CVE-2026-107572 Progressive Robot Ltd CVE debrief

Who should care

Defenders responsible for hMailServer deployments, especially those with authenticated users who have access to Sieve filters, should assess exposure and prioritize mitigation. This includes verifying affected versions, restricting filter access, and implementing compensating controls. Security teams and vulnerability management teams should also review the CVE record and source item details to understand the operational impacts and prioritize remediation.

Why it matters

CVE-2026-107572 allows authenticated users to cause denial of service in hMailServer via inefficient Sieve filter evaluation. Defenders should verify affected versions, restrict filter access, and prioritize mitigation.

• Denial of service for mail services
• Potential performance degradation due to inefficient filter evaluation
• Need for verification of affected versions and user access to Sieve filters
• Priority for upgrading to version 6.3.6 or later if available

Technical summary

The Sieve filter evaluation in hMailServer 6.2.24 through 6.3.5 has an inefficient complexity issue that allows an authenticated account holder to make mail services unavailable with their own filter. A ':matches' pattern with a large number of wildcards can cause a significant delay, and 'deleteheader' can erase fields one at a time, leading to O(N^2) complexity. The vulnerability affects hMailServer deployments with Sieve filter access granted to authenticated users. Defenders should prioritize verifying and mitigating this vulnerability.

Defensive priority

Defenders should prioritize verifying and mitigating this vulnerability in hMailServer deployments, especially for authenticated users with Sieve filter access.

Recommended defensive actions

• Verify hMailServer version and check if it is 6.2.24 through 6.3.5
• Restrict Sieve filter access to trusted users and monitor filter usage
• Implement compensating controls to mitigate denial of service
• Upgrade to version 6.3.6 or later if available
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record and source item provide details on the inefficient regular expression complexity in hMailServer's Sieve filter evaluation, which can lead to denial of service for mail services. The vulnerability affects hMailServer versions 6.2.24 through 6.3.5. Defenders should verify affected versions, restrict filter access, and prioritize mitigation. The CVE record was published on 2026-10-08T11:46:41.080Z and has not been modified since then. Evidence limits suggest verifying hMailServer deployments, especially those with Sieve  

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.