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Work around key algorithm naming change in OpenSSH <= 7.7.

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When you send a "publickey" USERAUTH_REQUEST containing a certified
RSA key, and you want to use a SHA-2 based RSA algorithm, modern
OpenSSH expects you to send the algorithm string as
rsa-sha2-NNN-cert-v01@openssh.com. But 7.7 and earlier didn't
recognise those names, and expected the algorithm string in the
userauth request packet to be ssh-rsa-cert-v01@... and would then
follow it with an rsa-sha2-NNN signature.

OpenSSH itself has a bug workaround for its own older versions. Follow
suit.
This commit is contained in:
Simon Tatham
2023-05-04 18:24:18 +01:00
parent cfe6fd95a7
commit d663356634
8 changed files with 82 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

35
ssh/userauth2-client.c
View File

@ -135,6 +135,8 @@ static void ssh2_userauth_ki_write_responses(
static void ssh2_userauth_final_output(PacketProtocolLayer *ppl);
static void ssh2_userauth_print_banner(struct ssh2_userauth_state *s);
static ptrlen workaround_rsa_sha2_cert_userauth(
struct ssh2_userauth_state *s, ptrlen id);
static const PacketProtocolLayerVtable ssh2_userauth_vtable = {
.free = ssh2_userauth_free,
@ -2381,9 +2383,19 @@ static void ssh2_userauth_add_alg_and_publickey(
* bare key algorithms, or nothing useful will happen. */
const ssh_keyalg *pkalg_base =
pkalg->base_alg ? pkalg->base_alg : pkalg;
/* Construct an algorithm string that includes both the
* signature subtype (e.g. rsa-sha2-512) and the
* certificate-ness. Exception: in earlier versions of
* OpenSSH we don't want to do that, and must send just
* ssh-rsa-cert-... even when we're delivering a non-SHA-1
* signature. */
const ssh_keyalg *output_alg =
ssh_keyalg_related_alg(certalg, pkalg_base);
put_stringz(pkt, output_alg->ssh_id);
ptrlen output_id = ptrlen_from_asciz(output_alg->ssh_id);
output_id = workaround_rsa_sha2_cert_userauth(s, output_id);
put_stringpl(pkt, output_id);
}
put_stringpl(pkt, ptrlen_from_strbuf(s->detached_cert_blob));
done = true;
@ -2430,11 +2442,30 @@ static void ssh2_userauth_add_alg_and_publickey(
return;
}
/* In all other cases, just put in what we were given. */
/* In all other cases, basically just put in what we were given -
* except for the same bug workaround as above. */
alg = workaround_rsa_sha2_cert_userauth(s, alg);
put_stringpl(pkt, alg);
put_stringpl(pkt, pkblob);
}
static ptrlen workaround_rsa_sha2_cert_userauth(
struct ssh2_userauth_state *s, ptrlen id)
{
if (!(s->ppl.remote_bugs & BUG_RSA_SHA2_CERT_USERAUTH))
return id;
/*
* No need to try to do this in a general way based on the
* relations between ssh_keyalgs; we know there are a limited
* number of affected versions of OpenSSH, so this doesn't have to
* be futureproof against later additions to the family.
*/
if (ptrlen_eq_string(id, "rsa-sha2-256-cert-v01@openssh.com") ||
ptrlen_eq_string(id, "rsa-sha2-512-cert-v01@openssh.com"))
return PTRLEN_LITERAL("ssh-rsa-cert-v01@openssh.com");
return id;
}
/*
* Helper function to add an SSH-2 signature blob to a packet. Expects
* to be shown the public key blob as well as the signature blob.

22
ssh/verstring.c
View File

@ -612,6 +612,28 @@ static void ssh_detect_bugs(struct ssh_verstring_state *s)
bpp_logevent("We believe remote version requires us to "
"filter our KEXINIT");
}
if (conf_get_int(s->conf, CONF_sshbug_rsa_sha2_cert_userauth) == FORCE_ON ||
(conf_get_int(s->conf, CONF_sshbug_rsa_sha2_cert_userauth) == AUTO &&
(wc_match("OpenSSH_7.[2-7]*", imp)))) {
/*
* These versions have the bug in which using RSA/SHA-2
* authentication with a certified key requires the key
* algorithm to be sent as ssh-rsa-cert-... instead of
* rsa-sha2-NNN-cert-...
*
* OpenSSH 7.8 wants rsa-sha2-NNN-cert-...:
* https://github.com/openssh/openssh-portable/commit/4ba0d54794814ec0de1ec87987d0c3b89379b436
* (also labelled "OpenBSD-Commit-ID:
* c6e9f6d45eed8962ad502d315d7eaef32c419dde")
*
* OpenSSH 7.2 was the first release supporting RSA/SHA-2
* at all, so this bug is irrelevant to anything before that.
*/
s->remote_bugs |= BUG_RSA_SHA2_CERT_USERAUTH;
bpp_logevent("We believe remote version has SSH-2 "
"RSA/SHA-2/certificate userauth bug");
}
}
const char *ssh_verstring_get_remote(BinaryPacketProtocol *bpp)