GatewayRunner._prepare_inbound_message_text() interpolated
source.user_name — the platform-supplied, user-settable display name —
directly into the message text of every turn in a shared multi-user
session: f"[{source.user_name}] {message_text}". Shared sessions are the
default for any threaded conversation (thread_sessions_per_user defaults
to False) and apply to any group with group_sessions_per_user=False, so
no special configuration is needed to reach this path.
An unescaped display name containing embedded newlines could therefore
masquerade as a new markdown section (a fake "## Override" heading)
inside the live conversation the model reads on every turn — the same
indirect-prompt-injection vector gateway/session.py's
build_session_context_prompt() already guards against for the identical
user_name field via _format_untrusted_prompt_value(), which was never
applied to this sibling call site.
Add neutralize_untrusted_inline_text() alongside the existing helper in
gateway/session.py: it collapses embedded newlines/control characters to
a single inert line without JSON-quoting, so inline "[Name] message"
formatting is preserved byte-for-byte for the common case (unlike
reusing _format_untrusted_prompt_value directly, which would add visible
quote marks to every sender prefix). Wire it into the sender-prefix
construction in gateway/run.py.
170959d80a