CVE-2026-40019 in Dovecot
Summary
by MITRE • 08/28/2026
An unauthenticated attacker can send a truncated quoted argument to the ManageSieve login process, which makes it spin in an infinite loop consuming CPU. This can cause degradation or denial of service for Sieve script management, and repeated connections can consume all available CPU on the server. Monitor system for abnormal CPU usage and kill the offending process. Restrict network access to the ManageSieve service to trusted clients. Update to non-vulnerable version. No publicly available exploits are known.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 08/28/2026
The vulnerability in question involves a critical flaw within the ManageSieve login mechanism, specifically affecting how the service processes incoming authentication requests. An unauthenticated attacker can exploit this weakness by sending a truncated quoted argument during the initial handshake phase of the connection. This malformed input triggers an infinite loop within the server-side processing logic, causing the affected process to consume CPU resources continuously without terminating or returning control to the system scheduler. The root cause lies in insufficient validation and boundary checking for string arguments passed through the ManageSieve protocol interface, allowing a specifically crafted packet to bypass standard termination conditions expected by the parser.
From an operational perspective, this flaw results in a severe denial of service condition. As the vulnerable process spins indefinitely, it monopolizes CPU cycles on the host system. If multiple connections are established simultaneously using this technique, the cumulative effect can exhaust all available processing power on the server. This leads to a complete degradation of service for legitimate users attempting to manage Sieve scripts or access other services hosted on the same machine. The impact is particularly acute because it requires no authentication credentials, making it accessible to any network actor with connectivity to the ManageSieve port, typically TCP 4190.
In terms of industry classification standards, this vulnerability aligns closely with CWE-835, which describes a loop involving an uncontrolled resource consumption scenario often referred to as a DoS via infinite loop or CPU exhaustion. It also relates to CWE-20 regarding improper input validation where the system fails to verify that user-supplied data meets expected format and length constraints before processing. From a tactical standpoint within the MITRE ATT&CK framework, this behavior is consistent with techniques used in resource hijacking for denial of service, specifically falling under T1499 Endpoint Denial of Service or potentially T1053 Scheduled Task/Job if the loop leads to persistent system instability affecting availability metrics.
Mitigation strategies should focus on both immediate containment and long-term remediation. Immediate actions include monitoring server performance metrics for abnormal spikes in CPU usage associated with ManageSieve processes and terminating any such offending processes manually when detected. Network-level controls are highly recommended; administrators should restrict access to the ManageSieve service using firewall rules or network segmentation, allowing connections only from trusted client IP addresses known to be legitimate sources of mail management traffic. Furthermore, deploying intrusion detection systems capable of identifying malformed protocol packets can provide an additional layer of defense by dropping suspiciously formatted requests before they reach the application logic.
The definitive resolution requires updating the ManageSieve software or its underlying server implementation (such as Dovecot) to a version that includes patches for this input validation flaw. Since no public exploits are currently known, organizations have a window of opportunity to apply these updates proactively without facing immediate targeted attacks from automated botnets. Regular patch management cycles and vulnerability scanning should be maintained to ensure timely identification and remediation of similar flaws in other network-facing services.