MISP Guard Incomplete Tag Validation Allows Non-Allowed Taxonomy Tags to Bypass Feed Policy Enforcement

Overview

MISP Guard, a proxy that enforces security policies on MISP feed data, contained an incomplete validation check for required taxonomy tags. When a feed event or its attributes carried tags belonging to a required taxonomy (for example, TLP), the guard only verified that at least one tag of that taxonomy matched the configured allowed list. It did not verify that every tag of the required taxonomy present on the event or attribute was in the allowed list. As a result, an event or attribute could simultaneously carry an allowed tag (e.g., tlp:clear) and a non-allowed tag (e.g., tlp:red or tlp:amber) and still pass the guard's policy check. This allowed data marked with a higher classification or sensitivity level to be distributed through feeds configured to only distribute lower-classification data, effectively bypassing the intended data-handling restrictions. The vulnerability affected both event-level and attribute-level tag validation, including attributes nested within objects and shadow attributes. An attacker with the ability to author or modify MISP events in a feed source could include non-allowed taxonomy tags alongside allowed ones to exfiltrate more sensitive data to feed consumers than the guard policy intended to permit.

Affected

• misp-guard < 1.4

Remediation

The fix introduces a new validation function, check_required_taxonomy_tags_allowed, that iterates over every tag belonging to a required taxonomy and verifies each one is present in the configured allowed-tags list, raising a ForbiddenException if any tag is not allowed. This check is applied at both the event level and the attribute level. Additionally, an inherit_event_taxonomies flag is introduced so that attributes without their own required-taxonomy tags inherit the event's already-validated taxonomy, while attributes that do carry tags of the required taxonomy are still fully validated against the allowed list. This ensures that no non-allowed tag of a required taxonomy can pass the guard regardless of whether an allowed tag is also present.