hMailServer: Allocation of Resources Without Limits or Throttling
CVE-2026-126613 is a high-severity allocation of resources without limits or throttling vulnerability in hMailServer from Progressive Robot, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected hMailServer versions: 6.3.2 through 6.3.7. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.47% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Check which version of hMailServer you run against the affected versions below, and install the vendor's latest release.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Progressive Robot | hMailServer | 6.3.2 through 6.3.7 | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by CNA (gitlab.com).
Official description
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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Allocation of Resources Without Limits or Throttling
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
How developers prevent this class of flaw
- Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
- Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
- For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
More Allocation of Resources Without Limits or Throttling vulnerabilities (CWE-770)
Timeline
- CVE record published.
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Other references
Other vulnerabilities in hMailServer
- 7.3 High severity
- 7.5 High severity
- 7.3 High severity
- 7.5 High severity
- 8.8 High severity
- 7.8 High severity
Recent Allocation of Resources Without Limits or Throttling vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 8.6 High severity
- 7.0 High severity
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