AsyncSSH vulnerable to Prefix Truncation Attack (a.k.a. Terrapin Attack) against ChaCha20-Poly1305 and Encrypt-then-MAC

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

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary AsyncSSH v2.14.1 and earlier is vulnerable to a novel prefix truncation attack (a.k.a. Terrapin attack), which allows a man-in-the-middle attacker to strip an arbitrary number of messages right after the initial key exchange, breaking SSH extension negotiation (RFC8308) in the process and thus downgrading connection security. Mitigations To mitigate this protocol vulnerability, OpenSSH suggested a so-called "strict kex" which alters the SSH handshake to ensure a Man-in-the-Middle attacker cannot introduce unauthenticated messages as well as convey sequence number manipulation across handshakes. Support for strict key exchange has been added to AsyncSSH in the patched version. Warning: To take effect, both the client and server must support this countermeasure. As a stop-gap measure, peers may also (temporarily) disable the affected algorithms and use unaffected alternatives like AES-GCM instead until patches are available. Details The SSH specifications of ChaCha20-Poly1305…

Published by the source

18 Dec 2023, 19:21 UTC

Last modified by the source

7 Dec 2024, 05:40 UTC

Source last checked

26 Oct 2026, 13:12 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-hfmc-7525-mj55

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-hfmc-7525-mj55

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# Patch Next advisory brief: GHSA-hfmc-7525-mj55

Advisory: AsyncSSH vulnerable to Prefix Truncation Attack (a.k.a. Terrapin Attack) against ChaCha20-Poly1305 and Encrypt-then-MAC
IDs: GHSA-hfmc-7525-mj55
Record: https://patchnext.com/advisories/ghsa-hfmc-7525-mj55 (revision 2, source checked 26 Oct 2026, 13:12 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 2.14.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI asyncssh: affected <= 2.14.1; fixed in 2.14.2

## 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://osv.dev/vulnerability/GHSA-hfmc-7525-mj55 (OSV record GHSA-hfmc-7525-mj55 lists affected ranges: PyPI asyncssh >=0 <2.14.2 (fixed))
- Verified: https://github.com/advisories/GHSA-hfmc-7525-mj55 (GitHub-reviewed advisory GHSA-hfmc-7525-mj55 lists affected ranges: PyPI asyncssh >=0 <=2.14.1)

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