Prerequisites: What configuration and user actions are required? If the attack depends on unusual options or deliberately malformed input, explain why those conditions could arise in a real application.
• Impact: What confidentiality, integrity, or availability boundary is crossed, and what can the attacker achieve as a result?

A vulnerability involving an optional or less common feature can still qualify if these conditions are met. Conversely, the existence of a defensive check in the code does not, by itself, establish that every bypass of that check has a realistic security impact.

Do not disclose suspected exploitable vulnerabilities through public GitHub issues, pull requests, or discussions before coordinating with the maintainers.

Report privately through one of the following channels:

1. GitHub Security Advisories (preferred): https://github.com/postalsys/ee-client/security/advisories/new.
2. Email: andris@postalsys.com, as listed in SECURITY.txt. Encrypt sensitive details if possible.

Please provide enough information to evaluate both the defect and its security impact. Reports should include:

1. The affected @postalsys/ee-client version or commit, browser or Node.js version, and relevant configuration.
2. A concise description of the issue and the affected public API or code path (for example message list or message viewer rendering, the compose modal, attachment or source download, the API request layer, or session token keep-alive handling. For XSS, name the specific email field and payload that is not correctly escaped).
3. A minimal, tested reproduction, including steps to run it, expected behavior, and observed behavior.
4. A concrete threat model: attacker control, prerequisites, trust boundary, and resulting impact. Clearly distinguish what your reproduction proves from what you believe might be possible.
5. An explanation of why the issue affects real deployments beyond a deliberately constructed test setup, where this is not obvious.

A suggested fix is welcome but does not replace an explanation of exploitability. A complete real-world exploit is not always necessary, but the report must make a credible security impact clear.

The following generally do not qualify for private security handling unless you can demonstrate an additional, meaningful security boundary crossing:

• Issues that require the attacker to control trusted application code, administrator-only configuration, or the secret they supposedly extract.
• Problems caused solely by application-level misuse, exposed credentials, or insecure deployment choices outside @postalsys/ee-client's control.
• Contrived proof-of-concept setups where the tester controls both sides of a request, without explaining how a separate attacker could trigger the issue.
• Parser quirks, unexpected behavior, and bypasses of internal checks with no demonstrated security impact.
• Speculative findings based only on static analysis or an automated tool's assessment, without a verified reproduction and credible attack scenario.

These may still be valid bugs and may be considered through the normal issue tracker. Please coordinate before making any potentially exploitable details public. Classification as a bug does not guarantee a fix or a particular priority.

Automated analysis and AI tools are welcome as research aids, but the person submitting a report is responsible for verifying its claims. Do not submit bulk, unverified, or speculative AI-generated findings. Test each finding against a supported release and establish the actual security impact. Reports that do not meet the requirements above may be closed without further investigation or individual feedback; they can be resubmitted with stronger evidence.

@postalsys/ee-client is maintained by a small team, and there is no guaranteed response or resolution time. Reports are prioritized based on demonstrated impact, exploitability, affected usage, and available maintenance resources.

A private report does not automatically become a security advisory. Confirmed vulnerabilities may be fixed and disclosed through GitHub Security Advisories; other findings may be closed, deferred, or handled as ordinary bugs. Please do not disclose details of an unresolved, potentially exploitable issue publicly without coordinating with the maintainers.

We do not request or manage CVE identifiers. A report being acknowledged, a bug being fixed, or a pull request being merged does not mean that @postalsys/ee-client has confirmed a security vulnerability or endorsed a CVE assignment. We do not guarantee advisories, CVE identifiers, or public credit for reports. For confirmed vulnerabilities disclosed through an advisory, substantive reporters may be credited if they wish.

In scope: the @postalsys/ee-client library source in this repository: the message browser UI rendering (folder list, message list, message viewer, compose modal), the escaping of untrusted email fields before insertion into the DOM, the API request layer, attachment and message-source download, and access token handling including the sess_ keep-alive mechanism.

Out of scope:

• Vulnerabilities in your own application code that embeds this library.
• The behavior of the EmailEngine server itself, or any API it exposes. Report those against EmailEngine, not this client.
• Rendering of a message's own HTML body: the client displays the HTML part of an email as delivered. Sanitizing or sandboxing hostile HTML mail (e.g. via an iframe or a sanitizer) is the responsibility of the embedding application and its deployment; this is a documented design boundary, not a client bug. Report a concrete bypass of the fields the client does escape instead.
• Misconfiguration of your deployment, for example serving the client over plain HTTP, or exposing access tokens in a shared or logged context.
• Missing security headers without a demonstrated, concrete impact.
• Social-engineering reports.

Thank you for helping keep @postalsys/ee-client and its users safe.