Overhaul TOFU checking code.
Main motivation for this was to switch from keying the cache cert database off hostname + address to hostname + port. While making the necessary changes I refactored to reduce code duplication and make the overall flow of the TOFU checks more transparent. The check of whether the "previous certificate" has expired has been changed from using the most frequently seen previous cert to the most recently seen, which makes a *lot* more sense and is arguably a bug fix. The address column of the DB is now used only for reporting, but the column is not maintained well, or rather, the semantics are currently "address cert was first received from", and we may want something less static?
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parent
fc056ef680
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88daabe091
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@ -571,7 +571,7 @@ you'll be able to transparently follow links to Gopherspace!""")
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# Do TOFU
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if self.options["tls_mode"] == "tofu":
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cert = s.getpeercert(binary_form=True)
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self.tofu_store.validate_cert(address[4][0], host, cert)
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self.tofu_store.validate_cert(address[4][0], address[4][1], host, cert)
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# Send request and wrap response in a file descriptor
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ui_out.debug("Sending %s<CRLF>" % gi.url)
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252
src/av98/tofu.py
252
src/av98/tofu.py
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@ -15,6 +15,7 @@ try:
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except ModuleNotFoundError:
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_HAS_CRYPTOGRAPHY = False
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import av98.util as util
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ui_out = logging.getLogger("av98_logger")
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class TofuStore:
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@ -22,20 +23,38 @@ class TofuStore:
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def __init__(self, config_dir):
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self.config_dir = config_dir
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self.certdir = os.path.join(config_dir, "cert_cache")
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if not os.path.exists(self.certdir):
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os.makedirs(self.certdir)
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db_path = os.path.join(self.config_dir, "tofu.db")
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self.db_conn = sqlite3.connect(db_path)
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self.db_cur = self.db_conn.cursor()
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self.create_db()
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self.update_db()
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def create_db(self):
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self.db_cur.execute("""CREATE TABLE IF NOT EXISTS cert_cache
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(hostname text, address text, fingerprint text,
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(hostname text, port integer, address text, fingerprint text,
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first_seen date, last_seen date, count integer)""")
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def update_db(self):
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# Update 1 - check for port column
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try:
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self.db_cur.execute("""SELECT port FROM cert_cache where 1=0""")
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has_port = True
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except sqlite3.OperationalError:
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has_port = False
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if not has_port:
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self.db_cur.execute("""ALTER TABLE cert_cache ADD COLUMN port integer""")
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self.db_cur.execute("""UPDATE cert_cache SET port= 1965 WHERE count > 0""")
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def close(self):
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self.db_conn.commit()
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self.db_conn.close()
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def validate_cert(self, address, host, cert):
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def validate_cert(self, address, port, host, cert):
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"""
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Validate a TLS certificate in TOFU mode.
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@ -46,115 +65,146 @@ class TofuStore:
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Whether the cryptography module is installed or not, check the
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certificate's fingerprint against the TOFU database to see if we've
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previously encountered a different certificate for this IP address and
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hostname.
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previously encountered a different certificate for this hostname and
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port
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"""
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now = datetime.datetime.utcnow()
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# Do 'advanced' checks if Cryptography library is installed
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if _HAS_CRYPTOGRAPHY:
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# Using the cryptography module we can get detailed access
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# to the properties of even self-signed certs, unlike in
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# the standard ssl library...
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c = x509.load_der_x509_certificate(cert, _BACKEND)
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# Check certificate validity dates
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if c.not_valid_before >= now:
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raise ssl.CertificateError("Certificate not valid until: {}!".format(c.not_valid_before))
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elif c.not_valid_after <= now:
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raise ssl.CertificateError("Certificate expired as of: {})!".format(c.not_valid_after))
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# Check certificate hostnames
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names = []
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common_name = c.subject.get_attributes_for_oid(x509.oid.NameOID.COMMON_NAME)
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if common_name:
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names.append(common_name[0].value)
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try:
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names.extend([alt.value for alt in c.extensions.get_extension_for_oid(x509.oid.ExtensionOID.SUBJECT_ALTERNATIVE_NAME).value])
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except x509.ExtensionNotFound:
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pass
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names = set(names)
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for name in names:
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try:
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ssl._dnsname_match(name, host)
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break
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except Exception:
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continue
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else:
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# If we didn't break out, none of the names were valid
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raise ssl.CertificateError("Hostname does not match certificate common name or any alternative names.")
