Comment by mykhal

from test/verify_extra_test.c:

    Test for CVE-2015-1793 (Alternate Chains Certificate Forgery)
   
    Chain is as follows:
   
    rootCA (self-signed)
      |
    interCA
      |
    subinterCA       subinterCA (self-signed)
      |                   |
    leaf ------------------
      |
    bad
   
    rootCA, interCA, subinterCA, subinterCA (ss) all have CA=TRUE
    leaf and bad have CA=FALSE
   
    subinterCA and subinterCA (ss) have the same subject name and keys
   
    interCA (but not rootCA) and subinterCA (ss) are in the trusted store
    (roots.pem)
    leaf and subinterCA are in the untrusted list (untrusted.pem)
    bad is the certificate being verified (bad.pem)
   
    Versions vulnerable to CVE-2015-1793 will fail to detect that leaf has
    CA=FALSE, and will therefore incorrectly verify bad