CVE-2026-40013 in Dovecot Pro
Summary
by MITRE • 08/28/2026
An attacker that has valid credentials can submit a Sieve script containing an extreme numeric literal, which causes an out-of-bounds write when the ManageSieve service compiles the script. This causes memory corruption and an observed crash of the ManageSieve process, resulting in denial of service for script management. This might be able to be used for remote code execution. Disable the ManageSieve service if users do not need remote Sieve script management. Update to non-vulnerable version. No publicly available exploits are known.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability described involves a critical memory corruption flaw within the ManageSieve service, specifically triggered during the compilation phase of user-submitted Sieve scripts. This issue arises when an authenticated attacker submits a script containing extreme numeric literals that exceed expected bounds or processing limits. The underlying technical failure is classified as an out-of-bounds write, which occurs because the application fails to properly validate or sanitize these large numerical values before allocating memory or writing data structures associated with the compiled script state. This lack of rigorous input validation allows maliciously crafted data to overwrite adjacent memory locations that are not intended for modification by the Sieve compiler logic.
From a technical perspective, this flaw aligns closely with CWE-787: Out-of-bounds Write and CWE-20: Improper Input Validation. The ManageSieve protocol is designed to allow remote management of mail filtering scripts, but it relies on the integrity of its internal data structures during compilation. When an extreme numeric literal is processed, the resulting memory corruption can corrupt heap metadata or overwrite critical control flow pointers within the process space. This type of vulnerability is particularly dangerous because it bypasses authentication requirements by leveraging valid credentials, thereby operating under the context and permissions of the authenticated user account rather than as a fully unauthenticated remote attacker.
The operational impact of this vulnerability primarily manifests as a denial of service due to the immediate crash of the ManageSieve process upon script compilation failure caused by memory corruption. However, given that out-of-bounds writes can manipulate program execution flow, there is a significant potential for remote code execution if an attacker can carefully craft the payload to overwrite function pointers or return addresses with shellcode or ROP gadgets. This transforms what appears as a simple service crash into a high-severity security risk capable of granting full control over the mail server infrastructure. In terms of attack classification, this scenario relates to ATT&CK technique T1059: Command and Scripting Interpreter, specifically through the abuse of legitimate administrative tools like ManageSieve for malicious purposes after initial access is gained via valid credentials.
Mitigation strategies should focus on both immediate containment and long-term remediation. The most effective immediate measure is to disable the ManageSieve service entirely if remote script management is not a required business function, thereby removing the attack surface from exposure. For environments where ManageSieve must remain active, it is imperative to update the software to a patched version that includes proper bounds checking and input sanitization for numeric literals within Sieve scripts. Additionally, implementing strict network segmentation can limit access to the ManageSieve port only to trusted administrative subnets, reducing the risk of exploitation by compromised user accounts or lateral movement from other parts of the network. Regular auditing of script submissions and monitoring for unusual process crashes in system logs can also aid in early detection of attempted exploits.