OfflineIMAP STARTTLS Downgrade Credential Disclosure

Executive Summary

This vulnerability in OfflineIMAP before version 8.0.3 involves the client trusting the email server's advertised STARTTLS capability before authentication. If the server does not advertise STARTTLS, the client may send credentials in plaintext without enforcing encryption. This allows an attacker to perform a STRIPTLS or man-in-the-middle attack by removing the STARTTLS capability from the server's response, intercepting and extracting account credentials in cleartext.

The core issue is that OfflineIMAP does not enforce the use of STARTTLS even when configured, leading to insecure transmission of passwords if the server does not advertise STARTTLS. This behavior is uncommon among email clients and exposes users to credential theft.

Useful 0 Not Useful 0

Compliance Impact

The vulnerability in OfflineIMAP allows credentials to be transmitted in cleartext due to the lack of enforced STARTTLS, which exposes sensitive user information to man-in-the-middle attacks.

This exposure of sensitive data such as account credentials in plaintext can lead to non-compliance with common security requirements found in standards and regulations like GDPR and HIPAA, which mandate the protection of personal and sensitive information during transmission.

Because the vulnerability undermines the confidentiality of user credentials, organizations using vulnerable versions of OfflineIMAP may fail to meet the encryption and data protection controls required by these regulations.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can lead to your email account credentials being intercepted by an attacker through a man-in-the-middle attack. Because OfflineIMAP may send your password in cleartext if the server does not advertise STARTTLS, an attacker can strip the STARTTLS capability and capture your login information.

The impact includes unauthorized access to your email account, potential data theft, and compromise of sensitive information contained in your emails.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability can be detected by monitoring whether OfflineIMAP connections are using STARTTLS properly or if credentials are being sent in plaintext due to STARTTLS capability stripping.

One way to detect this is to capture and analyze network traffic between the OfflineIMAP client and the email server to check if the STARTTLS command is issued and if the connection is encrypted afterward.

You can use tools like tcpdump or Wireshark to capture traffic and look for the presence or absence of STARTTLS in the server's capability response and whether the authentication credentials are sent in cleartext.

• Use tcpdump to capture traffic on the relevant port (usually 143 for IMAP): tcpdump -i <interface> -s 0 -w capture.pcap port 143
• Open the capture in Wireshark and filter for IMAP protocol to check if STARTTLS is advertised by the server and if the client issues the STARTTLS command.
• Look for any authentication commands (e.g., LOGIN) sent before STARTTLS or without TLS encryption, which indicates credentials are sent in plaintext.

Additionally, reviewing OfflineIMAP client logs for warnings about STARTTLS capability missing or skipped TLS negotiation can help detect this vulnerability.

Useful 0 Not Useful 0

Mitigation Strategies

To mitigate this vulnerability, ensure that OfflineIMAP enforces the use of STARTTLS when configured and does not silently skip TLS negotiation if the server does not advertise STARTTLS.

Update OfflineIMAP to a version that includes the fix where the client logs a warning and attempts STARTTLS regardless of the server's advertised capabilities, failing the connection if STARTTLS is not supported.

• Configure OfflineIMAP with 'starttls = yes' to require STARTTLS.
• If the server does not support STARTTLS, either switch to implicit TLS (e.g., using port 993 for IMAPS) or confirm explicitly with the user before sending credentials in plaintext.

These steps prevent man-in-the-middle STRIPTLS attacks by ensuring credentials are not sent in cleartext and that TLS negotiation is properly enforced.

Useful 0 Not Useful 0