CVE-2026-40016Network attack vector

Service degradation via unchecked Sieve matcher loops

Published May 12, 2026 · Updated May 12, 2026

Resource exhaustion in Open-Xchange Dovecot Pro 2.3.0 and earlier allows remote authenticated users to degrade server performance. The Pigeonhole :contains and :matches matchers use naive nested search loops that check sieve_max_cpu_time only between bytecode operations. Remote authenticated users or local users who can install a crafted Sieve script can consume up to 130 times the configured CPU limit, delaying mail service.

CVSS severity5.3
Medium
EPSS probability0.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Resource exhaustion in Open-Xchange Dovecot Pro 2.3.0 and earlier allows remote authenticated users to degrade server performance. The Pigeonhole :contains and :matches matchers use naive nested search loops that check sieve_max_cpu_time only between bytecode operations. Remote authenticated users or local users who can install a crafted Sieve script can consume up to 130 times the configured CPU limit, delaying mail service.

The record

CVE
CVE-2026-40016
Published
May 12, 2026
Updated
May 12, 2026
Vendor
Open-Xchange GmbH
Product
Dovecot Pro
Classifications
CWE-400, T1499.003
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 12, 2026CVE publishedPublication date reported by the CVE source.
  2. May 12, 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=0 <=2.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Open-Xchange GmbH · Dovecot Proversion=0 <=2.3.0

Published CVSS scores

6.5NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
5.3Open-XchangeCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.003Application Exhaustion Flood

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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