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self.check_cert_expiry_and_names(cert, host, now)
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# Compute SHA256 fingerprint
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sha = hashlib.sha256()
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sha.update(cert)
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fingerprint = sha.hexdigest()
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# Have we been here before?
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self.db_cur.execute("""SELECT fingerprint, first_seen, last_seen, count
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FROM cert_cache
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WHERE hostname=? AND address=?""", (host, address))
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self.db_cur.execute("""SELECT fingerprint, address, first_seen, last_seen, count
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FROM cert_cache WHERE hostname=? AND port=?""", (host, port))
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cached_certs = self.db_cur.fetchall()
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# If so, check for a match
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if cached_certs:
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max_count = 0
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most_frequent_cert = None
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for cached_fingerprint, first, last, count in cached_certs:
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if count > max_count:
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max_count = count
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most_frequent_cert = cached_fingerprint
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if fingerprint == cached_fingerprint:
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# Matched!
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ui_out.debug("TOFU: Accepting previously seen ({} times) certificate {}".format(count, fingerprint))
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self.db_cur.execute("""UPDATE cert_cache
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SET last_seen=?, count=?
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WHERE hostname=? AND address=? AND fingerprint=?""",
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(now, count+1, host, address, fingerprint))
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self.db_conn.commit()
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break
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else:
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certdir = os.path.join(self.config_dir, "cert_cache")
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if _HAS_CRYPTOGRAPHY:
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# Load the most frequently seen certificate to see if it has
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# expired
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with open(os.path.join(certdir, most_frequent_cert+".crt"), "rb") as fp:
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previous_cert = fp.read()
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previous_cert = x509.load_der_x509_certificate(previous_cert, _BACKEND)
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previous_ttl = previous_cert.not_valid_after - now
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print(previous_ttl)
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ui_out.debug("TOFU: Unrecognised certificate {}! Raising the alarm...".format(fingerprint))
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print("****************************************")
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print("[SECURITY WARNING] Unrecognised certificate!")
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print("The certificate presented for {} ({}) has never been seen before.".format(host, address))
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print("This MIGHT be a Man-in-the-Middle attack.")
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print("A different certificate has previously been seen {} times.".format(max_count))
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if _HAS_CRYPTOGRAPHY:
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if previous_ttl < datetime.timedelta():
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print("That certificate has expired, which reduces suspicion somewhat.")
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else:
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print("That certificate is still valid for: {}".format(previous_ttl))
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print("****************************************")
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print("Attempt to verify the new certificate fingerprint out-of-band:")
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print(fingerprint)
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choice = input("Accept this new certificate? Y/N ").strip().lower()
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if choice in ("y", "yes"):
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self.db_cur.execute("""INSERT INTO cert_cache
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VALUES (?, ?, ?, ?, ?, ?)""",
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(host, address, fingerprint, now, now, 1))
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self.db_conn.commit()
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with open(os.path.join(certdir, fingerprint+".crt"), "wb") as fp:
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fp.write(cert)
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else:
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raise Exception("TOFU Failure!")
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# If not, cache this cert
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else:
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# If not, cache this first cert and we're done
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if not cached_certs:
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ui_out.debug("TOFU: Blindly trusting first ever certificate for this host!")
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self.db_cur.execute("""INSERT INTO cert_cache
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VALUES (?, ?, ?, ?, ?, ?)""",
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(host, address, fingerprint, now, now, 1))
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self.db_conn.commit()
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certdir = os.path.join(self.config_dir, "cert_cache")
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if not os.path.exists(certdir):
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os.makedirs(certdir)
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with open(os.path.join(certdir, fingerprint+".crt"), "wb") as fp:
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fp.write(cert)
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self.cache_new_cert(cert, host, port, address, fingerprint, now)
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return
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# If we have, check the received cert against the cache
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if self.find_cert_in_cache(host, port, fingerprint, cached_certs, now):
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return
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# Handle an unrecognised cert
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ui_out.debug("TOFU: Unrecognised certificate {}! Raising the alarm...".format(fingerprint))
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## Find the most recently seen previous cert for reporting
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most_recent = None
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for cached_fingerprint, cached_address, first, last, count in cached_certs:
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if not most_recent or last > most_recent:
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most_recent = last
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most_recent_cert = cached_fingerprint
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most_recent_address = cached_address
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most_recent_count = count
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## Report the situation
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print("****************************************")
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print("[SECURITY WARNING] Unrecognised certificate!")
