[gnutls-devel] GnuTLS | outer AlgorithmIdentifer parameters are not verified against tbsCertificate (#698)
Development of GNU's TLS library
gnutls-devel at lists.gnutls.org
Tue Feb 5 23:22:13 CET 2019
New Issue was created.
Issue 698: https://gitlab.com/gnutls/gnutls/issues/698
Author: Tavis Ormandy
## Description of problem:
I think GnuTLS might not be conforming with [RFC5280 section 22.214.171.124](https://tools.ietf.org/html/rfc5280#section-126.96.36.199).
> This field MUST contain the same algorithm identifier as the
signature field in the sequence tbsCertificate (Section 188.8.131.52).
GnuTLS does not consider the parameters of the `signatureAlgorithm` to be part of the `AlgorithmIdentifier`, and therefore the outer `signatureAlgorithm` can be different from the `tbsCertificate` `signatureAlgorithm`. This behavior does not match the behaviour of OpenSSL and NSS who do consider the parameters part of the identifier.
I think that this isn't a security vulnerability on it's own (although, I am not a cryptographer), because most (do any?) algorithms don't specify any parameters. However, you can modify and add new objects to signed certificates and they will still verify.
That ***really*** feels like it might have significant consequences, even though I don't have the crypto skills to analyze it.
## Version of gnutls used:
## Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)
Built from source
## How reproducible:
I modified [this certificate](https://crt.sh/?id=18) to change the outer `signatureAlgorithm` to have a 1 byte BIT STRING object (rather than the RFC-required NULL object).
I could make the object larger because the signature starts with an empty byte I can truncate (maybe there are certificates with more room).
Steps to Reproduce:
* Download the attached pem certificate.
* Observe that the file is different from the original.
* Notice that gnutls still verifies the certificate.
OpenSSL does not consider it valid.
## Actual results:
$ certtool --verify-allow-broken --verify --infile AlgorithmIdentifer.parameters.pem
Loaded system trust (299 CAs available)
Subject: CN=RapidSSL CA,O=GeoTrust\, Inc.,C=US
Issuer: CN=GeoTrust Global CA,O=GeoTrust Inc.,C=US
Checked against: CN=GeoTrust Global CA,O=GeoTrust Inc.,C=US
Signature algorithm: RSA-SHA1
Output: Verified. The certificate is trusted.
Chain verification output: Verified. The certificate is trusted.
## Expected results:
$ openssl verify AlgorithmIdentifer.parameters.pem
C = US, O = "GeoTrust, Inc.", CN = RapidSSL CA
error 7 at 0 depth lookup: certificate signature failure
error AlgorithmIdentifer.parameters.pem: verification failed
If you look at the diff between the two certificates, you can see the changes I made:
--- /dev/fd/63 2019-02-05 14:13:31.409400948 -0800
+++ /dev/fd/62 2019-02-05 14:13:31.409400948 -0800
@@ -1,4 +1,4 @@
- 0:d=0 hl=4 l= 981 cons: SEQUENCE
+ 0:d=0 hl=4 l= 982 cons: SEQUENCE
4:d=1 hl=4 l= 701 cons: SEQUENCE
8:d=2 hl=2 l= 3 cons: cont [ 0 ]
10:d=3 hl=2 l= 1 prim: INTEGER :02
@@ -62,7 +62,7 @@
655:d=4 hl=2 l= 52 cons: SEQUENCE
657:d=5 hl=2 l= 8 prim: OBJECT :Authority Information Access
667:d=5 hl=2 l= 40 prim: OCTET STRING [HEX DUMP]:3026302406082B060105050730018618687474703A2F2F6F6373702E67656F74727573742E636F6D
- 709:d=1 hl=2 l= 13 cons: SEQUENCE
+ 709:d=1 hl=2 l= 14 cons: SEQUENCE
711:d=2 hl=2 l= 9 prim: OBJECT :sha1WithRSAEncryption
- 722:d=2 hl=2 l= 0 prim: NULL
- 724:d=1 hl=4 l= 257 prim: BIT STRING
+ 722:d=2 hl=2 l= 1 prim: OCTET STRING :A
+ 725:d=1 hl=4 l= 257 prim: BIT STRING
Notice I just added an OCTET STRING where the parameters are supposed to be (in fact, the RFC says it SHALL be NULL, so this is non-conforming)
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