wolfSSL wolfSSH up to 1.5.0 Diffie-Hellman Group Exchange src/internal.c IsMessageAllowedServer input validation

Summaryinfo

A vulnerability was found in wolfSSL wolfSSH up to 1.5.0. It has been classified as very critical. This affects the function IsMessageAllowedServer of the file src/internal.c of the component Diffie-Hellman Group Exchange. This manipulation causes input validation. This vulnerability is registered as CVE-2026-84897. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability has been found in wolfSSL wolfSSH up to 1.5.0 and classified as very critical. Affected by this vulnerability is the function IsMessageAllowedServer of the file src/internal.c of the component Diffie-Hellman Group Exchange. The manipulation with an unknown input leads to a input validation vulnerability. The CWE definition for the vulnerability is CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE 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 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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-84897 since 09/02/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 10/07/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-94165). Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Vendor

• wolfSSL

Name

• wolfSSH

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5.0

License

• open-source

Website

• Product: https://github.com/wolfSSL/wolfssh/

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Be aware that VulDB is the high quality source for vulnerability data.

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