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print("The certificate presented for {}:{} ({}) has never been seen before.".format(host, port, address))
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print("This MIGHT be a Man-in-the-Middle attack.")
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print("A different certificate has previously been seen {} times.".format(most_recent_count))
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if _HAS_CRYPTOGRAPHY:
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previous_ttl = self.get_cached_cert_expiry(most_recent_cert) - now
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if previous_ttl < datetime.timedelta():
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print("That certificate has expired, which reduces suspicion somewhat.")
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else:
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print("That certificate is still valid for: {}".format(previous_ttl))
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if most_recent_address == address:
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print("The new certificate is being served from the same IP address as the previous one.")
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else:
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print("The new certificate is being served from a DIFFERNET IP address as the previous one.")
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print("****************************************")
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print("Attempt to verify the new certificate fingerprint out-of-band:")
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print(fingerprint)
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## Ask the question
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if util.ask_yes_no("Accept this new certificate?"):
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self.cache_new_cert(cert, host, port, address, fingerprint, now)
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else:
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raise Exception("TOFU Failure!")
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def cache_new_cert(self, cert, host, port, address, fingerprint, now):
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"""
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Accept a new certificate for a given host/port combo.
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"""
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# Save cert to disk
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with open(os.path.join(self.certdir, fingerprint+".crt"), "wb") as fp:
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fp.write(cert)
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# Record in DB
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self.db_cur.execute("""INSERT INTO cert_cache
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(hostname, port, address, fingerprint, first_seen, last_seen, count)
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VALUES (?, ?, ?, ?, ?, ?, ?)""",
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(host, port, address, fingerprint, now, now, 1))
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self.db_conn.commit()
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def check_cert_expiry_and_names(self, cert, host, now):
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"""
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- Check the certificate Common Name or SAN matches `host`
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- Check the certificate's not valid before date is in the past
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- Check the certificate's not valid after date is in the future
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"""
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c = x509.load_der_x509_certificate(cert, _BACKEND)
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# Check certificate validity dates
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if c.not_valid_before >= now:
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raise ssl.CertificateError("Certificate not valid until: {}!".format(c.not_valid_before))
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elif c.not_valid_after <= now:
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raise ssl.CertificateError("Certificate expired as of: {})!".format(c.not_valid_after))
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# Check certificate hostnames
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names = []
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common_name = c.subject.get_attributes_for_oid(x509.oid.NameOID.COMMON_NAME)
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if common_name:
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names.append(common_name[0].value)
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try:
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names.extend([alt.value for alt in c.extensions.get_extension_for_oid(x509.oid.ExtensionOID.SUBJECT_ALTERNATIVE_NAME).value])
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except x509.ExtensionNotFound:
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pass
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names = set(names)
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for name in names:
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try:
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ssl._dnsname_match(name, host)
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break
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except Exception:
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continue
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else:
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# If we didn't break out, none of the names were valid
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raise ssl.CertificateError("Hostname does not match certificate common name or any alternative names.")
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def find_cert_in_cache(self, host, port, fingerprint, cached_certs, now):
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"""
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Try to find a cached certificate for the given host:port matching the
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given fingerprint. If one is found, update the "last seen" DB value.
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"""
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for cached_fingerprint, cached_address, first, last, count in cached_certs:
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if fingerprint == cached_fingerprint:
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# Matched!
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ui_out.debug("TOFU: Accepting previously seen ({} times) certificate {}".format(count, fingerprint))
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self.db_cur.execute("""UPDATE cert_cache
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SET last_seen=?, count=?
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WHERE hostname=? AND port=? AND fingerprint=?""",
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(now, count+1, host, port, fingerprint))
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self.db_conn.commit()
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return True
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return False
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def get_cached_cert_expiry(self, fingerprint):
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"""
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Parse the stored certificate with a given fingerprint and return its
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expiry date.
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"""
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with open(os.path.join(self.certdir, fingerprint+".crt"), "rb") as fp:
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previous_cert = fp.read()
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previous_cert = x509.load_der_x509_certificate(previous_cert, _BACKEND)
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return previous_cert.not_valid_after
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