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CVE-2026-62949 medium CVSS 6.5

Vulnerability in asyncssh (CVE-2026-62949)

Summary

Vulnerability in asyncssh (CVE-2026-62949). Risk of unauthorized operations or information disclosure. Exploitable via SSH_MSG_CHANNEL_OPEN_CONFIRMATION. Mitigation: upgrade to 2.24.0 or later.

Original description

The description as published by the source.

Summary

A malicious SSH server can wedge an AsyncSSH client, and an authenticated
client can wedge an AsyncSSH server, by sending a channel maximum packet size of 0 in SSH_MSG_CHANNEL_OPEN_CONFIRMATION (server→client) or
SSH_MSG_CHANNEL_OPEN (client→server). AsyncSSH stores the peer-supplied value
verbatim with no lower-bound check; the first time channel data is written,
SSHChannel._flush_send_buf enters a synchronous infinite loop that cannot
be interrupted by asyncio.wait_for or any timeout. The loop body has no
await, so it blocks the entire asyncio event loop — for a server, one
malicious authenticated channel freezes all current and future connections.

RFC 4254 §5.1 leaves receiver behavior for a peer-reported "maximum packet
size = 0" undefined, so the value must be rejected rather than stored.

Root cause

asyncssh/channel.py:

# process_open (server side)        -- line 465
self._send_pktsize = send_pktsize    # peer value, no >= 1 check

# process_open_confirmation (client) -- line 528
self._send_pktsize = send_pktsize    # peer value, no >= 1 check

# _flush_send_buf                    -- lines 305-320
while self._send_buf and self._send_window:
    pktsize = min(self._send_window, self._send_pktsize)  # 0 when peer sends 0
    buf, datatype = self._send_buf[0]
    if len(buf) > pktsize:          # True for any buffered data
        data = buf[:pktsize]        # empty (b'')
        del buf[:pktsize]           # no-o…

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-2026-62949 has been found in asyncssh.
Attackers can target a specific entry point like SSH_MSG_CHANNEL_OPEN_CONFIRMATION over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 6.5/10.
What to do: upgrade asyncssh to 2.24.0 or later.
If unsure, ask your IT team or search "asyncssh CVE-2026-62949" on the vendor's site.

CVE-2026-62949 (asyncssh) — CWE-835 / CVSS v3 6.5
Attack vector: remote (network-reachable) / no user interaction
Attack surface: SSH_MSG_CHANNEL_OPEN_CONFIRMATION / SSH_MSG_CHANNEL_OPEN / SSHChannel._flush_send_buf / asyncio.wait_for
Patched: 2.24.0 — 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-835) exists in asyncssh. Attackers reach the vulnerable code path via SSH_MSG_CHANNEL_OPEN_CONFIRMATION.

📍 Affected scope

Check the vendor advisory for the affected versions of asyncssh. Attack surface: SSH_MSG_CHANNEL_OPEN_CONFIRMATION / SSH_MSG_CHANNEL_OPEN / SSHChannel._flush_send_buf / asyncio.wait_for.

🔥 Severity

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

🔧 How to fix

Update to 2.24.0.

🛡️ 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 'asyncssh' . 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 'asyncssh' . | 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 asyncssh 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 asyncssh to 2.24.0

    ステージング環境で 2.24.0 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

pip asyncssh
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.24.0"}]}]

References

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