# CVE-2026-18020: IBM i off-by-one error in bounds checking denial of service

Source: https://junglewise.ai/threats/cve-2026-18020-ibm-i-off-by-one-error-in-bounds-checking-denial-of-service  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-13. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-18020 · Severity: medium · CVSS 5.3 · Published 2026-08-13

IBM i is an enterprise operating system used to run business-critical applications and data management systems. A remote attacker can trigger a denial of service by exploiting an off-by-one error in the NetServer bounds checking logic, disrupting access to services and potentially impacting business operations.

Technologies: [IBM I 7.3](https://junglewise.ai/threats/technologies/i-7-3), [IBM I 7.4](https://junglewise.ai/threats/technologies/i-7-4), [IBM I 7.5](https://junglewise.ai/threats/technologies/i-7-5), [IBM I 7.6](https://junglewise.ai/threats/technologies/i-7-6). Vendors: [IBM](https://junglewise.ai/threats/vendors/ibm).

## Quick answers

**Is CVE-2026-18020 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-18020 being exploited in the wild.

**How severe is CVE-2026-18020?** Medium severity, with a CVSS score of 5.3, published 13 August 2026.

**Which products and versions are affected?** IBM I 7.3 7.3; IBM I 7.4 7.4; IBM I 7.5 7.5; IBM I 7.6 7.6.

**Is there a fix for CVE-2026-18020?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-18020 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-18020?** Check whether you run IBM I 7.3, IBM I 7.4, IBM I 7.5 or one of the others listed 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 is an off-by-one error in bounds checking within IBM i NetServer (CWE-125: Out-of-bounds Read). A remote, unauthenticated attacker can send a crafted network request to trigger an out-of-bounds read condition, causing the NetServer service to crash or become unresponsive. The attack requires network connectivity to the affected system and no special privileges or user interaction. The impact is limited to availability; no confidentiality or integrity compromise is possible. IBM has released PTF MJ10939 for IBM i 7.6 and corresponding patches for earlier versions.

## Affected products

- IBM i 7.6 7.6
- IBM i 7.5 7.5
- IBM i 7.4 7.4
- IBM i 7.3 7.3

## Timeline

- 2026-08-13: disclosed

## References

- [https://www.ibm.com/support/pages/node/7283573](https://www.ibm.com/support/pages/node/7283573)

## Related threats

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- [IBM i session IP binding bypass in Navigator for i](https://junglewise.ai/threats/cve-2026-18065-ibm-i-session-ip-binding-bypass-in-navigator-for-i)
- [IBM i integer underflow in memory handling](https://junglewise.ai/threats/cve-2026-18341-ibm-i-integer-underflow-in-memory-handling)
- [IBM i memory leak in Debug Server denial of service](https://junglewise.ai/threats/cve-2026-18076-ibm-i-memory-leak-in-debug-server-denial-of-service)
- [IBM i buffer overflow in NFS](https://junglewise.ai/threats/cve-2026-17207-ibm-i-buffer-overflow-in-nfs)