misp-stix EUVD-2026-63883

Exploitation requires two specific, non-default conditions to hold simultaneously: first, the consuming application must reuse the same misp-stix parser instance across two or more independent STIX document conversions - standard conversion entry points that instantiate a new parser per file are explicitly not affected per the CVE description; second, the attacker must be able to influence the content of at least one STIX document processed by that long-lived parser instance before the target document is processed, and must be able to control or predict document processing order. The AT:P (attack precondition present) metric in the CVSS 4.0 vector formally encodes the parser-reuse requirement. Exploitation is strictly bounded to partial integrity contamination and limited disclosure of prior-document metadata; no code execution, privilege escalation, authentication bypass, or availability impact is possible per the description and confirmed by the patch diffs.

The CVSS 4.0 vector (AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N) scores 6.3, reflecting a network-reachable attack surface bounded by a required attack precondition (AT:P) - specifically, the consuming application must reuse parser instances across unrelated documents, which is explicitly a non-default usage pattern. Impacts are limited to partial confidentiality and integrity of the vulnerable system with no availability or scope change. No EPSS data was provided in the input, and the vulnerability is not listed in CISA KEV. No public proof-of-concept exploit code has been identified. The 'RCE' tag present in the input data is inconsistent with both the CVE description and the patch diffs, which explicitly state 'No direct availability impact or code execution is involved' - this tag should be disregarded as likely mislabeled metadata and does not alter the risk calculus. Real-world risk is low-to-moderate for most environments but operationally significant in threat-intelligence pipelines that process documents from mixed-trust sources with a shared parser instance, where false associations or metadata leakage between events could mislead analysts or violate distribution controls.

An attacker with the ability to inject a crafted STIX document into a threat-intelligence ingestion pipeline that reuses a misp-stix parser instance submits a bundle containing fabricated galaxy clusters, passive DNS records, or false timestamps targeting a known processing queue. When a legitimate high-priority STIX document is subsequently processed by the same instance, the resulting MISP event inherits the attacker's injected artifacts, causing false threat-actor associations or incorrect indicator attribution to propagate downstream to analysts and automated response systems. No public proof-of-concept exploit code has been identified at time of analysis.