Raw vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:NSummary
CVE-2026-27856 is a high-severity Improper Authentication (CWE-287) vulnerability in Open-Xchange Dovecot. Its CVSS base score is 7.4 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Password Guessing (T1110.001); ranked at the 32th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to IA-2 (Identification and Authentication (Organizational Users)) and IA-3 (Device Identification and Authentication) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
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.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-16565
Vulnerability Data
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…
more
the doveadm http service port, install fixed version. No publicly available exploits are known.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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- 8 hardening rules · 5 OS baselines
V11.2.4V6.4.4V6.5.4V6.5.5
Mitigating Controls (NIST 800-53 r5) AI
Mandates unique identification and authentication of organizational users before access, directly stopping improper authentication.
Requires unique identification and authentication of devices before establishing connections, preventing unauthenticated device claims.
Mandates unique identification and authentication of non-organizational users, directly addressing the weakness for external actors.
Manages authenticators with verification and secure distribution, reducing opportunities for improper authentication.
Developer testing can include timing analysis or side-channel test cases that reveal observable timing discrepancies.
Enforces access only after approved authorizations, which presupposes correct authentication has occurred.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
PR.AA-03 directly enforces authentication mechanisms that eliminate most improper-authentication defects, yet CWE-287 spans additional vectors (missing checks, flawed protocols, session handling) that one control does not fully close.
PR.AA-04 directly enables verification of identity assertions (mostly preventing CWE-287 in that scope) yet leaves many other authentication failure modes unaddressed (only partial prevention overall).
Secure SDLC practices directly require constant-time implementations that eliminate observable timing discrepancies.
PR.AA-01 supplies and governs credentials/tokens that authentication relies on, removing some weak-credential cases, yet leaves verification logic, missing checks, and protocol flaws untouched.
PR.AA-02 ensures valid enrollment and unique credential binding, which reduces some improper-auth risks at issuance time but leaves runtime claim verification untouched, so each direction only partially addresses the other.
PR.PS-01 can enforce auth-related settings via hardened baselines and default reviews, blocking some config-based instances of CWE-287, yet leaves code-level auth flaws untouched so neither direction reaches mostly.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Requiring authentication methods whose strength matches the sensitivity of the data and mandating multi-factor authentication directly blocks attempts to access resources without proving identity.
Verifying user identity before issuing replacement credentials and forcing changes after compromise reduces the likelihood that authentication bypass can be achieved through stolen or weak credentials.
Mandating segregated approval and oversight for non-human identities reduces the chance that weak or orphaned credentials can be exploited for unauthorized authentication.
Requiring authentication mechanisms and technical parameters for secure connections ensures that network services verify user identity before granting access, preventing exploitation of missing or weak authentication.
Security requirements specified early and verified through testing drive the consistent implementation of authentication mechanisms, decreasing the likelihood that authentication steps are omitted or incorrectly applied.
Defining the required level of trust in entity identity and the authentication mechanisms to achieve it ensures that authentication is explicitly addressed rather than omitted.
Hardening callouts derived
Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (authoritative rows only; rows rated `none` are excluded).
Oracle Linux 8 (1 rule)
- V-248827 OL 8 must not have the rsh-server package installed. prevents CWE-287
RHEL 7 (3 rules)
- V-204425 The Red Hat Enterprise Linux operating system must be configured so that the SSH daemon does not allow authentication using an empty password. prevents CWE-287
- V-204442 The Red Hat Enterprise Linux operating system must not have the rsh-server package installed. prevents CWE-287
- V-204424 The Red Hat Enterprise Linux operating system must not allow accounts configured with blank or null passwords. prevents CWE-287
RHEL 8 (1 rule)
- V-230492 RHEL 8 must not have the rsh-server package installed. prevents CWE-287
Ubuntu 22.04 (1 rule)
- V-260470 Ubuntu 22.04 LTS, when booted, must require authentication upon booting into single-user and maintenance modes. prevents CWE-287
Ubuntu 24.04 (2 rules)
- V-270675 Ubuntu 24.04 LTS when booted must require authentication upon booting into single-user and maintenance modes. prevents CWE-287
- V-270714 Ubuntu 24.04 LTS must not allow accounts configured in Pluggable Authentication Modules (PAM) with blank or null passwords. prevents CWE-287