CVE-2026-77761: Cross-Document Parser State Contamination in misp-stix

A parser state isolation vulnerability in misp-stix could cause data from a previously processed STIX document to be retained and incorporated into the MISP event generated from a subsequent document when the same parser instance is reused.

Several STIX 1 and STIX 2 parser components maintained per-document state that was not completely cleared between conversions. In the STIX 2 parser, galaxy and galaxy-cluster information, including custom galaxy clusters, could survive a parser reset and subsequently be associated with objects from another bundle.

The STIX 1 parsers were affected by the same underlying state-management issue. Depending on the parser type, retained information could include galaxies, references, passive DNS bookkeeping, package titles, dates, and timestamps. As a result, parsing a second STIX package with an already-used parser could produce a MISP event containing information that was present only in the previously processed package. For example, a generated event could inherit passive DNS records from an earlier document, reference unrelated galaxy information, combine titles from different packages, or use timestamps originating from another conversion.

The issue primarily affects applications using the misp-stix API directly and reusing parser instances across independent STIX documents. Normal conversion entry points that instantiate a new parser for each file are not affected by this particular reuse scenario.

An attacker able to influence documents processed by such a long-lived parser could potentially cause information from one conversion to contaminate a subsequent MISP event. This can affect the integrity of generated threat intelligence, resulting in incorrect associations, misleading contextual information, or unrelated indicators being attributed to an event. In environments where consecutive documents have different access controls or distribution scopes, the retained state could additionally result in limited disclosure of information from a previously processed document.

Successful exploitation depends on the consuming application reusing the same parser instance and on the ordering of processed documents, which increases attack complexity. No direct availability impact or code execution is involved.