CVE-2026-40015: IMAP Hibernate Service DOS Vulnerability

CVE-2026-40015 Overview

CVE-2026-40015 affects the Dovecot imap-hibernate service, distributed by Open-Xchange. An authenticated attacker with valid credentials can open many connections to the service and send invalid commands. The malformed input intermittently triggers an out-of-bounds read [CWE-125], which crashes the process. The crash interrupts hibernated IMAP sessions handled by the affected worker, causing degradation of service for IMAP users. No publicly available exploits are known at the time of publication.

Critical Impact
Authenticated attackers can crash the imap-hibernate process, disrupting hibernated IMAP sessions and degrading mail service availability for legitimate users.

Affected Products

Open-Xchange Dovecot (versions with IMAP hibernation enabled — see vendor advisory)
Deployments using the imap-hibernate service
IMAP mail platforms built on Dovecot Pro

Discovery Timeline

2026-08-28 - CVE-2026-40015 published to NVD
2026-09-03 - Last updated in NVD database

Technical Details for CVE-2026-40015

Vulnerability Analysis

The imap-hibernate service in Dovecot manages IMAP sessions that have been suspended to conserve memory and file-descriptor resources. When a client resumes activity, the hibernated state is rehydrated and passed back to an imap worker. This process depends on a compact command parser that reads client input from the persistent connection.

The vulnerability manifests when an authenticated user opens many concurrent connections and issues invalid commands. The parser performs an out-of-bounds read [CWE-125] on attacker-controlled input, dereferencing memory outside the intended buffer boundary. The condition is intermittent, indicating it depends on adjacent memory layout or specific input framing.

When the read touches unmapped memory, the process crashes. Every hibernated IMAP session multiplexed through that worker is torn down, forcing clients to reconnect and reauthenticate. Repeated crashes produce sustained denial of service against the mail platform.

Root Cause

The root cause is missing bounds validation during command parsing inside the imap-hibernate service. The parser advances past valid buffer boundaries when it encounters malformed input, resulting in a read from unintended memory. See the Open-Xchange Security Advisory OXDC-ADV-2026-0003 for vendor-supplied technical details.

Attack Vector

Exploitation requires network access to the IMAP service and valid account credentials. The attacker establishes multiple simultaneous connections to the imap-hibernate endpoint and transmits crafted invalid IMAP commands. Because the crash affects the shared hibernation process rather than a single session, one authenticated user can disrupt service for many unrelated users routed through the same worker.

No code execution, data disclosure, or integrity impact is associated with this issue. The impact is limited to availability of hibernated IMAP sessions.

Detection Methods for CVE-2026-40015

Indicators of Compromise

Unexpected termination or restart events for the imap-hibernate process in system logs.
Bursts of connections to the IMAP hibernation endpoint originating from a single authenticated account.
Elevated volume of IMAP client reconnection and reauthentication events following worker crashes.

Detection Strategies

Monitor Dovecot logs for imap-hibernate process crash entries, segmentation faults, and abnormal exit codes.
Correlate authenticated IMAP sessions with connection-rate anomalies per account to identify abusive clients.
Alert on repeated invalid IMAP command sequences originating from the same user or source IP.

Monitoring Recommendations

Ingest Dovecot service logs and Linux journald records into a centralized log platform for retention and correlation.
Track process restart counts for imap-hibernate as a service-health metric and alert on deviations from baseline.
Establish per-account rate baselines for concurrent IMAP connections and flag sudden spikes.

How to Mitigate CVE-2026-40015

Immediate Actions Required

Apply the fixed Dovecot version referenced in the Open-Xchange advisory as soon as testing permits.
If patching is not immediately feasible, disable IMAP hibernation to remove the vulnerable code path.
Review authentication logs for accounts exhibiting abnormal connection patterns and rotate credentials where abuse is suspected.

Patch Information

Open-Xchange has published fixed builds and remediation guidance in Open-Xchange Security Advisory OXDC-ADV-2026-0003. Administrators should upgrade to the non-vulnerable version identified in that advisory and restart affected Dovecot services after deployment.

Workarounds

Disable IMAP hibernation in the Dovecot configuration until patched builds are deployed.
Enforce per-user connection limits at the IMAP proxy or firewall to reduce the blast radius of a single abusive account.
Rate-limit invalid IMAP command sequences at network intermediaries where feasible.

bash

# Disable IMAP hibernation in Dovecot until patched
# Edit /etc/dovecot/conf.d/20-imap.conf
protocol imap {
  imap_hibernate_timeout = 0
}

# Reload Dovecot to apply the change
doveadm reload