CVE-2026-27856

High

Published: 27 March 2026

Published

27 March 2026

Modified

30 March 2026

KEV Added

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Patch

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CVSS Score 7.4 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS Score 0.0002 3.9th percentile

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

Summary

CVE-2026-27856 is a high-severity Improper Authentication (CWE-287) vulnerability in Open Xchange (inferred from references). Its CVSS base score is 7.4 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 3.9th 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-7 (Boundary Protection) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique. 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 mitigates the timing oracle vulnerability in doveadm credential verification by requiring organizations to identify, report, and remediate flaws through installation of the fixed Dovecot version.

SC-7 Boundary Protection good match

prevent

Prevents remote attackers from reaching the vulnerable doveadm HTTP service port by monitoring and controlling communications at system boundaries, such as via firewalls.

IA-5 Authenticator Management partial match

prevent

Addresses improper authenticator verification susceptible to timing attacks by requiring secure management and protection of authenticators from unauthorized disclosure and modification.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1110.001 Password Guessing Credential Access

Adversaries with no prior knowledge of legitimate credentials within the system or environment may guess passwords to attempt access to accounts.

attack.mitre.org →

Why these techniques?

Timing oracle in exposed doveadm HTTP auth directly enables remote password guessing (T1110.001) against a public-facing service (T1190); obtained credentials then permit Valid Accounts usage.

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

NVD Description

Doveadm credentials are verified using direct comparison which is susceptible to timing oracle attack. An attacker can use this to determine the configured credentials. Figuring out the credential will lead into full access to the affected component. Limit access to…

the doveadm http service port, install fixed version. No publicly available exploits are known.

Deeper analysisAI

CVE-2026-27856 is a vulnerability in the Dovecot mail server, specifically affecting the doveadm HTTP service where credentials are verified through direct comparison, making it susceptible to timing oracle attacks. This flaw, cataloged under CWE-287 (Improper Authentication), allows an attacker to infer configured credentials by measuring response time differences during authentication attempts. The vulnerability was published on 2026-03-27 and carries a CVSS v3.1 base score of 7.4 (AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N).

A remote attacker with network access to the doveadm HTTP service port, requiring no privileges or user interaction but high attack complexity, can exploit this timing oracle to determine valid credentials. Successful exploitation grants full access to the affected component, enabling high-impact confidentiality and integrity violations such as unauthorized data access or manipulation, though availability remains unaffected.

The Open-Xchange advisory recommends limiting access to the doveadm HTTP service port and installing a fixed version of Dovecot to mitigate the issue. No publicly available exploits are known at this time.