GHSA-47hw-gvq5-r2gm CVE-2026-102823
russh: Client-side channel-scoped Handler callbacks fire for channel IDs the client never opened
- Severity
- 7.5 / 10
- High · CVSS 3.1
- Exploit chance (EPSS)
- 0.32%
- Chance of exploitation in the next 30 days. Higher than 24.67% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-20 Improper Input Validation
- Fix
0.63.1- Upgrade russh to this version or later
Description
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
Affected versions
crates.io
| Package | Vulnerable | Fixed in |
|---|---|---|
| russh | < 0.63.1 |
0.63.1 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
russh crates.io → 0.63.1
- cargo
cargo update -p russh --precise 0.63.1
Severity breakdown
CVSS 3.1 7.5
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- High
- Availability
- None
Weakness
CWE-20: Improper Input Validation
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
How to prevent it
- Architecture and Design: Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build "recognizers" for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of allowing parser code to be scattered throughout the program, where it could be subject to errors or inconsistencies that create weaknesses. [REF-1109] [REF-1110] [REF-1111]
- Architecture and Design: Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
- Architecture and Design, Implementation: Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server. Even though client-side checks provide minimal benefits with respect to server-side security, they are still useful. First, they can support intrusion detection. If the server receives input that should have been rejected by the client, then it may be an indication of an attack. Second, client-side error-checking can provide helpful feedback to the user about the expectations for valid input. Third, there may be a reduction in server-side processing time for accidental input errors, although this is typically a small savings.
- Implementation: When your application combines data from multiple sources, perform the validation after the sources have been combined. The individual data elements may pass the validation step but violate the intended restrictions after they have been combined.
- Implementation: Be especially careful to validate all input when invoking code that crosses language boundaries, such as from an interpreted language to native code. This could create an unexpected interaction between the language boundaries. Ensure that you are not violating any of the expectations of the language with which you are interfacing. For example, even though Java may not be susceptible to buffer overflows, providing a large argument in a call to native code might trigger an overflow.
- Implementation: Directly convert your input type into the expected data type, such as using a conversion function that translates a string into a number. After converting to the expected data type, ensure that the input's values fall within the expected range of allowable values and that multi-field consistencies are maintained.
- Implementation: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180, CWE-181). Make sure that your application does not inadvertently decode the same input twice (CWE-174). Such errors could be used to bypass allowlist schemes by introducing dangerous inputs after they have been checked. Use libraries such as the OWASP ESAPI Canonicalization control. Consider performing repeated canonicalization until your input does not change any more. This will avoid double-decoding and similar scenarios, but it might inadvertently modify inputs that are allowed to contain properly-encoded dangerous content.
- Implementation: When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-vp5q-ff62-jwqv russh: Agent server never calls `Agent::confirm`, so keys added with the confirm constraint (`ssh-add -c`) sign without confirmation | High | Also affects russh |
| GHSA-fw5c-8223-6h42 russh: peer-declared zero maximum packet size permanently stalls channel writes (silent AsyncWrite hang) | Medium | Also affects russh |
| GHSA-9wfm-jcj8-jfv9 russh's SSH agent server is susceptible to unbound allocations via repeated SSH_AGENT_CONSTRAIN_CONFIRM messages | Low | Also affects russh |
| GHSA-4p9f-7p9m-5vfg russh's SSH client is susceptible to unbounded allocations via EXT_INFO packets | Medium | Also affects russh |
| GHSA-4wc5-f2rc-q74m russh: Unbounded memory allocation in agent protocol server via crafted constraint records | Medium | Also affects russh |
| GHSA-j7h2-66j4-ghjf A russh SSH server accepts a publickey authentication whose signature uses a different algorithm than the one named in the request | Low | Also affects russh |
| GHSA-g6xm-f9xp-qq35 Russh: Configured server auth-attempt cap is not enforced in the USERAUTH_REQUEST runtime path | Low | Also affects russh |
| GHSA-w3jg-pjxf-73p4 Russh: Missing X25519 zero-point validation in hybrid ML-KEM key exchange | Medium | Also affects russh |
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