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GHSA-vcgp-9326-pqcp

HighCVSS 7.6 / 10
Published May 4, 2026·Last modified May 5, 2026
Affected Components(0)

No affected components available

Description

Summary

A man-in-the-middle attacker can cause Net::IMAP#starttls to return "successfully", without starting TLS.

Details

When using Net::IMAP#starttls to upgrade a plaintext connection to use TLS, a man-in-the-middle attacker can inject a tagged OK response with an easily predictable tag. By sending the response before the client finishes sending the command, the command completes "successfully" before the response handler is registered. This allows #starttls to return without error, but the response handler is never invoked, the TLS connection is never established, and the socket remains unencrypted.

This allows man-in-the-middle attackers to perform a STARTTLS stripping attack, unless the client code explicitly checks Net::IMAP#tls_verified?.

Impact

TLS bypass, leading to cleartext transmission of sensitive information.

Mitigation

  • Upgrade to a patched version of net-imap that raises an exception whenever #starttls does not establish TLS.
  • Connect to an implicit TLS port, rather than use STARTTLS with a cleartext port. This is strongly recommended anyway:
    • RFC 8314: Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access
    • NO STARTTLS: Why TLS is better without STARTTLS, A Security Analysis of STARTTLS in the Email Context
  • Explicitly verify Net::IMAP#tls_verified? is true, before using the connection after #starttls.
Risk Scores
Base Score
7.6

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights.

Threat Intelligence
4.9

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.31%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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High severity
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