# CVE-2026-88831: BusyBox httpd IP deny rules CIDR prefix parsing bypass

Source: https://junglewise.ai/threats/cve-2026-88831-busybox-httpd-ip-deny-rules-with-invalid-cidr-prefix-lengths-fail  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-23. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-88831 · Severity: medium · CVSS 5.3 · Published 2026-09-23

BusyBox's embedded web server has a flaw in how it processes IP address filtering rules. When an administrator configures access controls to block certain IP addresses using invalid CIDR notation, the filter fails to work as intended, allowing blocked clients to connect. This could permit unauthorized access to systems that rely on BusyBox httpd for network-based administration or service delivery.

Vendors: [Busybox](https://junglewise.ai/threats/vendors/busybox).

## Quick answers

**Is CVE-2026-88831 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-88831 being exploited in the wild.

**How severe is CVE-2026-88831?** Medium severity, with a CVSS score of 5.3, published 23 September 2026.

**Which products and versions are affected?** Busybox Httpd 1.37.0 and earlier.

**Is there a fix for CVE-2026-88831?** A patch was published on 2026-09-25: Fix included in BusyBox 1.37.0-9.1.hum1 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-88831 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-88831?** Check whether you run Busybox Httpd 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

The vulnerability exists in BusyBox httpd's IP deny rule parsing when invalid CIDR prefix lengths are provided. Invalid prefixes result in a zeroed mask that fails to match any client, causing the deny rule to be ineffective. An attacker on the network can bypass IP-based access controls by sending requests that should have been blocked, provided the rule was configured with an invalid CIDR prefix.

## Affected products

- BusyBox httpd 1.37.0 and earlier

## Timeline

- 2026-09-23: disclosed
- 2026-09-25: advisory
- 2026-09-25: patched: Fix included in BusyBox 1.37.0-9.1.hum1

## References

- [https://catalog.redhat.com/software/containers/](https://catalog.redhat.com/software/containers/)
- [https://access.redhat.com/errata/RHSA-2026:72111](https://access.redhat.com/errata/RHSA-2026:72111)

## Related threats

- [BusyBox httpd authentication bypass with yescrypt hashes](https://junglewise.ai/threats/cve-2026-88837-busybox-httpd-treats-yescrypt-y-password-hashes-as-plaintext)