misc: Add function to check username syntax

Malicious code can be injected using the username with metacharacters,
therefore the username must be validated before using it with any %u.

Signed-off-by: Norbert Pocs <norbertpocs0@gmail.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/misc.h

@@ -123,6 +123,7 @@ ssize_t ssh_readn(int fd, void *buf, size_t nbytes);
 ssize_t ssh_writen(int fd, const void *buf, size_t nbytes);
 
 int ssh_check_hostname_syntax(const char *hostname);
+int ssh_check_username_syntax(const char *username);
 
 #ifdef __cplusplus
 }

src/misc.c

@@ -2187,4 +2187,36 @@ int ssh_check_hostname_syntax(const char *hostname)
     return SSH_OK;
 }
 
+/**
+ * @brief Checks syntax of a username
+ *
+ * This check disallows metacharacters in the username
+ *
+ * @param username The username to be checked, has to be null terminated
+ *
+ * @return SSH_OK if the username passes syntax check
+ *         SSH_ERROR otherwise or if username is NULL or empty string
+ */
+int ssh_check_username_syntax(const char *username)
+{
+    size_t username_len;
+
+    if (username == NULL || *username == '-') {
+        return SSH_ERROR;
+    }
+
+    username_len = strlen(username);
+    if (username_len == 0 || username[username_len - 1] == '\\' ||
+        strpbrk(username, "'`\";&<>|(){}") != NULL) {
+        return SSH_ERROR;
+    }
+    for (size_t i = 0; i < username_len; i++) {
+        if (isspace(username[i]) != 0 && username[i + 1] == '-') {
+            return SSH_ERROR;
+        }
+    }
+
+    return SSH_OK;
+}
+
 /** @} */

tests/unittests/torture_misc.c

@@ -1058,6 +1058,39 @@ static void torture_ssh_check_hostname_syntax(void **state)
     assert_int_equal(rc, SSH_ERROR);
 }
 
+static void torture_ssh_check_username_syntax(void **state) {
+    int rc;
+    (void)state;
+
+    rc = ssh_check_username_syntax("username");
+    assert_int_equal(rc, SSH_OK);
+    rc = ssh_check_username_syntax("Alice");
+    assert_int_equal(rc, SSH_OK);
+    rc = ssh_check_username_syntax("Alice and Bob");
+    assert_int_equal(rc, SSH_OK);
+    rc = ssh_check_username_syntax("n4me?");
+    assert_int_equal(rc, SSH_OK);
+
+    rc = ssh_check_username_syntax("alice&bob");
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax("backslash\\");
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax("&var|()us\"<ha`r{}'");
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax(" -");
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax("me and -");
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax("los -santos");
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax("- who?");
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax(NULL);
+    assert_int_equal(rc, SSH_ERROR);
+    rc = ssh_check_username_syntax("");
+    assert_int_equal(rc, SSH_ERROR);
+}
+
 static void torture_ssh_is_ipaddr(void **state) {
     int rc;
     char *interf = malloc(64);
@@ -1123,6 +1156,7 @@ int torture_run_tests(void) {
         cmocka_unit_test(torture_ssh_readn),
         cmocka_unit_test(torture_ssh_writen),
         cmocka_unit_test(torture_ssh_check_hostname_syntax),
+        cmocka_unit_test(torture_ssh_check_username_syntax),
         cmocka_unit_test(torture_ssh_is_ipaddr),
     };