CVE-2026-42246 — net-imap
Fix: ruby/net-imap@0ede4c4CVE-2026-42246 is a CWE-392 vulnerability in net-imap. A fix is available for net-imap — see the affected versions and patch details below.
net-imap vulnerable to STARTTLS stripping via invalid response timing
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 CVE-2026-42246.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
net-imap💎net-imap💎net-imap💎net-imapReal-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
#starttlsdoes not establish TLS. - Connect to an implicit TLS port, rather than use
STARTTLSwith 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?istrue, before using the connection after#starttls.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | net-imap | ≥ 0.6.0&&< 0.6.4 | 0.6.4bundle update net-imap --conservative |
| 💎RubyGems | net-imap | ≥ 0.5.0&&< 0.5.14 | 0.5.14bundle update net-imap --conservative |
| 💎RubyGems | net-imap | ≥ 0.4.0&&< 0.4.24 | 0.4.24bundle update net-imap --conservative |
| 💎RubyGems | net-imap | all versions | 0.3.10bundle update net-imap --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
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.
Fix
Update net-imap to 0.6.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42246 is resolved across your whole dependency graph.
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-42246 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-42246. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
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…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 10 | ruby4.0-0:4.0.3-35.el10_2 | RHSA-2026:33540 |
| Red Hat Enterprise Linux 10 | ruby-0:3.3.10-13.el10_2 | RHSA-2026:33565 |
| Red Hat Enterprise Linux 10.0 Extended Update Support | ruby-0:3.3.10-11.el10_0.2 | RHSA-2026:35895 |
| Red Hat Enterprise Linux 7 Extended Lifecycle Support | ruby-0:2.0.0.648-39.el7_9.1 | RHSA-2026:37397 |
| Red Hat Enterprise Linux 8 | ruby:2.5-8100020260615131019.489197e6 | RHSA-2026:33514 |
| Red Hat Enterprise Linux 8 | ruby:3.3-8100020260615131010.489197e6 | RHSA-2026:33515 |
| Red Hat Enterprise Linux 8.4 Advanced Mission Critical Update Support | ruby:2.5-8040020260630124058.522a0ee4 | RHSA-2026:35866 |
| Red Hat Enterprise Linux 8.6 Advanced Mission Critical Update Support | ruby:2.5-8060020260630123825.ad008a3a | RHSA-2026:35867 |
Frequently Asked Questions
Is CVE-2026-42246 in your dependencies?
O3 Security finds CVE-2026-42246 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.