CVE-2020-37248 Overview
OfflineIMAP versions before 8.0.3 contain a STARTTLS stripping vulnerability that exposes user credentials to network attackers. The client trusts the server's advertised STARTTLS capability before authentication occurs, allowing a man-in-the-middle (MITM) attacker to strip the TLS upgrade from the IMAP session. When the negotiation downgrades to plaintext, the attacker captures account credentials in cleartext and can hijack the connection. This issue is tracked under [CWE-348: Use of Less Trusted Source] and affects all OfflineIMAP deployments relying on opportunistic TLS through STARTTLS rather than implicit TLS on port 993.
Critical Impact
Network attackers positioned between the client and IMAP server can extract IMAP account credentials and read or modify mail traffic.
Affected Products
- OfflineIMAP versions prior to 8.0.3
- OfflineIMAP3 (downstream Python 3 port) prior to the fix commit 46505c53
- Any IMAP synchronization configuration relying on opportunistic STARTTLS
Discovery Timeline
- 2026-06-08 - CVE-2020-37248 published to NVD
- 2026-06-09 - Last updated in NVD database
Technical Details for CVE-2020-37248
Vulnerability Analysis
OfflineIMAP synchronizes remote IMAP mailboxes to local Maildir or IMAP stores. When connecting to a server on the standard IMAP port, the client issues a CAPABILITY command and inspects the server's response for the STARTTLS token before attempting to upgrade the cleartext socket to TLS. The flaw stems from trusting this pre-authentication, unauthenticated capability advertisement. An attacker controlling the network path can rewrite the server response to omit STARTTLS, causing the client to proceed without encryption. The client then transmits the username and password using LOGIN or AUTHENTICATE PLAIN over the unencrypted channel.
Root Cause
The root cause is missing enforcement of mandatory TLS in the connection handler. OfflineIMAP did not provide a configuration assertion that required STARTTLS to succeed before submitting credentials. The fix landed in commit 46505c53ef995455d66c685f9ec3ff6ea93dbb74, which changes the client to fail closed when TLS cannot be negotiated on a connection the user intended to protect. This aligns with RFC 8314 guidance that recommends implicit TLS over opportunistic STARTTLS for mail submission and retrieval.
Attack Vector
Exploitation requires a network-adjacent position such as a hostile Wi-Fi access point, a compromised upstream router, or BGP/DNS interception. The attacker intercepts the IMAP TCP stream, removes the STARTTLS capability from the server greeting, and forwards the rewritten response. Because OfflineIMAP runs unattended, no user interaction warns the operator that the channel is unencrypted. Captured credentials enable persistent access to the victim mailbox and can be reused against other services through credential stuffing. Technical discussion is available in the OfflineIMAP GitHub issue tracker and the OfflineIMAP3 issue thread.
No verified public proof-of-concept code is published. The vulnerability mechanism is documented in the upstream patch commit and the OSS-Security mailing list disclosure.
Detection Methods for CVE-2020-37248
Indicators of Compromise
- Outbound IMAP traffic on port 143 from hosts running OfflineIMAP where TLS handshakes are absent after the initial banner
- IMAP LOGIN or AUTHENTICATE PLAIN commands observed in cleartext on the network
- OfflineIMAP log entries reporting unexpected loss of the STARTTLS capability between sessions
- Unexpected authentication events from new geographies on accounts synchronized by OfflineIMAP
Detection Strategies
- Inspect IMAP sessions at network egress to verify a STLS/STARTTLS exchange precedes any authentication command
- Alert on any IMAP LOGIN command bytes appearing in unencrypted TCP payloads to known mail provider IP ranges
- Compare historical OfflineIMAP session metadata to identify capability strings that drop STARTTLS between runs from the same server
Monitoring Recommendations
- Forward OfflineIMAP logs to a centralized log platform and alert on connection downgrades or TLS negotiation failures
- Monitor for credential reuse from unexpected source IPs on accounts synchronized by OfflineIMAP clients
- Track installed package versions across managed Linux endpoints to identify hosts still running OfflineIMAP below 8.0.3
How to Mitigate CVE-2020-37248
Immediate Actions Required
- Upgrade OfflineIMAP to version 8.0.3 or later, or update OfflineIMAP3 to a build containing commit 46505c53
- Reconfigure accounts to use implicit TLS on port 993 with ssl = yes rather than opportunistic STARTTLS
- Rotate IMAP account passwords for any account previously synchronized over an untrusted network
Patch Information
The fix is distributed through the OfflineIMAP PyPI release history starting at version 8.0.3. Downstream packagers should pull the OfflineIMAP3 commit 46505c53ef995455d66c685f9ec3ff6ea93dbb74 referenced in the vendor commit. Validate the installed version with offlineimap --version after upgrading.
Workarounds
- Disable STARTTLS by switching the account configuration to implicit TLS using ssl = yes and remoteport = 993
- Pin the server certificate with sslcacertfile or cert_fingerprint to prevent transparent interception
- Restrict OfflineIMAP execution to trusted networks until the client is patched
# Configuration example: enforce implicit TLS in ~/.offlineimaprc
[Repository Remote-Example]
type = IMAP
remotehost = imap.example.com
remoteport = 993
ssl = yes
sslcacertfile = /etc/ssl/certs/ca-certificates.crt
cert_fingerprint = <sha256-fingerprint-of-server-cert>
starttls = no
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