fix: correct inverted expiry check in mint quote validation

[zk-bits]

Summary

The expiry check in mint() was using > instead of <, causing the mint to:

• Reject valid quotes (where expiry is in the future)
• Accept expired quotes (where expiry is in the past)

The Bug

quote.expiry is a Unix timestamp representing when the quote expires (set as invoice.date + invoice.expiry).

Previous (buggy) code:

if quote.expiry and quote.expiry > int(time.time()):
    raise TransactionError("quote expired")

This raises "quote expired" when quote.expiry > now, meaning when the expiry is in the future (quote is still valid).

Fixed code:

if quote.expiry and quote.expiry < int(time.time()):
    raise TransactionError("quote expired")

This correctly raises "quote expired" when quote.expiry < now, meaning when the expiry is in the past (quote has actually expired).

Impact

• Severity: Denial of Service for mint operations
• Not exploitable for fund theft because the quote.paid check at line 497 happens first, ensuring the Lightning invoice was actually paid before reaching the expiry check
• Any mint with expiry set on quotes would reject all valid mint attempts

Test

Simple proof that the logic was inverted:

import time

# Quote created now with 1 hour expiry
quote_expiry = int(time.time()) + 3600
now = int(time.time())

# Buggy: quote_expiry > now = True → raises "quote expired" (WRONG - quote is valid!)
# Fixed: quote_expiry < now = False → quote is valid (CORRECT)

[TheRealCheebs]

Good find! I would include now as also an expired time, so update < to <= .