CVE-2026-27858

High

Published: 27 March 2026

Published

27 March 2026

Modified

30 April 2026

KEV Added

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Patch

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CVSS Score 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Score 0.0002 6.3th percentile

Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-27858 is a high-severity Uncontrolled Resource Consumption (CWE-400) vulnerability in Open-Xchange Dovecot. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 6.3th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SC-5 (Denial-of-service Protection) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Application or System Exploitation (T1499.004). What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly addresses the vulnerability by requiring timely installation of the fixed version to remediate the memory allocation flaw in managesieve.

SC-5 Denial-of-service Protection good match

prevent

Implements denial-of-service protections such as rate limiting and resource controls to mitigate unauthenticated crafted message floods causing memory exhaustion and crashes.

SC-7 Boundary Protection partial match

prevent

Enforces boundary protections to restrict network access to the managesieve protocol, reducing exposure to remote unauthenticated attackers as recommended in the advisory.

MITRE ATT&CK Enterprise TechniquesAI

T1499.004 Application or System Exploitation Impact

Adversaries may exploit software vulnerabilities that can cause an application or system to crash and deny availability to users.

attack.mitre.org →

Why these techniques?

The CVE describes remote unauthenticated exploitation of a memory exhaustion flaw (CWE-400) in a network-exposed service (managesieve), directly enabling adversaries to crash the process and deny availability via application/system exploitation.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

Attacker can send a specifically crafted message before authentication that causes managesieve to allocate large amount of memory. Attacker can force managesieve-login to be unavailable by repeatedly crashing the process. Protect access to managesieve protocol, or install fixed version. No…

publicly available exploits are known.

Deeper analysisAI

CVE-2026-27858 is a denial-of-service vulnerability in the managesieve component of Dovecot, stemming from CWE-400 (Uncontrolled Resource Consumption). An attacker can send a specifically crafted message prior to authentication, causing the managesieve process to allocate a large amount of memory. This issue received a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) and was published on 2026-03-27.

An unauthenticated attacker with network access can exploit this vulnerability by repeatedly sending the crafted message, forcing the managesieve-login process to crash and rendering it unavailable. The attack requires low complexity and no user interaction, enabling remote denial of service against the affected service.

The Open-Xchange Dovecot security advisory (oxdc-adv-2026-0001) recommends protecting access to the managesieve protocol or installing a fixed version to mitigate the issue. No publicly available exploits are known.