russh: Client-side channel-scoped Handler callbacks fire for channel IDs the client never opened

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Russh is a Rust SSH client and server library. Prior to 0.63.1, client_read_authenticated in russh/src/client/encrypted.rs forwards CHANNEL_DATA, CHANNEL_EXTENDED_DATA, CHANNEL_EOF, CHANNEL_CLOSE, CHANNEL_OPEN_FAILURE, CHANNEL_SUCCESS, CHANNEL_FAILURE, and CHANNEL_REQUEST subtypes exit-status, exit-signal, and xon-xoff to public client::Handler callbacks without confirming that the ChannelId belongs to a channel the client opened and established. A malicious SSH server can send lifecycle events for predicted, unopened, unconfirmed, or released channel identifiers, causing application panics or corrupting command completion and exit-code tracking. This issue is fixed in version 0.63.1.

Source published

29 Sep 2026 18:25:58 UTC

Source modified

2 Oct 2026 12:20:10 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Russh is a Rust SSH client and server library. Prior to 0.63.1, client_read_authenticated in russh/src/client/encrypted.rs forwards CHANNEL_DATA, CHANNEL_EXTENDED_DATA, CHANNEL_EOF, CHANNEL_CLOSE, CHANNEL_OPEN_FAILURE, CHANNEL_SUCCESS, CHANNEL_FAILURE, and CHANNEL_REQUEST subtypes exit-status, exit-signal, and xon-xoff to public client::Handler callbacks without confirming that the ChannelId belongs to a channel the client opened and established. A malicious SSH server can send lifecycle events for predicted, unopened, unconfirmed, or released channel identifiers, causing application panics or corrupting command completion and exit-code tracking. This issue is fixed in version 0.63.1.

Source published

Sep 29, 2026, 6:25:58 PM (timezone not stated)

Source modified

Oct 2, 2026, 12:20:10 PM (timezone not stated)

Observed here

11 Nov 2026 05:29:28 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Russh is a Rust SSH client and server library. Prior to 0.63.1, client_read_authenticated in russh/src/client/encrypted.rs forwards CHANNEL_DATA, CHANNEL_EXTENDED_DATA, CHANNEL_EOF, CHANNEL_CLOSE, CHANNEL_OPEN_FAILURE, CHANNEL_SUCCESS, CHANNEL_FAILURE, and CHANNEL_REQUEST subtypes exit-status, exit-signal, and xon-xoff to public client::Handler callbacks without confirming that the ChannelId belongs to a channel the client opened and established. A malicious SSH server can send lifecycle events for predicted, unopened, unconfirmed, or released channel identifiers, causing application panics or corrupting command completion and exit-code tracking. This issue is fixed in version 0.63.1.

Source published

29 Sep 2026 19:17:24 UTC

Source modified

2 Oct 2026 13:17:25 UTC

Observed here

25 Oct 2026 04:17:26 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

### Summary CVE-2026-68930 was fixed by adding `Session::is_established_channel()` in `russh/src/server/encrypted.rs`, which gates every channel-scoped SERVER-side message (CHANNEL_REQUEST, CHANNEL_DATA, CHANNEL_EOF, CHANNEL_CLOSE, CHANNEL_WINDOW_ADJUST, CHANNEL_EXTENDED_DATA) on `enc.channels.get(&channel).is_some_and(|c| c.confirmed)` before invoking any `Handler` callback. The identical validation was never added to the CLIENT side (`russh/src/client/encrypted.rs`), which processes channel-scoped messages sent by the SSH SERVER once the client has authenticated. ### Details In `client_read_authenticated` (`russh/src/client/encrypted.rs`, ~lines 431-757), for CHANNEL_DATA, CHANNEL_EXTENDED_DATA, CHANNEL_EOF, CHANNEL_CLOSE, CHANNEL_OPEN_FAILURE, CHANNEL_SUCCESS, CHANNEL_FAILURE, and the CHANNEL_REQUEST sub-types exit-status/exit-signal/xon-xoff, the code only optionally forwards the event to the internal per-channel mpsc sender via `if let Some(chan) = self.channels.get(&channel_num) { ... }` (a no-op if the channel is unknown), but then **unconditionally** calls the corresponding public `Handler` trait method (`client.data(...)`, `client.exit_status(...)`, `client.channel_close(...)`, `client.channel_success(...)`, etc.) regardless of whether `channel_num` corresponds to any channel the client ever opened or that was ever confirmed. Only CHANNEL_OPEN_CONFIRMATION (closes the connection with `Error::Inconsistent` if unknown) and CHANNEL_WINDOW_ADJUST (returns early with `Ok(())` if unknown) correctly validate channel existence before acting. Corroborating evidence this check was intended but never wired up: `crate::Error` defines a dedicated `WrongChannel` variant documented as "Message received/sent on unopened channel" (`russh/src/lib_inner.rs`, ~line 144-146), yet a repo-wide search shows this variant is never constructed or returned anywhere in the codebase — dead code left over from (or intended for) exactly this validation. Because `Session::new_channel_id()` (`russh/src/session.rs`, ~line 708) allocates channel IDs sequentially starting at 1, a malicious or compromised SSH server can trivially predict the ID of the client's next channel and inject spoofed lifecycle events for it before or interleaved with the real channel-open exchange, or replay events for already-closed channel IDs. ### PoC Many real-world consumers of russh-as-a-client (deployment/orchestration tools, CI runners connecting to build/bastion hosts, git-over-ssh style tooling, database/tunnel clients) implement the `client::Handler` trait directly and key their own state (e.g. `HashMap<ChannelId, CommandState>`, exit-code trackers, per-channel byte counters, completion futures) off the channel IDs the library hands them, trusting the documented contract that events like "The remote process has exited" (`exit_status`) or "Called when the server closes a channel" (`channel_close`) only fire for a channel the application itself opened. A malicious, MITM'd (via a compromised/rogue jump host the client is configured to trust), or simply hostile SSH server can send `SSH_MSG_CHANNEL_REQUEST` (exit-status/exit-signal), `SSH_MSG_CHANNEL_DATA`, `SSH_MSG_CHANNEL_CLOSE`, `SSH_MSG_CHANNEL_SUCCESS`/`FAILURE`, or `SSH_MSG_CHANNEL_OPEN_FAILURE` for an arbitrary/predicted/never-opened channel ID at any point after authentication completes. Because the library invokes the `Handler` callback unconditionally, this reaches application code with an ID it never registered. ### Impact (1) A reliable, purely protocol-level trigger for an application panic/DoS in any client that indexes per-channel state by `ChannelId` without itself re-checking channel validity — the exact class of bug CVE-2026-68930 fixed server-side; and (2) lets the server spoof exit-status/exit-signal/close/success/failure notifications for a channel the client has not yet opened or has already released, desynchronizing the client's command-completion bookkeeping (e.g. reporting a forged exit code 0 for a not-yet-run remote command, or a premature `channel_close` before real output/exit-status has arrived) — a business-logic-level integrity violation of the SSH channel lifecycle that automation built on russh implicitly relies on. Suggested fix: add the same `is_established_channel()`-style gate already used in `server/encrypted.rs` to `client/encrypted.rs`'s `client_read_authenticated`, checking `self.channels.get(&channel_num)` before invoking any `Handler` callback (not just the mpsc forward), for every channel-scoped message type. For credit/changelog purposes, please use: Yazan Balawneh, Cystack.ps

Source published

30 Sep 2026 23:27:17 UTC

Source modified

30 Sep 2026 23:45:04 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Publisher rating · HIGH