CVE-2026-40203Network attack vector

Guessed-message disclosure via reused IMAP compression state

Published Aug 28, 2026 · Updated Aug 28, 2026

Information disclosure in affected OX Dovecot CE and Pro releases allows remote attackers to confirm guessed message bodies via IMAP traffic sizes. The IMAP server reuses one compression state across responses, making compressed response lengths depend on attacker-supplied mail and other messages. The attacker must send mail to the user, observe that user's IMAP traffic sizes, and target a session with IMAP compression enabled; arbitrary unknown-content recovery was not demonstrated.

CVSS severity3.7
Low
EPSS probability0.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in affected OX Dovecot CE and Pro releases allows remote attackers to confirm guessed message bodies via IMAP traffic sizes. The IMAP server reuses one compression state across responses, making compressed response lengths depend on attacker-supplied mail and other messages. The attacker must send mail to the user, observe that user's IMAP traffic sizes, and target a session with IMAP compression enabled; arbitrary unknown-content recovery was not demonstrated.

The record

CVE
CVE-2026-40203
Published
Aug 28, 2026
Updated
Aug 28, 2026
Vendor
Open-Xchange GmbH
Product
OX Dovecot CE
Classifications
CWE-205, CWE-200, T1040
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 28, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 28, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=2.3.0 <2.4.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Open-Xchange GmbH · OX Dovecot CEversion=2.3.0 <2.4.5
Open-Xchange GmbH · OX Dovecot Proversion=2.3.0 <2.3.22.2; version=3.0.0 <3.0.7; version=3.1.0 <3.1.6

Published CVSS scores

3.7Red HatCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
3.7Open-Xchange GmbHCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-205Observable Behavioral Discrepancy
CWE-200Exposure of Sensitive Information to an Unauthorized Actor
T1040Network Sniffing

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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