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CVE-2026-54590 medium CVSS 5.9

Path Traversal in asyncssh (CVE-2026-54590)

Summary

Path traversal in asyncssh (CVE-2026-54590). Data can be tampered with by attackers. Exploitable via AuthorizedKeysFile. Mitigation: upgrade to 2.23.0 or later.

Original description

The description as published by the source.

Incomplete fix for CVE-2026-45309 (GHSA-g794-3fmp-753h). The
2.23.0 guard that sanitises the SSH username before %u substitution
in AuthorizedKeysFile blocks /, \ and .., but does not block a
leading ~ (or ${ENV}), both of which are re-introduced by later
expansion and reach the file open — defeating the guard.

Affected: asyncssh 2.23.0 and current develop (commit a60f863,
HEAD on 2026-05-29).

Summary

The fix for CVE-2026-45309 added a guard in
SSHServerConfig._set_tokens (asyncssh/config.py:715-716) that
rejects an SSH username containing /, \, or equal to .., before
it is substituted for the %u token in AuthorizedKeysFile:

  if self._user == '..' or '/' in self._user or '\\' in self._user:
      raise IllegalUserName('Unsafe username substitution')

However, the %u-substituted value is subsequently passed through
environment-variable expansion (_expand_val, config.py:145-149 —
token expansion then env expansion) and, at file-open time, through
expanduser() (read_authorized_keys → read_file →
open(Path(filename).expanduser()), auth_keys.py:348 →
misc.py:290). Both re-introduce the path control the guard was meant
to remove, so a username that contains no //\ can still cause the
server to read an authorized-keys file outside the intended per-user
directory.

The client-supplied username reaches this path pre-authentication:
_process_userauth_request takes…

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-2026-54590 has been found in asyncssh.
Attackers can target a specific entry point like AuthorizedKeysFile over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 5.9/10.
What to do: upgrade asyncssh to 2.23.0 or later.
If unsure, ask your IT team or search "asyncssh CVE-2026-54590" on the vendor's site.

CVE-2026-54590 (asyncssh) — CWE-22 / CVSS v3 5.9
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: AuthorizedKeysFile / develop / a60f863 / SSHServerConfig._set_tokens
Patched: 2.23.0 — 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

Path traversal (CWE-22) exists in asyncssh. Attackers reach the vulnerable code path via AuthorizedKeysFile without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of asyncssh. Attack surface: AuthorizedKeysFile / develop / a60f863 / SSHServerConfig._set_tokens.

🔥 Severity

Severity: Medium (CVSS 5.9/10). Data can be tampered with by attackers.

🔧 How to fix

Update to 2.23.0.

🛡️ 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 2.23.0

    ステージング環境で 2.23.0 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":"2.23.1"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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