CVE-2026-42393: Doveadm Timing Attack Information Disclosure

CVE-2026-42393 Overview

CVE-2026-42393 is a timing side channel vulnerability in the Dovecot doveadm service. The comparison routine used to validate the doveadm password and API key is not fully timing safe. An attacker on the same network segment can measure response timing across repeated requests to infer the length of the configured secret. The secret value itself is not disclosed, but knowing its length reduces the search space for brute-force guessing. The issue is tracked under CWE-200: Exposure of Sensitive Information and is documented in the Open-Xchange Security Advisory OXDC-ADV-2026-0003.

Critical Impact
An adjacent-network attacker can determine the length of the doveadm password or API key through response timing analysis, weakening downstream credential guessing resistance.

Affected Products

Dovecot doveadm service (Open-Xchange distribution)
Deployments exposing the doveadm administrative interface to networked clients
Versions prior to the fix referenced in advisory OXDC-ADV-2026-0003

Discovery Timeline

2026-08-28 - CVE-2026-42393 published to the National Vulnerability Database
2026-09-03 - Last updated in NVD database

Technical Details for CVE-2026-42393

Vulnerability Analysis

The doveadm service authenticates administrative requests using a shared password or API key. The comparison function used during validation returns early when the first mismatched byte is encountered rather than executing in constant time. That early exit produces measurable timing differences correlated with the length of the configured secret.

An attacker with network reachability to the doveadm port can submit crafted authentication attempts of varying lengths. By statistically analyzing round-trip response times across many repeated requests, the attacker narrows down the exact byte length of the stored secret. Knowledge of the secret length substantially reduces the entropy space for a follow-on brute-force or dictionary attack.

The scope is limited to information about secret length. No portion of the secret itself is returned, and exploitation requires accurate timing measurements over the network, which raises the practical bar. No public exploit code has been observed.

Root Cause

The underlying flaw is a non-constant-time string comparison used against sensitive credential material. Standard comparison operators short-circuit on the first differing character, which leaks length and, in some cases, prefix information through observable side channels [CWE-200].

Attack Vector

Exploitation requires adjacent-network access to the doveadm service and the ability to issue repeated authentication requests while accurately measuring response latency. The attacker does not need credentials or user interaction. Success yields only the length of the secret, not the secret itself. Because network jitter obscures fine-grained timing, the attacker typically needs a large number of samples and a stable network path to extract usable signal.

No verified proof-of-concept code is publicly available. Refer to the Open-Xchange Security Advisory for vendor technical detail.

Detection Methods for CVE-2026-42393

Indicators of Compromise

Unusually high volumes of failed authentication attempts against the doveadm service from a single source or subnet
Repeated doveadm requests using varying credential lengths from the same client within short time windows
Connections to the doveadm port from hosts that do not normally administer the mail platform

Detection Strategies

Enable verbose authentication logging on doveadm and alert on repeated failed authentications from the same peer
Correlate network flow telemetry with doveadm service logs to surface sources issuing statistically anomalous request rates
Deploy network intrusion detection signatures that flag high-frequency, uniform-size probing patterns against administrative mail ports

Monitoring Recommendations

Baseline normal administrative request volume for doveadm and alert on deviations
Monitor for connections to doveadm originating outside the trusted management network
Aggregate authentication failure events into a SIEM or data lake for long-window correlation across sources

How to Mitigate CVE-2026-42393

Immediate Actions Required

Restrict network access to the doveadm service so only trusted management hosts can reach it
Update Dovecot to a non-vulnerable version referenced in advisory OXDC-ADV-2026-0003
Rotate the doveadm password and API key after patching, and increase the secret length to reduce brute-force feasibility
Review authentication logs for prior evidence of repeated probing against doveadm

Patch Information

Open-Xchange has published fixed builds through the OXDC-ADV-2026-0003 advisory. Apply the vendor-supplied update, which replaces the affected comparison with a constant-time routine that does not leak length information.

Workarounds

Firewall the doveadm port so it is only reachable from a dedicated administrative network segment or jump host
Place the service behind a VPN or mutual TLS tunnel that terminates before the doveadm listener
Rate-limit authentication attempts at the network perimeter to reduce the number of samples an attacker can collect

bash

# Example iptables rule restricting doveadm access to a management subnet
iptables -A INPUT -p tcp --dport 24245 -s 10.10.50.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 24245 -j DROP