Progressive Robot hMailServer: Denial of service via unbounded JSON parsing
Summary
hMailServer 6.3.2 through 6.3.7 builds a JSON tree from requests and mailed TLS-RPT reports without limits on the number of values, and copies nested values once per level. A crafted JSON document of about 16 MB can raise memory by roughly 2 GB and block a thread for seconds to minutes. Because the REST API and the mail delivery threads run in the same process as SMTP, IMAP and POP3, this can make mail delivery, the REST API and webmail unavailable. Exploitation needs no account in some configurations, for example a mailed TLS-RPT report to a domain's report mailbox, while other paths need an account or a protected-message link.
Affected
- hMailServer (Progressive Robot fork) 6.3.2 through 6.3.7
- The TLS-RPT mail path applies from 6.3.4; the protected-message reply path applies from 6.3.4 when the REST API and a public base URL are set
- Default configuration is not affected
Fixed in
- Fixed in hMailServer 6.3.8
Workaround
- Until the update is applied on 6.3.4 to 6.3.7, clear each domain's report mailbox or move it off the domains the server hosts, so that no report is read by the server
- Switch OpenPGP in the webmail off (portal_enabled under /api/v1/settings/openpgp), or off for affected domains and accounts
- Keep the REST API off unless it is required
Exploitation
- Remote, no authentication needed for the mailed TLS-RPT and protected-message reply paths in the configurations described above
- Authenticated users can trigger the webmail send and draft routes on 6.3.2 to 6.3.7
- Proof-of-concept measurements by the vendor showed a worker held for seconds to minutes and about 2 GB of extra memory per request
- Unknown whether public exploitation has occurred
Our recommendation
- Upgrade all hMailServer servers on 6.3.2 to 6.3.7 to 6.3.8 (check database TLS settings before upgrading, as described in the 6.3.8 notes)
- If an immediate upgrade is not possible, apply the workarounds above
- Check whether TLS-RPT report mailboxes or the REST API are exposed externally
- Monitor memory use and service availability of the mail server
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- hMailServer 6.3.8 release (GitLab) gitlab.com
- GitLab work item 110: JSON reader gitlab.com
- Progressive Robot hMailServer 6.3.8 downloads www.progressiverobot.com
- CVE record mirror CVE-2026-126613 cve.threatint.com
- VulDB 435713 vuldb.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 11 Nov 2026, 10:26.
Original description (NVD)
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.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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