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net-imap vulnerable to STARTTLS stripping via invalid response timingGHSA-vcgp-9326-pqcp

Fix: ruby/net-imap@0ede4c4

GHSA-vcgp-9326-pqcp is a CWE-392 vulnerability in net-imap. A fix is available for net-imap — see the affected versions and patch details below.

Also known asCVE-2026-42246
Published
Updated
Affected
4 pkgs
Patched
4 / 4
Exploits
None indexed
Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • A successful exploit gives an attacker total control of the affected component, not partial access.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-vcgp-9326-pqcp.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs24th percentile — riskier than 24% of all scored CVEsHighest risk
0.00%0.30%0.60%0.90%0.0%0.3%Jun 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

4 pkgs affected
💎net-imap💎net-imap💎net-imap💎net-imap

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

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.

Affected Packages

4 total 4 fixed
EcosystemPackageVulnerable rangeFix
💎RubyGemsnet-imap≥ 0.6.0&&< 0.6.40.6.4bundle update net-imap --conservative
💎RubyGemsnet-imap≥ 0.5.0&&< 0.5.140.5.14bundle update net-imap --conservative
💎RubyGemsnet-imap≥ 0.4.0&&< 0.4.240.4.24bundle update net-imap --conservative
💎RubyGemsnet-imapall versions0.3.10bundle update net-imap --conservative

Affected Products

1 product · 4 configurations
Application
net\ruby-lang
≥ 0.6.0 && < 0.6.4
1 version
\

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for net-imap, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update net-imap to 0.6.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-vcgp-9326-pqcp is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

This vulnerability affects the STARTTLS functionality in the Ruby net-imap library. Red Hat Product Security has assessed this issue as an Important severity vulnerability. Attack Complexity is considered High (AC:H), because successful exploitation requires an attacker capable of intercepting and modifying network…

Workaround published by Red Hat
As a temporary workaround, Users are strongly encouraged to switch from explicit TLS upgrading mechanisms (STARTTLS on port 143) to Implicit TLS connections (such as IMAPS on port 993). By enforcing implicit TLS via port 993 from the initial socket creation step, the connection is mathematically protected against packet injection and connection degradation tactics entirely, bypassing the vulnerable implementation path.
Source: Red Hat security advisory for GHSA-vcgp-9326-pqcp (CC BY 4.0)
ProductFixed inAdvisory
Red Hat Enterprise Linux 10ruby4.0-0:4.0.3-35.el10_2RHSA-2026:33540
Red Hat Enterprise Linux 10ruby-0:3.3.10-13.el10_2RHSA-2026:33565
Red Hat Enterprise Linux 10.0 Extended Update Supportruby-0:3.3.10-11.el10_0.2RHSA-2026:35895
Red Hat Enterprise Linux 7 Extended Lifecycle Supportruby-0:2.0.0.648-39.el7_9.1RHSA-2026:37397
Red Hat Enterprise Linux 8ruby:2.5-8100020260615131019.489197e6RHSA-2026:33514
Red Hat Enterprise Linux 8ruby:3.3-8100020260615131010.489197e6RHSA-2026:33515
Red Hat Enterprise Linux 8.4 Advanced Mission Critical Update Supportruby:2.5-8040020260630124058.522a0ee4RHSA-2026:35866
Red Hat Enterprise Linux 8.6 Advanced Mission Critical Update Supportruby:2.5-8060020260630123825.ad008a3aRHSA-2026:35867

Frequently Asked Questions

### 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 t
O3 Security · Impact-Aware SCA

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