---
id: "CVE-2026-126613"
page: "https://infrastack.ai/live/CVE-2026-126613"
kind: "vulnerability"
aliases: ["CVE-2026-126613"]
title: "hMailServer: unbounded JSON parsing lets remote attackers exhaust memory and halt mail"
severity: "high"
cvss: 7.5
epss: null
exploited: false
status: "open"
category: "runtime"
products: [{"name": "hMailServer", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["hmailserver", "mail-server", "runtime", "self-hosted", "webmail"]
not_affected_if: ["no-tls-rpt-mailbox", "rest-api-disabled"]
published_at: "2026-11-11T09:17:13+00:00"
first_seen_at: "2026-11-11T09:26:48+00:00"
updated_at: "2026-11-11T09:28:39+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-126613", "first_seen_at": "2026-11-11T09:26:48+00:00"}]
exploit_sources: []
---

# hMailServer: unbounded JSON parsing lets remote attackers exhaust memory and halt mail

Link for people: https://infrastack.ai/live/CVE-2026-126613 (this .md is for agents)

hMailServer 6.3.2 through 6.3.7 reads JSON into a tree that grows far larger than its input and has no limit on the number of values. A single crafted document, such as a TLS-RPT report mailed to a hosted domain, can exhaust server memory and hold its threads, taking down mail delivery, the REST API and webmail.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:hMailServer affected range not listed fixed no fixed version listed

## How to check

- Check the installed hMailServer version against the affected range `6.3.2` through `6.3.7`.
- Check whether a TLS-RPT report mailbox is set for any hosted domain.
- Check whether the REST API is enabled and whether a public base URL for message links is set.

## Timeline

- 2026-11-11T09:26:48+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-126613

## From the sources

Allocation of resources without limits in the JSON reader of Progressive Robot hMailServer 6.3.2 through 6.3.7 allows a remote attacker to exhaust the server's memory and hold its threads, making mail delivery, the REST API and the webmail unavailable. The reader built a tree of values far larger than the text it read, copying each value into every level above it, and set no bound on the number of values, so a single JSON document of many small values, within the 16 MB the server reads, raised the service's memory by about 2 GB and held one thread for seconds to minutes. From 6.3.4 a remote unauthenticated attacker reaches the reader by mailing such a document as a TLS-RPT report, up to 16 MB once decompressed, to a hosted domain's report mailbox, where an administrator has set one; it is read on a mail delivery thread (ten by default), and those threads deliver all mail, local and outbound. From 6.3.4 it is also reached without an account through the reply to a password-protected message, with the message's link alone, where the REST API is on (it is off by default) and a public base URL for such links is set; and by any account, from 6.3.2 through the webmail's send and draft routes and from 6.3.4 through its other routes granted 16 MB and the send API; the webmail's send, draft and settings routes parsed the body again for each member read. The REST API serves every request on four worker threads, in the process that also serves SMTP, IMAP and POP3, so a few such documents at once make it unavailable and can exhaust the machine's memory and end the whole service. This is a separate defect from CVE-2026-107574, the reader's handling of duplicate member names, fixed in 6.3.6.

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