# CVE-2026-127122: Out-of-bounds write with too many MCR blocks in a Connect request

Status: published. Published: 2026-11-13. Updated: 2026-11-13.

P-Net before 1.0.4 does not check the number of MCR request blocks in a Connect request against the number of multicast communication relations that an Application Relationship has room for (PNET_MAX_MC_CR, 1 by default). Each further block is written past the end of the array, into the rest of the Application Relationship and, with more blocks, into the following one. The written data comes from the request. No established Application Relationship is needed, and the attacker requires ADJACENT network access. Builds with PNET_OPTION_MC_CR disabled are not affected.

## Remediation

- fix: Upgrade to P-Net 1.0.4 or later. (source: CNA: rt-labs)
- workaround: Build P-Net with PNET_OPTION_MC_CR disabled. Multicast communication relations are not supported by P-Net, and a Connect request with MCR blocks is then rejected. (source: CNA: rt-labs)

## Affected products and versions

- RT-Labs / P-Net: ≥ 0; < 1.0.4 (affected, semver). Source: CNA: rt-labs

## Severity and prioritization

- CVSS 4.0: 7.2 (high); CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:H/SC:N/SI:N/SA:N; source: CNA: rt-labs

- CVSS 4.0: 7.2 (high); CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: 4b71179a-3b92-4a9d-b9fc-af8b8ed579ee

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-787

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://rt-labs.com/wp-content/uploads/2026/11/RRTL-261109-01.pdf (CNA: rt-labs)

- https://rt-labs.com/wp-content/uploads/2026/11/RRTL-261109-01.pdf (4b71179a-3b92-4a9d-b9fc-af8b8ed579ee)

## Provenance

- CNA: rt-labs: https://www.cve.org/CVERecord?id=CVE-2026-127122; retrieved 2026-11-13; source updated 2026-11-13

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-127122; retrieved 2026-11-13; source updated 2026-11-13

Revision: 927a39b22b4d52e0aab7af8f16c263ae621d35b7f911c0036b6e4538479cc070

Canonical: https://y2.dev/cve/CVE-2026-127122

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