Understanding How TLS-RPT Enhances Email Security Through Reporting

Overview

TLS-RPT (SMTP TLS Reporting) is a mechanism that enables domain owners to monitor issues related to the Transport Layer Security (TLS) encryption for email communication. It works by having mail servers send detailed reports about failed or downgraded TLS connections back to the domain owner, allowing them to detect misconfigurations or attacks (such as Man-in-the-Middle (MitM) attacks) in real time.

The reports provide visibility into delivery problems encountered by receiving email servers when negotiating encrypted connections using SMTP MTA Strict Transport Security (MTA-STS). This reporting mechanism helps ensure that messages are transmitted securely, improving both compliance and email security.

Key Functions of TLS-RPT

1. Monitoring TLS Failures: Enables domain owners to receive daily reports of TLS connection failures and issues related to the security of their email transmissions.
2. Identifying Misconfigurations: Helps diagnose incorrect TLS settings on email servers or communication downgrades.
3. Preventing Downgrade Attacks: Detects when an attacker attempts to intercept or downgrade the security of an SMTP connection.

TLS-RPT Workflow

1. DNS Lookup for TLS-RPT Policy:
   • Similar to DMARC, TLS-RPT is configured through a DNS TXT record. When a mail server encounters an issue with TLS, it checks for the presence of a TLS-RPT policy in the sender's DNS to determine where to send failure reports.
2. Mail Server Negotiates TLS:
   • When an email is transmitted, the sending and receiving mail servers attempt to negotiate TLS for encryption. If the TLS negotiation fails or is downgraded (e.g., due to a Man-in-the-Middle (MitM) attack), a report is generated.
3. Failure Report Generated:
   • If a connection fails to establish TLS, the receiving server generates a TLS-RPT report, detailing the issues encountered during the transmission.
4. Report Sent to Designated Address:
   • The report is sent to the domain owner's specified email address (configured in the DNS record), where it can be analyzed to address potential security issues.

TLS-RPT DNS Record Details

The TLS-RPT policy is published in DNS under the _smtp._tls subdomain, and it defines the destination for TLS-RPT failure reports. The format of the DNS TXT record specifies where and how often reports are sent.

Name

_smtp._tls.[domain].[topdomain]

Example Value

v=TLSRPTv1; rua=mailto:[email protected]

Mandatory Tags

Tag	Description
v	Specifies the version of the TLS-RPT protocol. The current version is TLSRPTv1.
rua	Specifies the destination email address for aggregate reports of TLS failures. The format is rua=mailto:[address].

TLS-RPT Aggregate Reports

TLS-RPT reports are aggregate reports that summarize the results of TLS connection attempts. These reports are usually sent daily and include information about whether or not TLS was successfully negotiated during the transmission of email.

Report Format

Reports are generated in JSON format and typically include the following key information:

• Organization Name: The name of the organization responsible for generating the report.
• Date Range: The time period during which the TLS failures occurred.
• Sending IP Addresses: The IP addresses of the sending servers.
• Receiving Host: The email host (MX server) that attempted the TLS connection.
• Failure Details: Information about why the TLS connection failed or was downgraded (e.g., expired certificates, mismatched TLS versions).

Example Report Structure

{
  "organization-name": "Example Inc.",
  "date-range": {
    "start-datetime": "2023-09-01T00:00:00Z",
    "end-datetime": "2023-09-01T23:59:59Z"
  },
  "email-server": "mx.example.com",
  "failed-connections": [
    {
      "source-ip": "192.0.2.1",
      "receiving-host": "mx1.example.com",
      "failure-reason": "STARTTLS failure",
      "error-code": "TLS handshake failed"
    }
  ]
}

Benefits of TLS-RPT

1. Improved Security Visibility: By receiving reports of TLS issues, domain owners gain valuable insight into the security posture of their email traffic.
2. Detection of Misconfigurations: TLS-RPT helps diagnose configuration issues, such as expired certificates, that prevent secure email transmissions.
3. Prevention of Downgrade Attacks: TLS-RPT can alert domain owners to attempted downgrade attacks, where an attacker forces a connection to fall back to an unencrypted state.
4. Proactive Issue Resolution: Allows domain administrators to proactively resolve problems with TLS configurations and ensure continued secure email transmission.

TLS-RPT and MTA-STS

TLS-RPT is commonly used in conjunction with MTA-STS (SMTP MTA Strict Transport Security), which enforces the use of TLS for email transmissions. While MTA-STS ensures that email servers only communicate over secure connections, TLS-RPT reports provide insight into whether the policy is working as intended.

Additional Details

Report Frequency

The TLS-RPT record does not specify a frequency for sending reports, but they are typically sent daily by receiving servers.

Report Analysis

Reports help administrators detect recurring issues with specific mail servers or geographic regions, allowing them to address these problems and improve email deliverability and security.

Conclusion

TLS-RPT is an essential tool for maintaining secure email communications. By allowing domain owners to receive detailed reports on failed TLS connections, it helps improve the overall security of email traffic and prevent interception or manipulation of messages. Used alongside MTA-STS, it offers a comprehensive solution for enforcing and monitoring secure email delivery.