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libssh: add support for SHA256 host public keys

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Reported-by: Joshua Rogers
Fixes #21605

Closes #21607
This commit is contained in:
Viktor Szakats 2026-05-14 13:32:46 +02:00
parent 9135294115
commit eb9b253d66
No known key found for this signature in database
11 changed files with 85 additions and 48 deletions
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90
lib/vssh/libssh.c
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@ -57,6 +57,7 @@
#include "multiif.h"
#include "select.h"
#include "vssh/vssh.h"
#include "curlx/base64.h" /* for curlx_base64_encode() */
#ifdef HAVE_UNISTD_H
#include <unistd.h>
@ -109,12 +110,14 @@ static CURLcode sftp_error_to_CURLE(int err)
}
/* Multiple options:
* 1. data->set.str[STRING_SSH_HOST_PUBLIC_KEY_MD5] is set with an MD5
* 1. data->set.str[STRING_SSH_HOST_PUBLIC_KEY_SHA256] is set with a SHA256
* hash.
* 2. data->set.str[STRING_SSH_HOST_PUBLIC_KEY_MD5] is set with an MD5
* hash (90s style auth, not sure we should have it here)
* 2. data->set.ssh_keyfunc callback is set. Then we do trust on first
* 3. data->set.ssh_keyfunc callback is set. Then we do trust on first
* use. We even save on knownhosts if CURLKHSTAT_FINE_ADD_TO_FILE
* is returned by it.
* 3. none of the above. We only accept if it is present on known hosts.
* 4. none of the above. We only accept if it is present on known hosts.
*
* Returns SSH_OK or SSH_ERROR.
*/
@ -122,8 +125,10 @@ static int myssh_is_known(struct Curl_easy *data, struct ssh_conn *sshc)
{
int rc;
ssh_key pubkey;
size_t hlen;
unsigned char *hash = NULL;
unsigned char *hash_sha256 = NULL;
size_t hlen_sha256;
unsigned char *hash_md5 = NULL;
size_t hlen_md5;
char *found_base64 = NULL;
char *known_base64 = NULL;
int vstate;
@ -139,20 +144,75 @@ static int myssh_is_known(struct Curl_easy *data, struct ssh_conn *sshc)
if(rc != SSH_OK)
return rc;
if(data->set.str[STRING_SSH_HOST_PUBLIC_KEY_MD5]) {
int i;
char md5buffer[33];
const char *pubkey_md5 = data->set.str[STRING_SSH_HOST_PUBLIC_KEY_MD5];
if(data->set.str[STRING_SSH_HOST_PUBLIC_KEY_SHA256]) {
const char *pubkey_sha256 =
data->set.str[STRING_SSH_HOST_PUBLIC_KEY_SHA256];
char *fingerprint_b64 = NULL;
size_t fingerprint_b64_len;
size_t pub_pos = 0;
size_t b64_pos = 0;
rc = ssh_get_publickey_hash(pubkey, SSH_PUBLICKEY_HASH_MD5, &hash, &hlen);
if(rc != SSH_OK || hlen != 16) {
rc = ssh_get_publickey_hash(pubkey, SSH_PUBLICKEY_HASH_SHA256,
&hash_sha256, &hlen_sha256);
if(rc != SSH_OK || hlen_sha256 != 32) {
failf(data, "Denied establishing ssh session: "
"SHA256 fingerprint not available");
goto cleanup;
}
if(curlx_base64_encode((const uint8_t *)hash_sha256, 32, &fingerprint_b64,
&fingerprint_b64_len) != CURLE_OK) {
rc = SSH_ERROR;
goto cleanup;
}
infof(data, "SSH SHA256 fingerprint: %s", fingerprint_b64);
/* Find the position of any = padding characters in the public key */
while((pubkey_sha256[pub_pos] != '=') && pubkey_sha256[pub_pos]) {
pub_pos++;
}
/* Find the position of any = padding characters in the base64 coded
* hostkey fingerprint */
while((fingerprint_b64[b64_pos] != '=') && fingerprint_b64[b64_pos]) {
b64_pos++;
}
/* Before we authenticate we check the hostkey's SHA256 fingerprint
* against a known fingerprint, if available.
*/
if((pub_pos != b64_pos) ||
strncmp(fingerprint_b64, pubkey_sha256, pub_pos)) {
failf(data,
"Denied establishing ssh session: mismatch SHA256 fingerprint. "
"Remote %s is not equal to %s", fingerprint_b64, pubkey_sha256);
curlx_free(fingerprint_b64);
rc = SSH_ERROR;
goto cleanup;
}
curlx_free(fingerprint_b64);
rc = SSH_OK;
goto cleanup;
}
if(data->set.str[STRING_SSH_HOST_PUBLIC_KEY_MD5]) {
const char *pubkey_md5 = data->set.str[STRING_SSH_HOST_PUBLIC_KEY_MD5];
char md5buffer[33];
int i;
rc = ssh_get_publickey_hash(pubkey, SSH_PUBLICKEY_HASH_MD5,
&hash_md5, &hlen_md5);
if(rc != SSH_OK || hlen_md5 != 16) {
failf(data,
"Denied establishing ssh session: MD5 fingerprint not available");
goto cleanup;
}
for(i = 0; i < 16; i++)
curl_msnprintf(&md5buffer[i * 2], 3, "%02x", hash[i]);
curl_msnprintf(&md5buffer[i * 2], 3, "%02x", hash_md5[i]);
infof(data, "SSH MD5 fingerprint: %s", md5buffer);
@ -297,8 +357,10 @@ cleanup:
/* !checksrc! disable BANNEDFUNC 1 */
free(known_base64); /* allocated by libssh, deallocate with system free */
}
if(hash)
ssh_clean_pubkey_hash(&hash);
if(hash_sha256)
ssh_clean_pubkey_hash(&hash_sha256);
if(hash_md5)
ssh_clean_pubkey_hash(&hash_md5);
ssh_key_free(pubkey);
if(knownhostsentry) {
ssh_knownhosts_entry_free(knownhostsentry);