CVE-2026-61911: Cyrus IMAP Information Disclosure Flaw

CVE-2026-61911 Overview

CVE-2026-61911 is an information disclosure vulnerability in Cyrus IMAP versions prior to 3.12.4. The flaw allows an authenticated user to install a Sieve script that acts as a mailbox existence oracle. By observing which fileinto branch fires during Local Mail Transfer Protocol (LMTP) delivery, an attacker can probe whether another user's private mailbox exists or read the value of shared mailbox annotations. The issue is categorized under [CWE-497: Exposure of Sensitive System Information to an Unauthorized Control Sphere].

Critical Impact
Authenticated users can enumerate private mailboxes belonging to other users and disclose shared mailbox annotation values that should remain confidential.

Affected Products

Cyrus IMAP versions prior to 3.8.8
Cyrus IMAP versions prior to 3.10.4
Cyrus IMAP versions prior to 3.12.4

Discovery Timeline

2026-09-09 - CVE CVE-2026-61911 published to NVD
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-61911

Vulnerability Analysis

Cyrus IMAP supports server-side mail filtering through Sieve scripts. Users can install scripts that route incoming messages into specific mailboxes using the fileinto action. The vulnerability arises because the LMTP delivery engine reveals distinguishable behavior depending on whether a target mailbox exists or a shared annotation resolves to a specific value.

An authenticated attacker crafts a Sieve script containing conditional fileinto branches referencing another user's mailbox path or a shared annotation. When mail is delivered through LMTP, the server takes different execution paths based on the existence check. The attacker infers the mailbox layout of other accounts by observing which branch of the script fired.

This constitutes a side-channel disclosure. No direct read access to the target mailboxes is required. The attack scales because a single crafted script can probe many candidate paths per delivery cycle.

Root Cause

The Sieve interpreter within Cyrus IMAP does not enforce access-control boundaries when evaluating fileinto targets against the namespace of other users or shared resources. Mailbox existence checks and annotation reads leak through observable script outcomes rather than being suppressed with a uniform result.

Attack Vector

The attacker requires authenticated access to the Cyrus IMAP server and permission to install Sieve scripts, which is standard for mail users. Exploitation is remote over the network and does not require user interaction. The attacker installs a probing Sieve script, triggers or waits for LMTP delivery, and observes which fileinto branch executed. See the Cyrus IMAP 3.12.4 Release Notes for the vendor description of the fix.

Detection Methods for CVE-2026-61911

Indicators of Compromise

Sieve scripts referencing fileinto targets outside the authenticated user's own mailbox namespace
Sieve scripts containing large numbers of conditional branches enumerating candidate mailbox names
LMTP delivery logs showing repeated fileinto branch evaluations against non-owned mailboxes

Detection Strategies

Audit stored Sieve scripts on the server for references to other users' namespaces or shared annotation identifiers
Correlate script installation events with subsequent LMTP delivery anomalies that reveal branch-selection patterns
Flag accounts that repeatedly modify Sieve scripts in short intervals, a pattern consistent with iterative probing

Monitoring Recommendations

Enable verbose LMTP delivery logging to capture fileinto branch outcomes and correlate them with the source Sieve script
Monitor Cyrus authentication logs for accounts installing Sieve scripts that reference non-owned mailbox paths
Track shared annotation read attempts and generate alerts when they originate from unexpected user contexts

How to Mitigate CVE-2026-61911

Immediate Actions Required

Upgrade Cyrus IMAP to version 3.12.4, 3.10.4, or 3.8.8 depending on the deployed release branch
Review all user-installed Sieve scripts and remove any that reference mailboxes or annotations outside the owning user's namespace
Restrict Sieve script installation permissions to trusted users where operational policy allows

Patch Information

The Cyrus project fixed the issue in versions 3.12.4, 3.10.4, and 3.8.8. Details are available in the Cyrus IMAP 3.12.4 Release Notes, the Cyrus IMAP 3.10.4 Release Notes, and the Cyrus IMAP 3.8.8 Release Notes. Administrators should apply the update on the maintenance branch matching their deployment.

Workarounds

Disable the Sieve service for untrusted user populations until patching is complete
Enforce strict namespace separation and remove shared annotation visibility for non-privileged accounts
Audit and prune existing Sieve scripts to eliminate cross-user fileinto targets

bash

# Verify the installed Cyrus IMAP version
cyrus-imapd -V

# Example: disable the Sieve managesieve service in cyrus.conf until patched
# Comment out or remove the sieve line, then restart
systemctl restart cyrus-imapd