HighCVE-2026-42246

    net-imap vulnerable to STARTTLS stripping via invalid response timing (CVE-2026-42246)

    Published Last modified Sources: SUSE, Ubuntu, BSI, Red Hat, Msrc Update Guide, OSV, CVE.org, EUVD

    What is affected, what to do

    Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4, a man-in-the-middle attacker can cause Net::IMAP#starttls to return "successfully", without starting TLS.

    Update Ruby Net-imap to version 0.3.10.

    Affected
    • Ruby Net-imap gte 0.4.0, < 0.4.24..*
    • Ruby Net-imap lt *..0.3.10
    Fixed in
    • Ruby Net-imap 0.3.10

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    7.6

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v4

    7.4

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: SUSE, Ubuntu, BSI, Red Hat, Msrc Update Guide, OSV, CVE.org, EUVD

    0.3 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 24.2

    Show all metrics
    Severity
    High
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    59.82 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    7.6 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.4
    CVSS v4

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.6
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.3 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    24.2
    Elevated — this CVE ranks above at least 10% of all CVEs scored today.
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: none known · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 12 May 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-392, CWE-393, CWE-754, CWE-636, CWE-841

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4, a man-in-the-middle attacker can cause Net::IMAP#starttls to return "successfully", without starting TLS. This issue has been patched in versions 0.3.10, 0.4.24, 0.5.14, and 0.6.4.

    Source: suse_csaf, Ubuntu Security Notices, BSI WID Portal, BSI CSAF, redhat_csaf, Microsoft Security Update Guide, OSV (Open Source Vulnerabilities), cvelistv5, NVD (NIST), CVE.org (MITRE), ENISA EUVD

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    rubynet-imap
    • < 0.3.10
    • ≥ 0.4.0, < 0.4.24
    • 0.3.10
    • —
    ruby-langnet\
    • 0.5.0 – < 0.5.14
    • 0.6.0 – < 0.6.4
    • < 0.3.10
    • 0.4.0 – < 0.4.24
    • 0.5.14
    • 0.6.4
    • 0.3.10
    • 0.4.24
    HCLBigFix Compliance
    • < 2.0.18
    • 2.0.18
    Open SourceRuby
    • < 3.3.12
    • 3.3.12
    Red HatEnterprise Linux
    • 9
    • —
    rubygemsnet-imap
    • 0.5.0
    • 0.4.0
    • 0.3.5
    • 0.2.2
    • 0.4.12
    • 0.2.5
    • 0.5.5
    • 0.5.7
    • 0.3.6
    • 0.1.1
    • 0.4.6
    • 0.5.2
    • 0.4.23
    • 0.5.8
    • 0.5.4
    • 0.3.2
    • 0.3.7
    • 0.6.1
    • 0.2.4
    • 0.2.0
    • 0.4.4
    • 0.4.21
    • 0.4.9.1
    • 0.5.10
    • 0.6.0
    • 0.4.7
    • 0.4.2
    • 0.3.8
    • 0.4.9
    • 0.5.12
    • 0.3.4
    • 0.5.6
    • 0.3.9
    • 0.3.3
    • 0.4.17
    • 0.4.5
    • 0.4.1
    • 0.4.8
    • 0.2.3
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    Vendors and products

    Weakness classes (CWE)

    • CWE-392Missing Report of Error Condition

      Base

      The product encounters an error but does not provide a status code or return value to indicate that an error has occurred.

      Entry at MITRE CWE-392

    • CWE-393Return of Wrong Status Code

      Base

      A function or operation returns an incorrect return value or status code that does not indicate the true result of execution, causing the product to modify its behavior based on the incorrect result.

      Entry at MITRE CWE-393

    • CWE-754Improper Check for Unusual or Exceptional Conditions

      Class

      The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.

      Entry at MITRE CWE-754

    • CWE-636Not Failing Securely ('Failing Open')

      Class

      When the product encounters an error condition or failure, its design requires it to fall back to a state that is less secure than other options that are available, such as selecting the weakest encryption algorithm or using the most permissive access control restrictions.

      Entry at MITRE CWE-636

    • CWE-841Improper Enforcement of Behavioral Workflow

      Class

      The product supports a session in which more than one behavior must be performed by an actor, but it does not properly ensure that the actor performs the behaviors in the required sequence.

      Entry at MITRE CWE-841

    References & sources

    • github.comruby/net-imap/security/advisories/GHSA-vcgp-9326-pqcpweb, x_refsource_CONFIRM
    • NVD (NIST)vuln/detail/CVE-2026-42246advisory
    • github.comruby/net-imap/commit/0ede4c40b1523dfeaf95777b2678e54cc0fd9618web, x_refsource_MISC
    • github.comruby/net-imap/commit/24a4e770b43230286a05aa2a9746cdbb3eb8485eweb, x_refsource_MISC
    • github.comruby/net-imap/commit/97e2488fb5401a1783bddd959dde007d9fbce42cweb, x_refsource_MISC
    • github.comruby/net-imap/commit/f79d35bf5833f186e81044c57c843eda30c873daweb, x_refsource_MISC
    • github.comruby/net-imappackage
    • github.comruby/net-imap/releases/tag/v0.3.10web, x_refsource_MISC
    • github.comruby/net-imap/releases/tag/v0.4.24web, x_refsource_MISC
    • github.comruby/net-imap/releases/tag/v0.5.14web, x_refsource_MISC
    • github.comruby/net-imap/releases/tag/v0.6.4web
    • github.comrubysec/ruby-advisory-db/blob/master/gems/net-imap/CVE-2026-422…web
    • nostarttls.secvuln.infoweb
    • rfc-editor.orginfo/rfc8314web
    • access.redhat.comsecurity/cve/CVE-2026-42246Vulnerability database, x_refsource_REDHAT
    • bugzilla.redhat.comRHBZ#2468499Issue tracking, x_refsource_REDHAT
    • security.access.redhat.comdata/csaf/v2/vex/2026/cve-2026-42246.jsonx_sadp-csaf-vex
    • access.redhat.comerrata/RHSA-2026:37397Vendor advisory, x_refsource_REDHAT
    • access.redhat.comerrata/RHSA-2026:35895Vendor advisory, x_refsource_REDHAT
    • access.redhat.comerrata/RHSA-2026:33565Vendor advisory, x_refsource_REDHAT

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedRed Hat ADP

      1 individual change: Affected versions (publisher)

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)changed
      Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10.0 Extended Update Support (+31)
      Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10.0 Extended Update Support (+31)

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    2. Record modifiedRed Hat ADP

      1 individual change: Affected versions (publisher)

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)changed
      Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10.0 Extended Update Support (+31)
      Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 10.0 Extended Update Support (+31)

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    3. Record modifiedRed Hat ADP

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:35834
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:35866
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:35867
      Affected versions (publisher)changed
      Red Hat Enterprise Linux AppStream (v. 10), Red Hat Enterprise Linux AppStream (v. 8), Red Hat Enterprise Linux AppStream E4S (v.8.8) (+12)
      Red Hat Enterprise Linux AppStream (v. 10), Red Hat Enterprise Linux AppStream (v. 8), Red Hat Enterprise Linux AppStream AUS (v.8.4) (+17)

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

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