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fix: parse MXP the way the spec defines it - #40
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Six tasks against the MXP specification: share SGR decoding, decode ANSI inside MXP, implement the line-security model, let the negotiated option choose the parser, answer VERSION/SUPPORT, and document the result. The security task is the third. Findings and evidence are in the plan header.
Review found ProcessEscape only special-cased CSI (ESC [); OSC, DCS/SOS/PM/APC, and two-byte-intermediate escapes all fell into the generic two-byte-escape branch and leaked their payloads into the line as literal text (e.g. an OSC title-set). Mirrors AnsiParser's Escape/EscapeIntermediate/Osc/OscEscape state machine and terminator rules exactly, reusing the existing bounded _seq buffer. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
A <SEND> typed by one player into a public channel became a clickable command in every other player's client — the exploit the spec's own rationale names. MxpParser now implements the ESC[#z line tags, the open/secure/locked modes, RESET, TEMP SECURE and the three LOCK modes, and gates every tag dispatch on the mode. A tag the mode refuses is echoed as the literal text it arrived as, byte for byte from the unparsed tag body, so an injection attempt is visible to the player rather than silently swallowed — and so nothing a round trip would normalise (case, internal whitespace, quoting) can be smuggled through it. An unknown or unparseable ESC[#z number is ignored rather than guessed at. Pre-existing tests that fed a secure element (SEND, A, BR, and the unsupported tags that are consumed rather than rendered) with no mode tag now say so. None asserted that an unsecured SEND was honoured, so none was inverted. Two ParserBoundaryTests still passed but had gone vacuous — a refused SEND opens no interaction, so "nothing leaked to the next line" was true of nothing — and are secured to test leakage again. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
… close Two exploitable gaps found in review, and a direct test for the table the whole boundary rests on. TEMP SECURE was spent by a tag but not by anything in between, and the spec sentence the plan abbreviated away is "must be immediately followed by a '<' character to start a tag". So "\x1b[4zRivane says, '<SEND HREF=..." armed the line and the *player's* SEND spent the arming. Any character other than '<' now disarms, ESC excepted while it may still be resolving a line tag; every escape that turns out to be something else disarms for itself in the new EndSequence(). On a locked line no character starts a tag, not even '<', which closes the ESC[7z ... ESC[4z ... ESC[0z route through the unlock. A refused closing tag left its frame open, and a deferred <send> — one with no HREF, whose command is its enclosed text — then absorbed every span to the end of the line, so "\x1b[4z<send>Y</send> Rivane says, 'hi'" finalised as the command "Y</send> Rivane says, 'hi'". A close matching an open frame is now honoured whatever the mode: closing can only reduce privilege, and the worst a player achieves is truncating a clickable region the server drew. The mode gate still runs first and unconditionally, because a close is "the next tag" and must spend a pending TEMP SECURE either way. MxpTagCategory gains a direct test. The parser drives only eight of its fifteen entries — Canonical folds the alternative spellings away before the gate sees them — so STRONG, EM, STRIKEOUT, HIGH, FONT and every mixed-case spelling reached it from nowhere at all. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Three items from re-review. ProcessEscape's two-byte default arm was the sixth exit from the escape state machine and the only one still returning to text with a bare _mode = Mode.Text. ESC c is not resolving a line tag, so "\x1b[4z\x1bc" followed by a player's <SEND> re-enabled the exploit the previous commit closed, with one extra escape in front of it. There are six EndSequence() call sites now, and the parametrised test covers all six rather than the one arm that happened to be reported. Every line tag but ESC[4z now disarms too, including one this client cannot parse. Not player-reachable — anyone who can emit ESC[0z can emit ESC[1z and skip the gate — but it makes the model uniform: a sequence disarms unless it is ESC[4z, wherever it ends. Reset_ClearsEveryModeField pinned two of the three fields. _lineMode is re-read from _defaultMode only in CompleteLine, which does not run before the first line's SEND is gated, so a Reset that dropped the _defaultMode clear still passed. It feeds two lines now, and the mutant fails at the second. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
TelnetSession used to answer IAC WILL MXP with DO and then keep parsing the stream with whatever WorldDefinition.ContentFormat said (Ansi by default, since nobody sets it) — a live capture against tdome.nukefire.org:4000 showed the server's <send> tags rendering as literal text because of this. TelnetSession now raises MxpEnabled once MXP negotiates, and WorldSession swaps in MxpParser on it, but only when starting from ContentFormat.Ansi: an explicit Pueblo or Mxp choice is a user decision a stray negotiation must not overrule. The old parser is flushed first so a buffered partial line is delivered under the rules it arrived under, and style does not carry across the swap — MXP's own RESET re-establishes it. Also updates RecordingTelnetSession (Tui.Tests), the other ITelnetSession implementation, to satisfy the new interface member.
