The email reader folds quoted history into <details> summaries via
`_foldSummary()` (static/js/emailLibrary/signatureFold.js), which builds a
sender/date "meta" chip into the summary HTML and assigns it to innerHTML.
The server-side thread parser (`_extract_quote_meta`,
src/email_thread_parser.py) strips tags but then un-escapes HTML entities
and preserves `<...>` patterns, and that raw meta reaches `_foldSummary`
unescaped via `_renderTurnsFromServer` (`t.meta`) — so an inbound email
whose quoted attribution contains `From: <img src=x onerror=...>`
runs script when the victim merely opens the message (stored XSS).
Make `_foldSummary` the single escaping chokepoint: escape `primary` and
`subMeta` with the module's existing `_esc`. The client-side
`_extractQuoteMeta` previously pre-escaped its output, and every consumer
of it routes through `_foldSummary`, so drop that now-redundant escaping to
avoid double-encoding (e.g. "Ben & Jerry" -> "Ben &amp; Jerry").
Verified (jsdom): server-raw and client-extracted malicious metas yield 0
live elements and 0 event-handler attributes; benign "Ben & Jerry" renders
single-escaped.
Co-authored-by: Claude <noreply@anthropic.com>
7a830e504d