fix(pdu): retain unknown GCC flag bits instead of failing the settings exchange - #1844
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Marc-André Moreau (mamoreau-devolutions) merged 1 commit intoAug 30, 2026
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…s exchange Three GCC settings-block decoders rejected the whole Basic Settings Exchange when a flags field carried a bit outside the defined list: RedirectionFlags in Client Cluster Data, the per-monitor flags in Client Monitor Data, and transportFlags in the Multitransport Channel Data. [MS-RDPBCGR] 3.3.5.3.3's processing rules ask the server to validate the Client Network Data bounds among the settings blocks and nothing else, so the strictness was this library's own, and it failed the connection at GCC, before any channel was joined. The exposure is real in each case. The transportFlags list has grown before (TRANSPORTTYPE_UDP_PREFERRED and SOFTSYNC_TCP_TO_UDP are later additions), so an older IronRDP would have refused every client that advertised them. The monitor flags list defines only TS_MONITOR_PRIMARY, so any vendor or future bit refused a multimon client outright. The redirection flags sit next to a version mask that already anticipates growth. Switch all three sites to from_bits_retain, the crate-wide policy since Devolutions#1144: unknown bits are kept rather than dropped, so re-encoding a decoded block reproduces the wire value. The cluster version mask handling is untouched, and the separately parsed RedirectionVersion stays strict since it is a discriminant, not a bit set. This continues the interop line of Devolutions#1489 (unknown GCC user-data blocks), Devolutions#1458 (unknown security header flags), Devolutions#1837 (undefined channel option bits) and Devolutions#1843 (unknown ClientInfoFlags bits).
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…elds (#1846) The two GCC core-data fields that echo the X.224 negotiation values are decoded strictly: serverSelectedProtocol in the client core data and clientRequestedProtocols in the server core data both reject the whole settings exchange when the value carries a bit outside the five defined PROTOCOL_* flags. The negotiation layer itself already decodes this exact type with from_bits_retain in both the RDP Negotiation Request and Response, so a value the library accepts leniently at X.224 becomes connection-fatal when echoed back one phase later. The strictness also works against the field's purpose. [MS-RDPBCGR] 3.3.5.3.3's mandated handling of serverSelectedProtocol is a value comparison: if the field does not contain the same value the server transmitted in the RDP Negotiation Response, the server SHOULD drop the connection. That comparison needs the value to survive decode; rejecting unknown bits at parse time replaces the spec's check with a decode error. The client-side echo is symmetric: the client knows what it requested and compares, rather than validating the bit set. The exposure is demonstrated by history: the PROTOCOL_* list has grown three times (RDSTLS, HYBRID_EX, RDSAAD), and each addition would have made older strict decoders refuse newer peers whose negotiation had already succeeded. Both sites switch to from_bits_retain, the crate-wide policy since #1144; re-encoding a decoded block reproduces the wire value. This continues the interop line of #1489, #1458, #1837, #1843, #1844 and #1845. Tests: one per direction, each verifying that a fixture with an undefined protocol bit decodes, that the bit is retained, and that re-encoding reproduces the exact input bytes. Both fail with the strict from_bits restored. `cargo xtask check fmt/lints/tests/typos/locks` all pass.
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…urity data (#1845) ClientSecurityData::decode rejects the whole Basic Settings Exchange when the encryptionMethods field carries a bit outside the four defined flags. [MS-RDPBCGR] 2.2.1.3.3 defines the field as the set of methods the client supports, with the server obliged to "select one of the methods specified by the client", and [MS-RDPBCGR] 3.3.5.3.3's mandated handling for this field is to examine encryptionMethods and extEncryptionMethods for at least one valid flag, not to validate the bit set. The strict decode implemented neither half of that: it rejected a valid flag accompanied by an unknown bit, and accepted the zero-flags case the rule actually addresses. The sibling extEncryptionMethods field in the same structure is already read without validation. So the strictness is this library's own, it fails the connection at GCC, and it protects nothing: a server simply never selects a method it does not know. The advertisement site switches to from_bits_retain, the crate-wide policy since #1144, with the usual property that re-encoding a decoded block reproduces the wire value. The server-side selection in ServerSecurityData deliberately STAYS strict, and this change adds a comment making that asymmetry durable: that field is the server's selected method, not an advertisement, a conforming server can never send an unknown bit there, a client cannot implement a method it does not know, and the value drives the conditional serverRandom/serverCertificate parse. This continues the interop line of #1489, #1458, #1837, #1843 and #1844. Test: a Client Security Data fixture with an undefined method bit decodes, the bit is retained, and re-encoding reproduces the exact input bytes. Fails with the strict from_bits restored. `cargo xtask check fmt/lints/tests/typos/locks` all pass.
