Russh: Unbounded memory exhaustion via CHANNEL_OPEN flood during a client-stalled rekey

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-102821: 0.36% estimated exploitation activity in the next 30 days; percentile 29.63 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A russh server can be driven to unbounded heap growth (process OOM / kill) by a peer that speaks only standard SSH messages, in the default configuration. The peer starts a key re-exchange (sends SSHMSGKEXINIT) but never sends the follow-up SSHMSGKEXECDHINIT, leaving the server's kex state machine in SessionKexState::InProgress indefinitely. While a rekey is in progress the server's three message-drain paths are all gated off (if !self.kex.active()), but the network-read path stays active, so every SSHMSGCHANNELOPEN the peer sends is processed inline and appends one reply to an unbounded internal queue (priorityreceiver, an UnboundedReceiver) that is not dequeued until the rekey completes. Because the peer decides whether the rekey ever completes, the queue — and the server's memory — grows without bound. This is reproducible end-to-end against a real russh server over a real encrypted transport; see "Proof of concept". A one-line negative control (same flood, no rekey) keeps…

Published by the source

30 Sep 2026, 23:40 UTC

Last modified by the source

1 Oct 2026, 00:00 UTC

Source last checked

26 Oct 2026, 15:43 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-35g8-35p8-c8fw

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-35g8-35p8-c8fw

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# Patch Next advisory brief: CVE-2026-102821

Advisory: Russh: Unbounded memory exhaustion via CHANNEL_OPEN flood during a client-stalled rekey
IDs: CVE-2026-102821, GHSA-35g8-35p8-c8fw
Record: https://patchnext.com/advisories/cve-2026-102821 (revision 2, source checked 26 Oct 2026, 15:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: russh 0.63.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io russh: affected <= 0.63.1; fixed in 0.63.2

## Suggested inspection (read-only)
- crates.io russh: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on russh.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-35g8-35p8-c8fw (OSV record GHSA-35g8-35p8-c8fw lists affected ranges: crates.io russh >=0 <0.63.2 (fixed))
- Verified: https://github.com/advisories/GHSA-35g8-35p8-c8fw (GitHub-reviewed advisory GHSA-35g8-35p8-c8fw lists affected ranges: crates.io russh >=0 <=0.63.1)

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