CVE-2026-41016: Apache Airflow SMTP Information Disclosure

CVE-2026-41016 Overview

Apache Airflow's SMTP provider contains a critical certificate validation bypass vulnerability in its SmtpHook implementation. The SmtpHook component calls Python's smtplib.SMTP.starttls() without providing an SSL context, which means no certificate validation is performed during the TLS upgrade process. This oversight allows a man-in-the-middle attacker positioned between the Airflow worker and the SMTP server to present a self-signed or otherwise invalid certificate, successfully complete the STARTTLS upgrade, and subsequently capture SMTP credentials transmitted during the login() call.

Critical Impact

Attackers in a man-in-the-middle position can intercept SMTP credentials by exploiting the missing certificate validation during STARTTLS, potentially gaining unauthorized access to email infrastructure.

Affected Products

• Apache Airflow SMTP Provider (apache-airflow-providers-smtp)
• Apache Airflow installations using SmtpHook for email notifications

Discovery Timeline

• 2026-04-30 - CVE CVE-2026-41016 published to NVD
• 2026-04-30 - Last updated in NVD database

Technical Details for CVE-2026-41016

Vulnerability Analysis

This vulnerability falls under the category of Improper Certificate Validation (CWE-295). The root issue stems from how the SmtpHook class initiates TLS connections to SMTP servers. When establishing a secure connection, the hook invokes smtplib.SMTP.starttls() without passing an SSL context object. In Python's smtplib, when no SSL context is provided, the STARTTLS upgrade proceeds without verifying the server's certificate chain, hostname, or certificate validity.

This creates a significant security gap in the authentication flow. While the connection appears encrypted after the STARTTLS upgrade completes, the client has no assurance that it is communicating with the legitimate SMTP server. An attacker who can intercept network traffic between the Airflow worker and the SMTP server can exploit this by presenting any certificate—including self-signed certificates—to complete the TLS handshake.

Root Cause

The vulnerability originates from a missing SSL context parameter in the starttls() method call within the SmtpHook implementation. Python's smtplib.SMTP.starttls() method accepts an optional context parameter of type ssl.SSLContext. When this parameter is omitted or set to None, Python uses a default SSL context that does not perform certificate verification. The secure approach requires creating an SSL context with ssl.create_default_context() or manually configuring certificate verification settings, then passing this context to starttls().

Attack Vector

An attacker must position themselves in the network path between the Airflow worker and the target SMTP server to exploit this vulnerability. This man-in-the-middle (MITM) positioning can be achieved through various techniques including ARP spoofing on local networks, DNS hijacking, BGP route manipulation, or compromise of network infrastructure devices.

Once positioned, the attack proceeds as follows: The attacker intercepts the Airflow worker's connection to the SMTP server. When the worker sends the STARTTLS command, the attacker responds and presents their own certificate. Since no validation occurs, the Airflow worker accepts this certificate and establishes an encrypted channel with the attacker. The worker then sends SMTP authentication credentials via the login() call, which the attacker captures. The attacker can optionally forward traffic to the legitimate SMTP server to avoid detection.

The captured credentials can then be used for unauthorized access to the email infrastructure, potentially enabling further attacks such as sending phishing emails from legitimate organizational accounts or accessing sensitive email communications.

Detection Methods for CVE-2026-41016

Indicators of Compromise

• Unexpected certificate changes or certificate warnings in SMTP connection logs
• Network traffic showing TLS connections to SMTP servers using untrusted or self-signed certificates
• Authentication failures following successful STARTTLS upgrades that may indicate credential capture
• Anomalous network routing patterns between Airflow workers and SMTP servers

Detection Strategies

• Monitor network traffic for TLS handshakes where the server certificate does not match expected SMTP server certificates
• Implement certificate pinning monitoring to detect when connections accept unexpected certificates
• Deploy network intrusion detection systems (NIDS) to identify potential MITM attacks on SMTP traffic
• Review Airflow task logs for SMTP connection anomalies or unexpected authentication failures

Monitoring Recommendations

• Enable detailed logging for all SMTP operations within Airflow workflows
• Configure network monitoring to alert on certificate chain validation failures
• Implement TLS inspection on egress traffic to identify connections with untrusted certificates
• Establish baseline metrics for SMTP authentication success rates to detect anomalies

How to Mitigate CVE-2026-41016

Immediate Actions Required

• Upgrade to a patched version of apache-airflow-providers-smtp that includes the certificate validation fix
• Review network architecture to minimize exposure of Airflow workers to potential MITM attacks
• Rotate SMTP credentials that may have been exposed while using vulnerable versions
• Implement network segmentation to isolate Airflow infrastructure from untrusted network segments

Patch Information

Apache has released a fix for this vulnerability in the apache-airflow-providers-smtp package. The fix ensures that an appropriate SSL context with certificate validation is passed to the starttls() method. Users should upgrade to the latest version of the SMTP provider that contains this security fix. For technical details on the fix implementation, refer to the GitHub Pull Request for Airflow and the Apache Mailing List Discussion.

Workarounds

• Configure network-level controls such as VPN tunnels or IPsec between Airflow workers and SMTP servers to provide an additional layer of encryption
• Implement strict egress firewall rules limiting Airflow workers to communicate only with known, trusted SMTP server IP addresses
• Consider using alternative SMTP providers or direct SMTP connections with custom hooks that properly validate certificates
• Deploy network monitoring to detect and alert on potential MITM attacks while awaiting patch deployment

# Upgrade to patched version
pip install --upgrade apache-airflow-providers-smtp

# Verify installed version
pip show apache-airflow-providers-smtp