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Allow manually confirming and caching certified keys.

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In the case where a server presents a host key signed by a different
certificate from the one you've configured, it need not _always_ be
evidence of wrongdoing. I can imagine situations in which two CAs
cover overlapping sets of things, and you don't want to blanket-trust
one of them, but you do want to connect to a specific host signed by
that one.

Accordingly, PuTTY's previous policy of unconditionally aborting the
connection if certificate validation fails (which was always intended
as a stopgap until I thought through what I wanted to replace it with)
is now replaced by fallback handling: we present the host key
fingerprint to the user and give them the option to accept and/or
cache it based on the public key itself.

This means that the certified key types have to have a representation
in the host key cache. So I've assigned each one a type id, and
generate the cache string itself by simply falling back to the base
key.

(Rationale for the latter: re-signing a public key with a different
certificate doesn't change the _private_ key, or the set of valid
signatures generated with it. So if you've been convinced for reasons
other than the certificate that a particular private key is in the
possession of $host, then proof of ownership of that private key
should be enough to convince you you're talking to $host no matter
what CA has signed the public half this week.)

We now offer to receive a given certified host key type if _either_ we
have at least one CA configured to trust that host, _or_ we have a
certified key of that type cached. (So once you've decided manually
that you trust a particular key, we can still receive that key and
authenticate the host with it, even if you later delete the CA record
that it didn't match anyway.)

One change from normal (uncertified) host key handling is that for
certified key types _all_ the host key prompts use the stronger
language, with "WARNING - POTENTIAL SECURITY BREACH!" rather than the
mild 'hmm, we haven't seen this host before'. Rationale: if you
expected this CA key and got that one, it _could_ be a bold-as-brass
MITM attempt in which someone hoped you'd accept their entire CA key.
The mild wording is only for the case where we had no previous
expectations _at all_ for the host to violate: not a CA _or_ a cached
key.
This commit is contained in:
Simon Tatham
2022-07-16 11:23:13 +01:00
parent a50178eba7
commit 42740a5455
6 changed files with 82 additions and 25 deletions
Download Patch File Download Diff File Expand all files Collapse all files

50
ssh/common.c
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@ -855,8 +855,8 @@ bool ssh2_bpp_check_unimplemented(BinaryPacketProtocol *bpp, PktIn *pktin)
SeatPromptResult verify_ssh_host_key(
InteractionReadySeat iseat, Conf *conf, const char *host, int port,
ssh_key *key, const char *keytype, char *keystr, const char *keydisp,
char **fingerprints, void (*callback)(void *ctx, SeatPromptResult result),
void *ctx)
char **fingerprints, int ca_count,
void (*callback)(void *ctx, SeatPromptResult result), void *ctx)
{
/*
* First, check if the Conf includes a manual specification of the
@ -934,7 +934,51 @@ SeatPromptResult verify_ssh_host_key(
seat_dialog_text_append(
text, SDT_TITLE, "%s Security Alert", appname);
if (storage_status == 1) {
if (key && ssh_key_alg(key)->is_certificate) {
seat_dialog_text_append(
text, SDT_SCARY_HEADING, "WARNING - POTENTIAL SECURITY BREACH!");
seat_dialog_text_append(
text, SDT_PARA, "This server presented a certified host key:");
seat_dialog_text_append(
text, SDT_DISPLAY, "%s (port %d)", host, port);
if (ca_count) {
seat_dialog_text_append(
text, SDT_PARA, "which was signed by a different "
"certification authority from the %s %s is configured to "
"trust for this server.", ca_count > 1 ? "ones" : "one",
appname);
if (storage_status == 2) {
seat_dialog_text_append(
text, SDT_PARA, "ALSO, that key does not match the key "
"%s had previously cached for this server.", appname);
seat_dialog_text_append(
text, SDT_PARA, "This means that either another "
"certification authority is operating in this realm AND "
"the server administrator has changed the host key, or "
"you have actually connected to another computer "
"pretending to be the server.");
} else {
seat_dialog_text_append(
text, SDT_PARA, "This means that either another "
"certification authority is operating in this realm, or "
"you have actually connected to another computer "
"pretending to be the server.");
}
} else {
assert(storage_status == 2);
seat_dialog_text_append(
text, SDT_PARA, "which does not match the certified key %s "
"had previously cached for this server.", appname);
seat_dialog_text_append(
text, SDT_PARA, "This means that either the server "
"administrator has changed the host key, or you have actually "
"connected to another computer pretending to be the server.");
}
seat_dialog_text_append(
text, SDT_PARA, "The new %s key fingerprint is:", keytype);
seat_dialog_text_append(
text, SDT_DISPLAY, "%s", fingerprints[fptype_default]);
} else if (storage_status == 1) {
seat_dialog_text_append(
text, SDT_PARA, "The host key is not cached for this server:");
seat_dialog_text_append(