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…ropping the session (#1847) The fast-path input decoder rejects the whole event stream when a keyboard or synchronize event carries an eventFlags bit outside the defined set, and a decode error there is session-fatal mid-use: input events arrive continuously, so a single unknown flag bit from a client drops a live session with work in progress. The strictness is also internally inconsistent: the slow-path decoders for the very same events already tolerate unknown bits. Slow-path keyboard flags (scan_code.rs), synchronize toggle flags (sync.rs) and unicode keyboard flags (unicode.rs) all decode with from_bits_retain, so the identical keystroke is tolerated on one path and connection-fatal on the other. The spec's only mandated strictness in this area is about event types, not flags: [MS-RDPBCGR] 3.3.5.8.2 says the server SHOULD drop the connection when an event structure does not match one of the known types. That check is the existing eventCode rejection, which stays exactly as it is. The eventFlags tables (2.2.8.1.2.2.1 and 2.2.8.1.2.2.5) carry no receiver-validation language. All three flag sites (scancode, sync, unicode) switch to from_bits_retain, the crate-wide policy since #1144. The retained bits fit the 5-bit header field, so re-encoding reproduces the wire value, and the server-side consumers read these flags with contains(), so retained bits flow through harmlessly. This continues the interop line of #1489, #1458, #1837, #1843, #1844, #1845 and #1846, and it is the last flag-site conversion of that series. Tests: a keyboard event and a synchronize event, each with an undefined bit in the 5-bit field, decode successfully, retain the bit, and re-encode byte-identically. Both fail with the strict from_bits restored. `cargo xtask check fmt/lints/tests/typos/locks` all pass.
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Three GCC settings-block decoders reject the whole Basic Settings Exchange when a flags field carries a bit outside the defined list: RedirectionFlags in Client Cluster Data, the per-monitor flags in Client Monitor Data, and transportFlags in the Multitransport Channel Data. [MS-RDPBCGR] 3.3.5.3.3's processing rules mandate nothing for any of these three fields (its checks cover the Client Network Data bounds, the color depth fields, the serverSelectedProtocol echo, and the encryption method flags), so the strictness is this library's own, and it fails the connection at GCC, before any channel is joined.
The exposure is real in each case. The transportFlags list has grown before (TRANSPORTTYPE_UDP_PREFERRED and SOFTSYNC_TCP_TO_UDP are later additions), so an older IronRDP would have refused every client that advertised them. The monitor flags list defines only TS_MONITOR_PRIMARY, so any vendor or future bit refuses a multimon client outright. The redirection flags sit next to a version mask that already anticipates growth.
All three sites switch to from_bits_retain, the crate-wide policy since #1144: unknown bits are kept rather than dropped, so re-encoding a decoded block reproduces the wire value. The cluster version mask handling is untouched, and the separately parsed RedirectionVersion stays strict since it is a discriminant, not a bit set.
This continues the interop line of #1489 (unknown GCC user-data blocks), #1458 (unknown security header flags), #1837 (undefined channel option bits) and #1843 (unknown ClientInfoFlags bits).
Tests: one per site, each verifying that a fixture with an undefined bit decodes, that the bit is retained, and that re-encoding reproduces the exact input bytes. All three fail if the strict from_bits is restored.
cargo xtask check fmt/lints/tests/typos/locksall pass.