CVE-2026-40019: ManageSieve DOS Vulnerability

CVE-2026-40019 Overview

CVE-2026-40019 is a denial of service vulnerability affecting the ManageSieve login process in Dovecot, distributed by Open-Xchange. An unauthenticated remote attacker can send a truncated quoted argument during the login phase, causing the server process to enter an infinite loop and consume CPU resources. Repeated connections amplify the effect and can exhaust all available CPU on the host, degrading or preventing Sieve script management for legitimate users. The flaw is classified under CWE-400: Uncontrolled Resource Consumption. No authentication or user interaction is required to trigger the condition.

Critical Impact
Unauthenticated attackers can exhaust server CPU by repeatedly submitting malformed ManageSieve login arguments, causing denial of service against Sieve script management.

Affected Products

Open-Xchange Dovecot (ManageSieve service)
Refer to the vendor advisory for specific affected build ranges
Deployments exposing the ManageSieve port (typically TCP/4190) to untrusted networks

Discovery Timeline

2026-08-28 - CVE-2026-40019 published to NVD
2026-09-03 - Last updated in NVD database

Technical Details for CVE-2026-40019

Vulnerability Analysis

The ManageSieve protocol, defined in RFC 5804, allows clients to upload and manage Sieve mail filtering scripts on a server. During the login phase, clients send commands with arguments that may be encoded as quoted strings or literal strings. This vulnerability arises when the login process parses a truncated quoted argument. Instead of returning a parse error and terminating the connection, the parser fails to advance its position and re-reads the same input indefinitely. The result is an infinite loop that pegs a CPU core at full utilization.

Because the flaw is reachable pre-authentication, any network client that can open a TCP connection to the ManageSieve listener can trigger it. A single connection consumes one core. Multiple parallel connections proportionally exhaust additional cores until the server can no longer serve legitimate traffic.

Root Cause

The root cause is missing input validation in the ManageSieve command tokenizer. When the parser encounters a quoted argument that terminates before the closing quote or before the declared length is satisfied, it does not exit the parsing loop or flag the input as malformed. The loop condition depends on progress that never occurs, satisfying the definition of CWE-400.

Attack Vector

An attacker connects to the ManageSieve service over the network and issues a login command containing a malformed quoted string, for example a leading double quote without a matching terminator followed by a premature line ending. The server process accepts the bytes, enters the parsing routine, and never exits. The attacker can then close the socket and open new connections to consume additional CPU. No credentials, prior access, or user interaction are required. See the Open-Xchange Security Advisory for protocol-level details.

// No verified public exploit code is available.
// The trigger is a ManageSieve command containing a truncated
// quoted-string argument sent to the pre-authentication parser.

Detection Methods for CVE-2026-40019

Indicators of Compromise

Sustained 100% CPU utilization by one or more ManageSieve worker processes with no corresponding authenticated session activity
Elevated inbound connection rates to TCP/4190 from a small set of source IP addresses
ManageSieve login attempts that terminate abnormally without completing authentication
Growing process count or thread count for the Dovecot ManageSieve service without matching mail delivery load

Detection Strategies

Monitor per-process CPU time for ManageSieve workers and alert when a single process exceeds a defined threshold for longer than expected login duration
Correlate ManageSieve TCP connection counts with successful authentication counts; a large delta indicates abuse
Inspect Dovecot logs for repeated malformed command errors or truncated protocol frames from the same client

Monitoring Recommendations

Ingest Dovecot and host process telemetry into a centralized logging or SIEM platform for baseline comparison
Alert on ManageSieve service latency increases and connection queue saturation
Track network flow data for TCP/4190 to identify volumetric or low-and-slow connection patterns

How to Mitigate CVE-2026-40019

Immediate Actions Required

Apply the vendor-supplied Dovecot update referenced in the Open-Xchange Security Advisory as soon as it is available for your distribution
Restrict network access to the ManageSieve port so that only trusted client subnets or authenticated VPN users can reach it
Identify and terminate any ManageSieve worker processes that show sustained maximum CPU usage without an active authenticated session

Patch Information

Open-Xchange has published advisory oxdc-adv-2026-0003 describing the fixed versions of Dovecot ManageSieve. Administrators should consult the advisory for the exact fixed build corresponding to their deployment channel and upgrade to a non-vulnerable version. Verify the patch by confirming the running Dovecot version and by replaying a benign truncated-quoted-string test against a staging instance to ensure the parser now returns a protocol error.

Workarounds

Firewall the ManageSieve port at the network edge and expose it only to management networks until the patch is deployed
Enforce per-source connection rate limits and concurrent connection caps on the ManageSieve listener
Reduce the login timeout so that a stuck login worker is reaped more quickly by the parent process
Monitor for and automatically kill ManageSieve worker processes exceeding a CPU time budget during the pre-authentication phase

bash

# Example iptables restriction limiting ManageSieve to a trusted subnet
iptables -A INPUT -p tcp --dport 4190 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 4190 -j DROP

# Example connection rate limit per source IP
iptables -A INPUT -p tcp --dport 4190 -m conntrack --ctstate NEW \
  -m recent --set --name MSIEVE
iptables -A INPUT -p tcp --dport 4190 -m conntrack --ctstate NEW \
  -m recent --update --seconds 60 --hitcount 10 --name MSIEVE -j DROP