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CVE Record

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

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.

HighCVSS 7.6Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

Ruby applications using affected net-imap versions may believe an email connection is encrypted when TLS never started. An attacker positioned between the application and IMAP server could exploit response timing to expose or alter email traffic and credentials. Patched releases are available.

Executive priority

Treat as a high-priority upgrade for systems processing sensitive email through STARTTLS. Accelerate remediation where IMAP traffic crosses untrusted or externally controlled networks. There is no supplied evidence of active exploitation, so response should be urgent and risk-based rather than emergency incident handling by default.

Technical view

A man-in-the-middle can send an invalid response at a specific protocol stage, causing Net::IMAP#starttls to return successfully without establishing TLS. Affected branches are below 0.3.10, 0.4.24, 0.5.14, and 0.6.4. The CVSS 4.0 score is 7.6, reflecting high confidentiality and integrity impact.

Likely exposure

Exposure is limited to Ruby applications using an affected net-imap version and invoking STARTTLS. Risk is greatest where an attacker can intercept or manipulate traffic between the application and IMAP server. Applications using patched versions are not identified as affected.

Exploitation context

The attack requires a network-positioned adversary, protocol timing, and an application initiating STARTTLS. The supplied record is not in KEV and provides no evidence of active exploitation. Public exploit availability is not established by the supplied sources.

Researcher notes

The flaw concerns erroneous state handling around invalid STARTTLS responses and response timing. Relevant weakness categories include missing cryptographic protection, error-handling failures, and improper sequencing. Review the branch-specific commits for exact behavioral changes. The supplied material establishes impact and patches but not real-world exploitation prevalence.

Mitigation direction

  • Upgrade net-imap to 0.3.10, 0.4.24, 0.5.14, 0.6.4, or a later compatible release.
  • Review vendor guidance for Ruby or operating-system packaged versions.
  • Prioritize applications carrying sensitive mail, credentials, or traffic across untrusted networks.
  • Investigate potentially exposed credentials if unencrypted IMAP sessions may have occurred.

Validation and detection

  • Inventory deployed and bundled net-imap versions, including transitive dependencies.
  • Identify application paths that call Net::IMAP#starttls.
  • Confirm production workloads load a patched version, not only an updated lockfile.
  • Test that STARTTLS failures cannot be reported as successful TLS establishment.
  • Verify affected systems actually negotiate TLS before exchanging sensitive IMAP data.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-325: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-392: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-393: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-636: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-754: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-841: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.6 (4.0)
Known Exploited
No
Published

Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
2ADP providers
32Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: total

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.6CVSS 4.0HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N——GitHub_M
7.4CVSS 3.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N2.25.2redhat-SADP

Vulnerability scoring details

Base CVSS 4.0 score

7.6High
CVSS 4.0 vector shape for CVE-2026-42246Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPnet-imap: ruby: Net::IMAP: Information disclosure via man-in-the-middle attack bypassing TLS
other:Red Hat severity ratingcvssV3_1
  • 2026-05-09T20:01:04.782Z: Reported to Red Hat.
  • 2026-05-09T19:33:17.880Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
rubynet-imap< 0.3.10, >= 0.4.0, < 0.4.24, >= 0.5.0, < 0.5.14, >= 0.6.0, < 0.6.4Listed
Weakness

CWE details

CWE-325CWE-392CWE-393CWE-636CWE-754CWE-841

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.