Review caught that the comment claimed the flush protects a line straddling the negotiation moment. It doesn't: WorldSession.OnOutputReceived already Feed()s and Flush()es the parser on every call, so nothing genuinely undelivered survives between calls, and MxpEnabled only ever fires between them. The real hazard is TelnetSession._pending spanning the negotiation and being submitted as one parsed-wholesale chunk — out of this task's scope, recorded as a deferred item, not fixed here. No behaviour change.
VERSION and SUPPORT replies must be sent as SECURE-tagged lines (ESC[1z prefix) per spec, or a server enforcing MXP line security refuses them as unsecured OPEN-line input. VERSION's attribute order was also wrong and STYLE was missing; both are now MXP=1.0 STYLE=0 CLIENT=... VERSION=... in the spec's exact order, with REGISTERED omitted as it is optional.
Document the allow-list line-security model (MxpLineMode/MxpTagCategory and the ESC[#z tags), why ANSI is decoded inside MxpParser instead of being chained with AnsiParser, why negotiation rather than ContentFormat picks the parser, and why the SUPPORTS list is held to the MTTS bit-vector honesty rule. Placed beside the GA/EOR prompt-marker entry in Other dependency notes. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
The telnet layer strips the line terminator, so MxpParser never saw a '\n' on a real connection and the line-security revert — which lived in CompleteLine() — never ran. One ESC[1z from a server left the session SECURE for the rest of the connection: a <SEND HREF="@shutdown"> another player typed into a public channel became a clickable command, and a <VERSION> they typed put a reply on the wire. Flush() was already the boundary for the tag stack. The line-end work is now a private EndLine() that both CompleteLine() and Flush() call, so one call is the boundary for everything. Deliberately not an EndLine() on ILineParser: a consumer that forgets to call one reproduces exactly this bug in silence, where one that does not flush gets no output at all. Also implements the spec's auto-close of unclosed OPEN tags — at a newline in OPEN mode, and on any mode change out of OPEN — which is the spec's own bound on how far player-authored markup reaches, and without which a <COLOR FORE=black BACK=black> typed into chat paints the rest of the session black on black and the tag stack grows without bound. Secure tags are never auto-closed, so it is a flag on the frame rather than a property of the tag name. Parser state no longer survives a reconnect: ConnectAsync rebuilds the parser from ContentFormat, which clears the modes *and* the negotiated MXP upgrade that Reset() could not. An unterminated escape string no longer swallows output across lines, in both AnsiParser and MxpParser — the two escape machines are near duplicates and are deliberately not consolidated, so both get the fix and both get a test. Plus: LockOpen_MakesOpenTheDefaultAgain now asserts on a further line, so it can fail; PuebloParser's header no longer claims ANSI is "handled upstream" when there is no upstream; the MXP protocol reply is no longer an unobserved fire-and-forget task; CLAUDE.md records the Flush-is-the- boundary fact, the deliberate escape-machine duplication, and the threat model (a player needs the server to have left a secure line standing, not to emit ESC[1z themselves).
