asyncssh has an incomplete fix for CVE-2026-45309 — AuthorizedKeysFile %u still escapes the intended directory via a leading ~ (and weakly via ${ENV}) username substitution

Exploitation

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

CVSS

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

EPSS

• CVE-2026-54590: 0.34% estimated exploitation activity in the next 30 days; percentile 27.09 (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.

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.settokens (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 (expandval, config.py:145-149 — token expansion then env expansion) and, at file-open time, through expanduser() (readauthorizedkeys → readfile →…

Published by the source

26 Aug 2026, 15:32 UTC

Last modified by the source

26 Aug 2026, 15:32 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-qr67-gv47-xwwh

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# Patch Next advisory brief: CVE-2026-54590

Advisory: asyncssh has an incomplete fix for CVE-2026-45309 — AuthorizedKeysFile %u still escapes the intended directory via a leading ~ (and weakly via ${ENV}) username substitution
IDs: CVE-2026-54590, GHSA-qr67-gv47-xwwh
Record: https://patchnext.com/advisories/cve-2026-54590 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: asyncssh 2.23.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI asyncssh: affected <= 2.23.0; fixed in 2.23.1

## Suggested inspection (read-only)
- `python -m pip show asyncssh`: prints the installed version. Run with the environment that serves production active.

## 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 asyncssh.
- 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://github.com/advisories/GHSA-qr67-gv47-xwwh (GitHub-reviewed advisory GHSA-qr67-gv47-xwwh lists affected ranges: PyPI asyncssh >=0 <=2.23.0)

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