DoS (nullptr dereference) 2/2 with CRLs in mod_tls of ProFTPD master HEAD

[debrouxl]

This is the 4th of 4 bugs in the tls_verify_crl() function. The code fails to take into account an empty CRL, for which sk_X509_REVOKED_value() returned NULL in my tests. It proceeds to dereferencing the NULL pointer, crashing the application.

My patch is as follows:

@@ -6005,7 +6014,8 @@
         ASN1_INTEGER *sn;
 
         revoked = sk_X509_REVOKED_value(X509_CRL_get_REVOKED(crl), j);
+        if (!revoked) continue;
 #if OPENSSL_VERSION_NUMBER >= 0x10100000L
         sn = X509_REVOKED_get0_serialNumber(revoked);
 #else
         sn = revoked->serialNumber;

Both of the other code bases which I noticed were getting the issuer CRL lookup right (second bug, issue #859) fail to check the return value against NULL as well:
https://github.com/CESNET/libnetconf2/blob/master/src/session_server_tls.c
https://github.com/copiousfreetime/stunnel/blob/master/src/verify.c (outdated stunnel < 5.24)

FWIW, 4 years ago, stunnel got rid of custom CRL handling code and started relying on OpenSSL's built-in handling instead. That was between 5.23 and 5.24, compare src/verify.c from https://www.usenix.org.uk/mirrors/stunnel/archive/5.x/stunnel-5.23.tar.gz and https://www.usenix.org.uk/mirrors/stunnel/archive/5.x/stunnel-5.24.tar.gz .

I hit this crash in the summer of 2018, after fixing the first crash (issue #858) when dealing with TLS CRLs using CentOS 7's ProFTPD 1.3.5e package against OpenSSL 1.0.2*.
I quickly reported the issues privately, but ProFTPD's TLS CRL handling remains broken on all branches more than a year later... I'm aware that TLS CRLs are highly unpopular, and that only system administrators are supposed to define them, but clearly, low-profile responsible disclosure didn't work here :)
Public reports, and CVE ID assignments (for which I'll use this issue as reference), piling onto the recent higher risk issue #846 (CVE-2019-18217) and older vulnerabilities should help the downstream propagation of all fixes, at least if any downstream provides security support for ProFTPD.

[Castaglia]

Fixed in master, and backported to the 1.3.6 branch. Thanks!

[debrouxl]

CVE-2019-19269 .