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DoS (nullptr dereference) 1/2 with CRLs in mod_tls of ProFTPD < 1.3.6 #858

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debrouxl opened this issue Nov 4, 2019 · 2 comments

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@debrouxl
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@debrouxl debrouxl commented Nov 4, 2019

For tracking purposes (this problem is fixed in 1.3.6+).
This is the 1st of 4 bugs in the tls_verify_crl() function. It's a direct nullptr dereference: variable initialized to NULL, then dereferenced. I fixed it on my side, then I noticed that it had been fixed upstream in 1.3.6. From the 1.3.5 -> 1.3.6 branches diff:

@@ -5899,14 +8979,15 @@ static int tls_verify_crl(int ok, X509_S
       X509_CRL *crl = NULL;
       EVP_PKEY *pubkey;
       char buf[512];
       int len;
       BIO *b = BIO_new(BIO_s_mem());
 
+      crl = sk_X509_CRL_value(crls, i);
       BIO_printf(b, "CA CRL: Issuer: ");
       X509_NAME_print(b, issuer, 0);
 
       BIO_printf(b, ", lastUpdate: ");
 #if OPENSSL_VERSION_NUMBER >= 0x10100000L
       ASN1_UTCTIME_print(b, X509_CRL_get_lastUpdate(crl));
 #else
       ASN1_UTCTIME_print(b, crl->crl->lastUpdate);

I'd have expected this kind of issues to be caught by static analysis.

Additionally, on my side, I used the following change:

-      X509_NAME_print(b, issuer, 0);
+      X509_NAME_print(b, X509_CRL_get_issuer(crl), 0); // Just in case...

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 .

My colleagues hit this crash in the summer of 2018 when dealing with TLS CRLs using CentOS 7's ProFTPD 1.3.5e package against OpenSSL 1.0.2*, and then I was tasked to debug the issue.
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.

@debrouxl

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@debrouxl debrouxl commented Nov 26, 2019

CVE-2019-19272 .

@Castaglia

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@Castaglia Castaglia commented Nov 27, 2019

The 1.3.5 release for ProFTPD is no longer supported; see the Versioning doc.

@Castaglia Castaglia closed this Nov 27, 2019
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