EUVD-2026-94864
GHSA-fh2m-q9cx-5h7c
Severity by source
AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Unauthenticated attacker emails the server (AV:N/PR:N/AC:L), but the quadratic decode fires only on recipient folder listing or IMAP PREVIEW (UI:R); impact is availability-only (A:H, C/I:N).
Primary rating from Vendor (GitLab).
CVSS VectorVendor: GitLab
Lifecycle Timeline
3DescriptionCVE.org
Inefficient algorithmic complexity in the REST API (6.3.3 through 6.3.5) and the IMAP PREVIEW response (6.2.22 through 6.3.5) of Progressive Robot hMailServer allows a remote unauthenticated attacker to make the webmail, the administration console and the REST API unavailable by sending a message. To show a snippet of each message in a folder's message list, the server decoded the character entity references of a message that has an HTML part and no text part with a string replacement whose work grew with the square of their number. A received HTML-only message holding a very large number of entity references therefore keeps one of the listener's four worker threads busy for minutes or longer each time the recipient's webmail lists the folder, without the message being opened, so that a few such listings leave the HTTP listener unable to answer anybody. The IMAP PREVIEW response read such text the same way, holding an IMAP thread for each client that asks for the preview of such a message. The flaw is in the server's shared string class, whose replace and remove both ran in quadratic time.
AnalysisAI
A remote, unauthenticated attacker can degrade or fully stall Progressive Robot hMailServer (6.2.22-pre1 through 6.3.5) by sending a single HTML-only email whose body contains a very large number of character entity references, because the server's shared string class performs entity decoding with a replace/remove implementation whose cost grows with the square of the number of entities (CWE-407). The expensive decode runs only when the server builds a folder snippet - when the recipient lists the folder in webmail via the REST API (6.3.3-6.3.5) or when an IMAP client issues a PREVIEW request (6.2.22-6.3.5) - and the message never has to be opened; each such listing can pin one of the listener's four worker threads for minutes, so a handful of crafted messages leaves webmail, the administration console and the REST API unable to answer anyone. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Requires that the attacker can deliver an email (open/receiving SMTP) to a mailbox on an affected hMailServer (6.2.22-pre1 through 6.3.5) whose message is HTML-only - it must have an HTML part and NO text part - and carries a very large number of HTML character entity references. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | This is a genuine but moderate-priority availability issue, not a confidentiality/integrity threat. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Upgrade to hMailServer 6.3.6, the vendor-released patch that eliminates the quadratic entity-decoding path (release notes: https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6; upstream work item: https://gitlab.com/hmailserver/hmailserver/-/work_items/67). … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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