The publickeyfromprivatekey function in libssh before 0.5.4, when no algorithm is matched during negotiations, allows remote attackers to cause a denial of service (NULL pointer dereference and crash) via a "Client: Diffie-Hellman Key Exchange Init" packet.

Description of problem:

Maybe it is possible to send a malicious kexinit package to eventually cause a server to a double-free.

I guess this is only a DoS.

Source code:

http://git.libssh.org/projects/libssh.git/

Patch:

From f6b9f851b962e3a587f3f99b7cb97d130f0c77b9 Mon Sep 17 00:00:00 2001 From: Jon Simons <jon> Date: Sat, 18 Oct 2014 23:23:26 -0700 Subject: [PATCH] kex: fixup error path in sshpacketkexinit

Before this change, dangling pointers can be unintentionally left in the respective nextcrypto kex methods slots. Ensure to set all slots to NULL in the error-out path.

Signed-off-by: Jon Simons <jon> --- src/kex.c | 5 +++++ 1 file changed, 5 insertions(+)

diff --git a/src/kex.c b/src/kex.c index f1a1b56..ee00ec3 100644 --- a/src/kex.c +++ b/src/kex.c @@ -443,6 +443,11 @@ SSHPACKETCALLBACK(sshpacketkexinit){ error: sshstringfree(str); for (i = 0; i < SSHKEXMETHODS; i++) { + if (serverkex) { + session->nextcrypto->clientkex.methods[i] = NULL; + } else { / client / + session->nextcrypto->serverkex.methods[i] = NULL; + } SAFEFREE(strings[i]); } -- 1.9.1