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WalkthroughMXP parsing now supports line-security modes, ANSI escape handling, tag authorization, secure protocol replies, and parser-state isolation. ANSI SGR decoding is shared. MXP negotiation switches eligible sessions to a fresh ChangesMXP conformance
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk:⚪ Minimal · up to The PR changes MXP parsing and session integration, while the remaining follow-ups are limited to documentation, an additional integration assertion, and clearer naming. No actionable merge-blocking risk remains after normal checks and review. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1📝 Generate docstrings 💡
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HarryCordewener
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Aug 20, 2026
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Actionable comments posted: 3
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Inline comments:
In `@docs/superpowers/plans/2026-08-20-mxp-conformance.md`:
- Around line 1087-1093: Update the documented snapshot-inspection workflow
around ansi_frame_to_image.py to include generating the required SVG from the
inspected HTML and explicitly providing the resulting .svg to the user, while
retaining HTML rendering for local inspection.
In `@src/SharpMUTerm.Core/Protocols/MxpParser.cs`:
- Around line 534-544: Rename TagIsAllowed to a name that explicitly conveys
consuming TEMP SECURE state, and update both corresponding call sites in
ProcessTag while preserving the existing one-call-per-tag behavior and logic.
In `@tests/SharpMUTerm.Core.Tests/Session/WorldSessionTests.cs`:
- Around line 345-378: Add a WorldSession integration test alongside
Mxp_ASecureLineFromTheServerDoesNotSecureTheLinesAfterIt that enables MXP, emits
a server-secured VERSION request, and asserts telnet.SentLines contains a
VERSION MXP response, covering the NewParser ClientReply subscription and
SendProtocolReply path.
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📒 Files selected for processing (22)
CLAUDE.mddocs/superpowers/plans/2026-08-20-mxp-conformance.mdsrc/SharpMUTerm.Core/Protocols/MxpLineMode.cssrc/SharpMUTerm.Core/Protocols/MxpParser.cssrc/SharpMUTerm.Core/Protocols/MxpTagCategory.cssrc/SharpMUTerm.Core/Protocols/PuebloParser.cssrc/SharpMUTerm.Core/Session/WorldSession.cssrc/SharpMUTerm.Core/Telnet/ITelnetSession.cssrc/SharpMUTerm.Core/Telnet/TelnetSession.cssrc/SharpMUTerm.Core/Text/AnsiParser.cssrc/SharpMUTerm.Core/Text/SgrCodes.cstests/SharpMUTerm.Core.Tests/Protocols/MxpLineModeTests.cstests/SharpMUTerm.Core.Tests/Protocols/MxpParserTests.cstests/SharpMUTerm.Core.Tests/Protocols/MxpTagCategoryTests.cstests/SharpMUTerm.Core.Tests/Protocols/ParserBoundaryTests.cstests/SharpMUTerm.Core.Tests/Session/FakeTelnetSession.cstests/SharpMUTerm.Core.Tests/Session/WorldSessionContentTests.cstests/SharpMUTerm.Core.Tests/Session/WorldSessionTests.cstests/SharpMUTerm.Core.Tests/Text/AnsiParserTests.cstests/SharpMUTerm.Core.Tests/Text/SgrCodesTests.cstests/SharpMUTerm.Tui.Tests/LinkSchemeSecurityTests.cstests/SharpMUTerm.Tui.Tests/RecordingTelnetSession.cs
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CodeRabbit findings on PR #40. TagIsAllowed read as a pure predicate and cleared _tempSecure. The whole TEMP SECURE rule is "the next tag, and only the next tag", so its correctness is exactly the property that it runs once per tag — and a later pre-check, log line or assertion would spend the arming silently, with no test able to name the mistake because the tag just renders as text. Renamed to ConsumeAuthorizationFor and the remark now says what it costs to call twice. The WorldSession MXP cohort had only negative assertions — that a reply does not go out — which a dropped ClientReply subscription satisfies perfectly. The upgrade path is the one every negotiating session takes, so it now has a test that fails when it breaks: verified by dropping the subscription, which fails the new test and nothing else. Plan's snapshot step rendered only the HTML; the repo's rule is that the SVG is what gets handed over and the HTML is what you inspect.
