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🛡️ CVE-2026-42246 — net-imap

🟠 CVSS 8.0 — High ✅ No Known Exploit CWE-392 NVD
8.0
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

net-imap vulnerable to STARTTLS stripping via invalid response timing

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](https://www.rfc-editor.org/info/rfc8314): Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access
  • [NO STARTTLS](https://nostarttls.secvuln.info/): 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.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is high, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality high, integrity high, availability none.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: High — the attacker first has to win a race, learn a secret or otherwise prepare the target.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: High — total loss, or loss the attacker controls.
  • Integrity impact: High — total loss, or loss the attacker controls.
  • Availability impact: None.

Weakness class

CVE-2026-42246 is classified as CWE-392: Missing Report of Error Condition. The product encounters an error but does not provide a status code or return value to indicate that an error has occurred.

Affected software

CVE-2026-42246 is recorded against 2 packages.

  • net-imap (fixed in 0.3.10)
  • net\ (from 0.6.0 up to 0.6.4)

Timeline and source

Published on 4 May 2026 and last revised on 14 May 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from NVD.

References

github.com (Web)
nvd.nist.gov (Advisory)
github.com (Web)
github.com (Web)
github.com (Web)
github.com (Web)
github.com (Package)
github.com (Web)
github.com (Web)
github.com (Web)
github.com (Web)
github.com (Web)
nostarttls.secvuln.info (Web)
www.rfc-editor.org (Web)

Other advisories for this package

net-imap has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-392: Missing Report of Error Condition) in other software:

CVE-2026-42246 on other distributions

Each distribution ships its own build and its own fixed version. Pick the one you run:

Details

Severity HIGH
CVSS Score 8.0
CVSS Vector CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
CWE CWE-392
Public Exploit ✅ No
Source NVD
Published 2026-05-04
Updated 2026-08-20
Modified 2026-05-14

Affected Packages

Software From version Fixed in
net-imap — 0.3.10
net\ 0.6.0 0.6.4

References

Similar Threats

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ⓘ Data Notice: The information presented above has been compiled from publicly available internet sources. Boteraser aggregates this data solely for informational purposes and does not independently classify, evaluate, or endorse any findings about the vulnerabilities listed. The accuracy and completeness of this information is the sole responsibility of the original publishers. Boteraser and its operators accept no liability for any decisions made based on this data.

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