Restructure threat-model sources by controller

[ppkarwasz]

Restructures the "Data sources" section of the common threat model into a Sources section organised by who controls each input, rather than a flat trusted/untrusted split.

The three categories are defined by their controller:

• Configuration (operator-controlled, trusted): environment variables, configuration properties, and configuration files, with the existing deployer responsibilities.
• Structural identifiers and control (developer-controlled, trusted): logger names, levels, markers, the identifiers and field names of a structured log message (for example the RFC 5424 MSGID and SD-ID fields), and the format string of a parameterized log statement.
• Content (user-controlled, untrusted): log messages, the string representation of log parameters, and thread context values.

Why

Triaging recent reports showed that reporters have a difficulty into classifying sources correctly, such as the structural identifiers of a structured message. Organising sources by controller makes the trust classification explicit and gives a clean basis for deciding which reports are in scope.

It also lets the model state the remediation rule that follows from the classification: trusted inputs may be rejected (fail-fast, for example by throwing), while user-controlled content must be accepted and never rejected, since rejecting it would turn a malicious value into a denial of service.

Thread context keys

Thread context values are classified as untrusted content. The trust level of thread context keys is deferred to discussion apache/logging-log4j2#4132 and marked in the document as a known open gap, rather than silently choosing a side.

Dependent changes

• The adversary-capabilities section now references the new content subsection, drops thread context keys from the in-scope channels pending the discussion above, and marks an adversary who controls developer-controlled structural or control inputs as out of scope.
• Internal cross-references updated to the new subsection anchors; the parent #threat-common-sources anchor is retained.

Out of scope for this PR

Sink-side concepts (structured versus unstructured layouts, active versus passive sinks, and downstream-destination trust) are intentionally left for a follow-up PR.

[Copilot]

Pull request overview

This PR restructures the "Data sources" section of the common threat model document into a "Sources" section organized by who controls each input (operator/developer/user), making the trust classification explicit. It also aligns the adversary-capabilities section with the new categories and defers the classification of thread context keys to an external discussion, marking it as a known open gap.

Changes:

• Replace the flat trusted/untrusted split with three controller-based subsections: Configuration (operator), Structural identifiers and control (developer), and Content (user).
• Add an explicit remediation rule (trusted inputs may be rejected; user-controlled content must not be rejected) and move the deserialization/object-stringification guidance under Content.
• Update adversary-capability cross-references to the new anchors, drop thread context keys from in-scope channels, and mark developer-controlled structural/control inputs as out of scope.

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[FreeAndNil]

Maybe we should put the string conversion topic in a separate note to move it a little bit away from the bullet points before about what we don't trust?

[FreeAndNil]

LGTM