# CVE-2026-38821: openNDS heap-based buffer overflow in http_microhttpd

Source: https://junglewise.ai/threats/cve-2026-38821-opennds-heap-based-buffer-overflow-in-http-microhttpd  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-28. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-38821 · Severity: high · CVSS 7.1 · Published 2026-08-28

openNDS is an open-source captive portal solution used to manage network access and user authentication on WiFi networks. A heap buffer overflow in the query string parser allows an unauthenticated attacker on the network to crash the service or potentially execute arbitrary code, disrupting internet access for all connected users.

Technologies: [openNDS](https://junglewise.ai/threats/technologies/opennds). Vendors: [openNDS](https://junglewise.ai/threats/vendors/opennds).

## Quick answers

**Is CVE-2026-38821 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-38821 being exploited in the wild.

**How severe is CVE-2026-38821?** High severity, with a CVSS score of 7.1, published 28 August 2026.

**Which products and versions are affected?** openNDS before 11.0.0.

**Is there a fix for CVE-2026-38821?** A patch was published on 2026: Fix available in version 11.0.0 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-38821 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-38821?** Check whether you run openNDS at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A heap-based buffer overflow vulnerability (CWE-122) exists in the query string handling code within http_microhttpd.c in openNDS versions before 11.0.0. The vulnerable component fails to properly validate and bound query string lengths during URL encoding operations, allowing an attacker to write past buffer boundaries. An unauthenticated attacker positioned on the captive portal network can craft a malicious HTTP request with an oversized query string to trigger the overflow. This can result in denial of service through daemon crash or potentially remote code execution. The fix increases the encoded query string buffer size from 1.333× to 1.5× the maximum query length (8192 bytes).

## Affected products

- openNDS openNDS before 11.0.0

## Timeline

- 2026-08-28: disclosed
- 2026: patched: Fix available in version 11.0.0

## References

- [https://github.com/openNDS/openNDS/commit/3b5f7ef40cd048826d3c4a16f61a73a1768fd5a9](https://github.com/openNDS/openNDS/commit/3b5f7ef40cd048826d3c4a16f61a73a1768fd5a9)

## Related threats

- [openNDS command injection in client_params.sh](https://junglewise.ai/threats/cve-2026-38822-opennds-command-injection-in-client-params-sh)
- [openNDS unauthenticated OS command injection in fas parameter](https://junglewise.ai/threats/cve-2026-38820-opennds-unauthenticated-os-command-injection-in-fas-parameter)
- [openNDS memory leak denial of service](https://junglewise.ai/threats/cve-2026-38819-opennds-memory-leak-denial-of-service)