A header only had to meet the target it declared for itself, so a fabricated one with
`bits = 0x207fffff` passed half the time, a jump of several blocks was accepted without
being linked to the tip (a fake header at a huge height locked `tip_height` and froze every
draw), and an ordinary one-block reorganisation closed the session on every server.
- draw.py: MAINNET_POW_LIMIT (00000000ffff...ffff); header_meets_its_own_target also
refuses a claimed target easier than POW_LIMIT.
- ElectrumListener._apply_header is async and takes the active client. A single block must
chain by prev_hash; a jump downloads the headers in between (blockchain.block.headers,
at most 2016 per request, ElectrumClient.get_block_headers) and each must carry valid
work and chain, otherwise HeaderValidationError ends the session.
- The listener keeps the last 10 headers (`_window`), built from the process's first header
and its 9 predecessors (which must chain into it). A header that does not join the tip is
walked back to its fork point inside the window and the server's branch is followed
instead of ending the session; a fork older than the window, or a branch that does not
end at the announced header, still ends it. The audit row and the pause for deep reorgs
are B-85; a competing header at the tip's own height is still ignored until then.
- _header_at also serves older heights from the window.
Tests changed because the spec changed: the mainnet powLimit cannot be mined in a test, so
tests/unit/conftest.py swaps in a regtest limit at import time (the new tests put the real
one back); the _apply_header tests are async; the two _run_once tests serve the first
header's 9 predecessors; test_apply_header_skips_linkage_check_across_a_height_gap is
replaced by the jump tests, since that behaviour is exactly what the finding removed.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
A single hostile Electrum server, or a MITM on the one active
connection, could fabricate the block header the draw's entropy comes
from and so pick the winner of every round: headers were accepted with
no proof-of-work check and no link to the previous tip.
app/rounds/draw.py adds header_meets_its_own_target (rejects a header
whose hash doesn't satisfy the difficulty target it claims) and
header_prev_hash. electrum/listener.py's _apply_header now rejects a
header failing either check by raising HeaderValidationError, which
ends the session the same way a dropped connection would so the
listener rotates to the next configured server.
ElectrumListener gains corroborate_header: before the draw uses a
block, it's independently checked against the other configured servers
and needs a majority to agree. rounds/scheduler.py's
_wait_for_next_block now calls this and, on failure, logs why and
waits for a further block instead of ever using an uncorroborated
header.
Certificate/hostname verification stays disabled, so this doesn't
cover an attacker able to MITM every configured server at once -
BUGS.md notes that as not covered.
Suite grows from 151 to 165 tests. BUGS.md moves B-28 to Previously
fixed.
Periodic scheduler (configurable round duration) that closes a round
only once all broadcast bets confirm, waits for the next block after
closing, draws a winner via block-hash-seeded modulo over participants
ordered by broadcast time, triggers the 70/30 payout, and only opens the
next round once that payout confirms. Draw logic is isolated in
draw.py as a deliberately simple, replaceable component.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
davideandClaude Sonnet 5.5