CVE-2026-52681: 
Dovecot vulnerability analysis and mitigation

CVE-2026-52681 is a Denial of Service vulnerability in Open-Xchange's Dovecot mail server affecting its Sieve script processing engine. An authenticated attacker can bypass the configured Sieve CPU limit by repeatedly changing the active script, resetting CPU usage accounting, and accumulating leftover compiled script files that are never cleaned up on deletion or rename. Affected products include OX Dovecot Pro (versions 2.3.15–2.3.22.2, 3.0.0–3.0.7, 3.1.0–3.1.6) and OX Dovecot CE (versions 2.3.15–2.4.5). The vulnerability was published on August 28, 2026, with a CVSS v3.1 base score of 3.1 (Low) (GitHub Advisory, Red Hat Bugzilla).

The root cause is improper resource accounting in Dovecot's Sieve script engine, classified under CWE-1050 (Excessive Platform Resource Consumption within a Loop) and CWE-770 (Allocation of Resources Without Limits or Throttling). CPU usage is tracked per compiled Sieve script, but switching the active script resets the counter, allowing an attacker to circumvent the configured CPU limit through repeated script changes. Additionally, compiled script files are not removed when a script is deleted or renamed, causing unbounded disk consumption over time. Exploitation requires valid user credentials and network access, making the attack complexity high but not requiring elevated privileges (GitHub Advisory, Red Hat Bugzilla).

Successful exploitation degrades mail delivery service availability through two mechanisms: sustained CPU consumption that bypasses configured Sieve CPU limits, and accumulation of orphaned compiled script files that increase disk usage over time. There is no confidentiality or integrity impact — the vulnerability is limited to availability (Low). The scope is unchanged, meaning impact is confined to the Dovecot mail server itself, without lateral movement potential (GitHub Advisory).

No publicly available proof-of-concept exploits are known, and there is no evidence of in-the-wild exploitation as of the disclosure date. The EPSS score is approximately 0.254% (17th percentile), indicating a low probability of exploitation in the near term. The NVD SSVC assessment classifies exploitation as "none" and the attack as non-automatable, reflecting the requirement for valid credentials and manual script manipulation. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog (GitHub Advisory, Red Hat Bugzilla).

1. Obtain valid credentials: The attacker must have a legitimate Dovecot account with Sieve script management permissions.
2. Connect to ManageSieve: Use a ManageSieve client (e.g., sieve-connect or a mail client with Sieve support) to authenticate to the target Dovecot server.
3. Upload and activate a CPU-intensive Sieve script: Create a Sieve script designed to consume significant CPU during evaluation (e.g., complex filtering rules) and set it as the active script.
4. Reset CPU accounting: Repeatedly switch the active script (e.g., alternate between two uploaded scripts) to reset the CPU usage counter each time, bypassing the configured Sieve CPU limit.
5. Accumulate disk waste: Delete or rename scripts without triggering cleanup of compiled script files, causing leftover compiled artifacts to accumulate on disk.
6. Sustain degradation: Continue the cycle to maintain elevated CPU load and disk consumption, degrading mail delivery service for other users (GitHub Advisory, Red Hat Bugzilla).

• System Resources: Abnormally high and sustained CPU usage by the Dovecot Sieve process without a corresponding spike in legitimate mail volume.
• File System: Accumulation of orphaned compiled Sieve script files (typically .svbin or similar) in the Sieve script storage directory that do not correspond to any active or listed scripts.
• Logs: Dovecot logs showing a single authenticated user repeatedly activating/deactivating or renaming Sieve scripts in rapid succession; ManageSieve session logs with frequent SETACTIVE or RENAMESCRIPT commands from the same account.
• Disk Usage: Unexpected growth in the Sieve script storage directory, particularly files not associated with currently listed scripts (GitHub Advisory).

Update to a patched version of Dovecot: OX Dovecot Pro 2.3.22.2 or later (for the 2.3.x branch), 3.0.7 or later (for the 3.0.x branch), or 3.1.6 or later (for the 3.1.x branch); OX Dovecot CE 2.4.5 or later. As an interim measure, monitor systems for abnormal CPU usage and disk consumption in Sieve script storage directories. Restricting Sieve script management access to trusted users only can reduce exposure until patching is complete (GitHub Advisory, Red Hat Bugzilla).

The vulnerability was disclosed by Open-Xchange GmbH via a CSAF advisory and reported to the oss-security mailing list and Full Disclosure list, indicating standard coordinated disclosure. Red Hat tracked the issue via Bugzilla with a low severity rating. No notable independent researcher commentary or significant social media discussion has been observed, consistent with the low CVSS score and absence of public exploits (Red Hat Bugzilla).