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CVE-2026-42246 ruby CVE debrief

CVE-2026-42246 is a high-severity flaw in Ruby's Net::IMAP client where a man-in-the-middle attacker can make Net::IMAP#starttls appear to succeed without actually negotiating TLS. If your application relies on IMAP STARTTLS to protect credentials or mail traffic, treat this as an urgent upgrade issue and verify that the connection is truly encrypted after the upgrade attempt.

Vendor
ruby
Product
net-imap
CVSS
HIGH 7.6
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-09
Original CVE updated
2026-08-24
Advisory published
2026-05-09
Advisory updated
2026-08-24

Who should care

Teams running Ruby applications that use Net::IMAP to connect to IMAP servers, especially over untrusted or enterprise networks where an on-path attacker could intercept traffic. This is most important for services that authenticate to mail servers or depend on STARTTLS to protect credentials and message contents.

Technical summary

Prior to versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4, Net::IMAP#starttls could return "successfully" even when TLS was not actually started, enabling a MITM attacker to defeat the expected security upgrade. The advisory references fixes in the listed release lines and associated GitHub commits.

Defensive priority

High. The issue can leave a connection in plaintext while the client believes TLS is active, which can expose authentication material and IMAP data to an on-path attacker. The supplied CVSS score is 7.6 (HIGH).

Recommended defensive actions

  • Upgrade Net::IMAP to a fixed release: 0.3.10, 0.4.24, 0.5.14, or 0.6.4, depending on your compatible branch.
  • Inventory Ruby services and jobs that call Net::IMAP#starttls so you can confirm they are using a patched version.
  • After upgrading, validate that STARTTLS actually results in an encrypted session before sending credentials or other sensitive data.
  • If you cannot upgrade immediately, reduce exposure by avoiding IMAP connections across untrusted networks until the fix is deployed.
  • Review any application logic that assumes a successful starttls return value alone is sufficient security confirmation.

Evidence notes

This debrief is based only on the supplied CVE/NVD record and the referenced GitHub Security Advisory materials. The source corpus identifies Ruby Net::IMAP as the affected project, describes a MITM attacker causing Net::IMAP#starttls to return success without starting TLS, and lists fixed versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4. The supplied metadata marks the vendor as unknown, so product attribution is taken from the advisory references rather than the vendor field.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-42246 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-42246

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-42246 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-42246

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.