Fix mbedtls fingerprint calculation

Commit 'Migrate to mbed TLS 2.x' (86d8cd68) introduced a bug in mbedtls
builds where we would calculate the certificate fingerprint over the
(too-short) 'to-be-signed' length of the certificate, rather than over the
certificate including the signature.  Fix that.

The security impact of the incorrect calculation is very minimal; the last
few bytes (max 4, typically 4) are not verified by the fingerprint.  We
expect no real-world impact, because users that used this feature before
will notice that it has suddenly stopped working, and users that didn't
will notice that connection setup fails.

Even if the user managed to somehow extract the incorrect hash (e.g. by
reading out the tls_digest_* env vars using a --tls-verify script), the
impact is miminal: the last 4 bytes must still be properly signed by the
CA, and typically contain extension fields, or the last bytes of the
public key (which are hard to choose).  The most important bits of the
certificate were always checked: the version, serial, signature algorithm,
issuer, validity and subject.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1495285075-4957-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14711.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/Changes.rst b/Changes.rst
index fbe0fc46f9a02d65145746159891f2733cac1930..94cba0703695060f94087de29febc58bbdc66c40 100644 (file)
--- a/Changes.rst
+++ b/Changes.rst
@@ -305,10 +305,26 @@ Maintainer-visible changes
 
 Version 2.4.3
 =============
+
+User-visible Changes
+--------------------
 - ``--verify-hash`` can now take an optional flag which changes the hashing
   algorithm. It can be either SHA1 or SHA256.  The default if not provided is
   SHA1 to preserve backwards compatibility with existing configurations.
 
+Bugfixes
+--------
+- Fix fingerprint calculation in mbed TLS builds.  This means that mbed TLS users
+  of OpenVPN 2.4.0, 2.4.1 and 2.4.2 that rely on the values of the
+  ``tls_digest_*`` env vars, or that use `--verify-hash` will have to change
+  the fingerprint values they check against.  The security impact of the
+  incorrect calculation is very minimal; the last few bytes (max 4, typically
+  4) are not verified by the fingerprint.  We expect no real-world impact,
+  because users that used this feature before will notice that it has suddenly
+  stopped working, and users that didn't will notice that connection setup
+  fails if they specify correct fingerprints.
+
+
 Version 2.4.1
 =============
 - ``--remote-cert-ku`` now only requires the certificate to have at least the

diff --git a/src/openvpn/ssl_verify_mbedtls.c b/src/openvpn/ssl_verify_mbedtls.c
index 27c5c3e149d24f16cc67c5784d4d05e8cdebeb48..2b7056c830b725436cd23b0a4df32a5d6b370457 100644 (file)
--- a/src/openvpn/ssl_verify_mbedtls.c
+++ b/src/openvpn/ssl_verify_mbedtls.c
@@ -208,7 +208,7 @@ x509_get_fingerprint(const mbedtls_md_info_t *md_info, mbedtls_x509_crt *cert,
 {
     const size_t md_size = mbedtls_md_get_size(md_info);
     struct buffer fingerprint = alloc_buf_gc(md_size, gc);
-    mbedtls_md(md_info, cert->raw.p, cert->tbs.len, BPTR(&fingerprint));
+    mbedtls_md(md_info, cert->raw.p, cert->raw.len, BPTR(&fingerprint));
     ASSERT(buf_inc_len(&fingerprint, md_size));
     return fingerprint;
 }