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What
SharpMUTerm's MXP parser honoured every markup tag on every line, decoded no ANSI, and — since
WorldSessionchose its parser from a static config field nobody sets whileTelnetSessionswallowed the negotiation with a no-op — never ran at all. That last part is why NukeFire's<send>Y</send>rendered as literal text after #39: the server negotiated MXP, we answeredDO, and then parsed the stream withAnsiParser.Implements https://www.zuggsoft.com/zmud/mxp.htm, in six tasks. Plan is in the branch at
docs/superpowers/plans/2026-08-20-mxp-conformance.md.The security boundary
MXP marks each line OPEN / SECURE / LOCKED with an
ESC[#ztag. Only a small allow-list of formatting tags may act on an OPEN line;<SEND>,<A>and the definition tags are SECURE. The spec's rationale is the point of the whole mechanism:Before this branch, a player typing
<SEND href="@shutdown">click</SEND>into a public channel became a clickable command in every other player's client.MxpTagCategoryis an allow-list, taken from the spec's own sentence, so a tag the spec gains later is secure by omission — in the safe direction.COLORan open tag.Three exploits were found and fixed during review, each with a reproduction:
ESC[4z(TEMP SECURE) was spent only by a tag, never by intervening text — soESC[4zRivane says, '<SEND HREF="@shutdown">…'let a player's SEND consume the arming.<send>builds its command from every span to the line end — so player text after the refused closer joined the command.CompleteLine(), reached only from a literal'\n'— but the telnet layer strips the terminator, so the parser never sees one. After any server line carryingESC[1z, the session stayed SECURE for the rest of the connection.Flush()is the real line boundary and now does the revert.That third one is the one worth knowing about: every cross-line test put
"\n"inside a singleFeedcall, an input shape the product never produces. The tests all passed and none of them touched the boundary the product has.Also
AnsiParser). It previously appended ESC bytes as literal text and lost the colour — the class doc claimed ANSI was "handled upstream" whenCreateParserreturns one parser or the other, never chained.SgrCodesis extracted so both share one SGR implementation.ContentFormat.Ansi— the default, meaning "nobody chose" — so an explicitPueblois never overruled.<VERSION>/<SUPPORT>are answered, prefixedESC[1zbecause the spec requires a secure-tagged reply; an unsecured one is refused by any server running the same model. TheSUPPORTSlist is honest:+highis omitted becauseH/HIGHis allow-listed but reaches no behaviour, held to the same rule as the MTTS bit vector.ESC ]from player chat used to swallow output across line boundaries until a BEL — fixed in both parsers.Verification
Core 1070 (was 950) · Tui 1828 · Graphics 83 · Scripting 42 · Web 37 — all green, build warning-free.
Every security test was mutation-checked: reverting the fix makes it fail. The
NukeFirePrompt_ParsesIntoTwoClickableAnswerscase replays the real capture fromtdome.nukefire.org:4000byte for byte.Known gaps, recorded rather than hidden
CLAUDE.mdcarries these. None is a security hole; all are fail-safe in direction.TelnetSession._pendingcan batch pre- and post-negotiation bytes into one submit, so text sent before MXP was advertised can be re-read as MXP. Lands on an OPEN line, so the worst case is a stripped formatting tag.<SUPPORT>accepts an argument list with per-tag+/-echoes; we always return the static list.<BR>reverts the line mode; the spec keys that on a newline.AnsiParserandMxpParserare near-duplicates, deliberately left unconsolidated — a fix to one needs applying to the other.&#nnn;, "values less than 32 are ignored") was never audited.IAC SB MXPintoBadSubNegotiationand has no client-side handler forIAC DO MXP.🤖 Generated with Claude Code
Summary by CodeRabbit
New Features
VERSIONandSUPPORTresponses.Bug Fixes