CVE-2026-61911 in IMAP
Summary
by MITRE • 09/09/2026
An issue was discovered in Cyrus IMAP before 3.12.4. There is a Sieve mailbox existence oracle. An authenticated user could install a Sieve script that probed whether another user's private mailbox existed, or read the value of shared mailbox annotations, by observing which fileinto branch fired during LMTP delivery.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified in Cyrus IMAP versions prior to 3.12.4 represents a significant information disclosure flaw rooted in the Sieve mail filtering language implementation. Sieve is an email processing language that allows users to define rules for how incoming messages are handled, such as filing them into specific folders or forwarding them elsewhere. In this context, the vulnerability acts as a mailbox existence oracle, enabling authenticated attackers to infer sensitive structural information about other user accounts within the same mail system. This flaw arises from the way the Local Mail Transfer Protocol (LMTP) delivery agent processes Sieve scripts and reports outcomes back to the client during message submission or delivery.
The technical mechanism of this exploitation relies on side-channel analysis through differential response times or status codes associated with specific sieve script branches. When an authenticated user installs a custom Sieve script, they can construct logic that attempts to file incoming messages into different mailboxes based on certain conditions. By carefully crafting these scripts and observing which branch executes successfully during LMTP delivery, the attacker can deduce whether a target mailbox exists or if shared annotations are present. For instance, if one branch of the sieve rule results in a successful delivery confirmation while another fails with an error indicating non-existence, the attacker gains definitive proof that the targeted private mailbox is active and accessible within the system architecture.
This capability poses severe operational risks to organizational security postures by facilitating user enumeration attacks. Attackers can map out the internal structure of the email infrastructure without needing direct access credentials for other accounts. This intelligence gathering phase is critical in advanced persistent threat scenarios, as it allows adversaries to identify high-value targets such as executives or administrators based on their mailbox existence and activity patterns. Furthermore, the ability to read shared mailbox annotations expands the scope of data leakage beyond simple account enumeration, potentially revealing collaboration structures, project assignments, or access permissions that were intended to remain confidential between specific groups of users.
The vulnerability aligns with CWE-203, which describes an observable discrepancy in system behavior when a condition is true versus false, and CWE-209 regarding the generation of error messages containing sensitive information. From an ATT&CK perspective, this flaw supports techniques under T1589, specifically Gather Victim Identity Information through email addresses or user enumeration. The exploitation does not require complex buffer overflows or memory corruption but rather a logical misunderstanding in how the mail server handles conditional logic and feedback mechanisms during message processing. This makes it particularly insidious as standard intrusion detection systems may not flag normal-looking LMTP traffic that merely triggers these differential responses.
Mitigation strategies must prioritize immediate upgrading to Cyrus IMAP version 3.12.4 or later, where this oracle behavior has been addressed by the developers. In environments where patching is delayed due to operational constraints, administrators should restrict Sieve script installation privileges to trusted users only and review existing scripts for any logic that could be abused for probing other mailboxes. Additionally, implementing strict rate limiting on LMTP connections can help mitigate automated enumeration attempts by slowing down the feedback loop required for successful oracle exploitation. Security teams should also monitor logs for unusual patterns in sieve execution errors or delivery statuses that correlate with non-existent mailbox references, as these may indicate active reconnaissance efforts against the email infrastructure.