CVE-2021-3634:libssh: SSH rekey digest size mismatch
A reconstruction of the flaw, and the evidence path Lachesis follows to make it visible.
With no family and no line seeded, the whole-taxonomy enumerator put its class-matched lead inside kex.c:732 (ssh_make_sessionid / rekey KEX digest handling) -- the.
That is the shape of the failure. The rest of this case file follows the evidence behind it.
What was known.
What Lachesis established.
Known before Lachesis ran
The CVE and vulnerable release were selected from public history. We are not claiming novel discovery.
- CVE
- CVE-2021-3634
- Vulnerable target
- libssh-0.9.5 (last-vulnerable) · vulnerable
- Prior knowledge
- CVE-2021-3634 · libssh-0.9.5 (last-vulnerable) is vulnerable
Independent re-detection
The experiment graph was built around the last-vulnerable release (libssh-0.9.5), scoped to src + include, so the vulnerable translation unit builds under the pass budget (build 25s, enrich 11056 nodes / 42359 edges), then enumerated BLIND over the whole constructor registry (no family hard-coded, no target line seeded). The census and candidate ids below are the enumerator's own verbatim output; prose in adjudication/trace is MY reading of the advisory + fix, clearly separated from tool output.
- Seeded inputs
- None
- Run timestamp
- 2026-09-03T00:00:00Z
- Evidence artifact
- cve-2021-3634.kuzu (built from libssh libssh-0.9.5, scoped to src + include; graph + sidecars deleted after capture)
The commands and outputs below come from this recorded Lachesis run.
What Lachesis reconstructed.
One historical repository. Four captured queries. A complete source-to-sink argument.
Lachesis follows the same evidence path through unfamiliar repositories.
Here is how the path becomes visible.
These are the recorded questions Lachesis asked of the historical vulnerable code, followed by the raw result and source location each query returned.
Captured runA real replay of Lachesis over the graph we built for this case (build 25s under the pass cap, bounded memory). Every value below is verbatim captured tool output; the graph was released immediately after capture.
Investigate the candidate
The narrowed scope builds the vulnerable TU under the pass cap.
build rc=0 25s
enrich rc=0 :: pass2 published 11056 nodes and 42359 edgesA small line with a large consequence.
the digest-length assumption is fixed at the initial KEX and not recomputed for the rekey's (possibly different-size) digest, so the length driving the hash copy no longer matches the destination buffer.
/* rekey: secret_hash/session_id length inherited from the initial KEX digest size, not the rekey's */
// 0.9.6 recomputes the digest/hash length for the rekey's negotiated KEX so secret_hash and session_id lengths always match the actual digest size.
The original record.
Read the historical advisory and vulnerability record behind this reconstruction.