EUVD-2026-42400Severity by source
AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
Network vector as SSH is remote; AC:H because %u-templated AuthorizedKeysFile is a prerequisite server configuration; PR:N as exploitation occurs during unauthenticated login; no confidentiality or availability impact.
Primary rating from Vendor (github).
CVSS VectorVendor: github
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
Lifecycle Timeline
2DescriptionCVE.org
AsyncSSH is a Python package which provides an asynchronous client and server implementation of the SSHv2 protocol on top of the Python asyncio framework. Version 2.23.0 contains an incomplete fix for CVE-2026-45309 in SSHServerConfig._set_tokens that blocks /, , and .. before %u substitution in AuthorizedKeysFile but does not block a leading ~ or ${ENV}, allowing later expansion in _expand_val and Path(filename).expanduser() to escape the intended authorized-keys directory. This issue is fixed in version 2.23.1.
AnalysisAI
{ENV}) unblocked, enabling escape via Python's Path.expanduser() and _expand_val() expansion. No public exploit has been identified at time of analysis, and CVSS AC:H reflects the prerequisite server-side configuration; however, successful exploitation achieves high integrity impact by enabling authentication with key material from arbitrary filesystem locations.
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires that the AsyncSSH SSH server be configured with an AuthorizedKeysFile directive that includes the %u username substitution token (e.g., AuthorizedKeysFile .ssh/authorized_keys_%u or a similar pattern). … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | CVSS 3.1 scores this at 5.9 Medium with vector AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker with network access to an AsyncSSH server configured with AuthorizedKeysFile using %u substitution connects and presents an SSH username crafted with a leading tilde, such as ~/.ssh/attacker_controlled, during the authentication handshake. The server's unpatched _set_tokens processes the %u substitution, and Python's Path.expanduser() subsequently resolves ~ to the SSH daemon's home directory, causing the server to consult an authorized-keys file outside the intended path. … |
| Remediation | The primary fix is to upgrade AsyncSSH to version 2.23.1, which resolves the incomplete sanitization in SSHServerConfig._set_tokens by additionally blocking leading ~ and ${ENV} patterns before %u substitution. … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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