CVE-2026-84897
Wolfssh: denial of service
- Risk score
- 40/100
- CVSS
- 5.3 v3.1
- Exploit chance (30 days)
- 0.5%
- Published
- 7 Oct 2026
Open in the live forecast Today's forecast
What to know
- What it is
- src/internal.c in wolfSSL wolfSSH through 1.5.0 admits the server-to-client Diffie-Hellman group exchange messages SSH_MSG_KEX_DH_GEX_GROUP (31) and SSH_MSG_KEX_DH_GEX_REPLY (33) when a server receives them from an unauthenticated client. IsMessageAllowedServer() applies no direction check to the key exchange message …
- Affected
- Wolfssh
- Fix
- Apply the vendor's security update; see the linked advisory.
- Exploitation
- No known exploitation
Story map
At a glance
Wolfssh: Amber, patch this week. In wolfSSH through 1.5.0 built with --enable-fwd, DoChannelOpen() in src/internal.c gates only direct-tcpip channel opens with the forwarding policy callback. forwarded-tcpip opens are admitted without an authorization check and are not capped in number, allowing a malicious SSH peer to make an endpoint allocate unbou…
The flaws
- CVE-2026-84897: Denial of service (CVSS 5.3 · EPSS 0%)
- CVE-2026-81535: Missing authorisation (CVSS 9.1 · EPSS 0%)
- CVE-2026-83540: Authentication bypass (CVSS 8.8 · EPSS 1%)
- CVE-2026-83742: Stack buffer overflow can lead to denial of service (CVSS 8.8 · EPSS 0%)
The fix
Fix: Apply the vendor's security update; see the linked advisory.
Who is exposed: Wolfssh
Where it stands
- No known exploitation
Connects to
- Same product: Earlier Wolfssh warnings. 1 in the last 90 days
Why it is Yellow
- CVSS 5.3 medium
References
Original description
src/internal.c in wolfSSL wolfSSH through 1.5.0 admits the server-to-client Diffie-Hellman group exchange messages SSH_MSG_KEX_DH_GEX_GROUP (31) and SSH_MSG_KEX_DH_GEX_REPLY (33) when a server receives them from an unauthenticated client. IsMessageAllowedServer() applies no direction check to the key exchange message range: when the peer is keying and no particular message is expected, which is the state a server is in for the whole window after it processes the client's KEXINIT because nothing sets handshake->expectMsgId there, the function falls out of its expectation branch without a verdict and reaches a numeric bound that admits every message id from 30 through 34. A client that negotiates diffie-hellman-group-exchange-sha256 and then sends message 31 makes the server run the client-side handler DoKexDhGexGroup(), which validates the attacker-supplied group with two 8-round Miller-Rabin primality tests, one on p and one on (p-1)/2, on a value of up to 8192 bits. The handler then returns success: the server stores the attacker's prime and generator, generates a Diffie-Hellman key pair in the attacker's group, and sends the client-role message SSH_MSG_KEX_DH_GEX_INIT (32) back to the attacker. Published RFC 3526 safe primes are the worst-case input and cost the attacker nothing to obtain. The primality validation was added in 1.5.0; versions from 1.2.0 through 1.4.22 admit the same message and enter the same client-role path without the primality cost. Message 33 is admitted as well, but on a server it is rejected before any cryptography because no public key check callback is registered, so it carries no comparable cost. Builds that define WOLFSSH_NO_DH_GEX_SHA256, which is implied by WOLFSSH_NO_DH or NO_SHA256, are unaffected.
Last activity 7 Oct 2026. Patch Forecast updates every hour.