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CVE-2026-102821 medium CVSS 6.5

Vulnerability in russh (CVE-2026-102821)

Summary

Vulnerability in russh (CVE-2026-102821). Risk of unauthorized operations or information disclosure. Exploitable via SSH_MSG_KEXINIT. Mitigation: upgrade to 0.63.2 or later.

Original description

The description as published by the source.

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 SSH_MSG_KEXINIT) but never sends the
follow-up SSH_MSG_KEX_ECDH_INIT, 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 SSH_MSG_CHANNEL_OPEN the peer
sends is processed inline and appends one reply to an unbounded internal
queue (priority_receiver, 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 memory flat, isolating the rekey window as the sole
trigger.

Impact

  • Availability / DoS. Server RSS climbs at roughly 3.3 KB per
    CHANNEL_OPEN, driven entirely by the peer, until the process is OOM-killed.
    In the reproduction one connection pushed the server from ~4 MB to 2.57 GB
    (and past 4.8 GB against the stock echoserver example) and it was still
    climbing when the flood was stopped.
  • **Reached with standard messages,…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-102821 has been found in russh.
Attackers can target a specific entry point like SSH_MSG_KEXINIT over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 6.5/10.
What to do: upgrade russh to 0.63.2 or later.
If unsure, ask your IT team or search "russh CVE-2026-102821" on the vendor's site.

CVE-2026-102821 (russh) — CWE-400 / CVSS v3 6.5
Attack vector: remote (network-reachable) / no user interaction
Attack surface: SSH_MSG_KEXINIT / SSH_MSG_KEX_ECDH_INIT / SSH_MSG_CHANNEL_OPEN / priority_receiver
Patched: 0.63.2 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-400) exists in russh. Attackers reach the vulnerable code path via SSH_MSG_KEXINIT.

📍 Affected scope

Check the vendor advisory for the affected versions of russh. Attack surface: SSH_MSG_KEXINIT / SSH_MSG_KEX_ECDH_INIT / SSH_MSG_CHANNEL_OPEN / priority_receiver.

🔥 Severity

Severity: Medium (CVSS 6.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 0.63.2.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'russh' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'russh' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で russh を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade russh to 0.63.2

    ステージング環境で 0.63.2 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

rust russh
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.63.2"}]}]
crates.io russh
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.63.2"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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