(Initially reported as SITIC Vulnerability Advisory SA04-001, redefined as bug 
after discussion with ASF httpd security team)

Apache's mod_ssl module suffers from a format string bug when logging
information about CRLs. If an administrator installs a malicious CRL file,
this bug can lead to the execution of arbitrary code.

The function ssl_callback_SSLVerify_CRL() in modules/ssl/ssl_engine_kernel.c
calls ap_log_error() with data from the CRL as the format string instead of
using the data as parameters, leading to a security breach.

This bug was discovered by Ulf Harnhammar for SITIC, Swedish IT 
Incident Centre.

The included patch "issue1.patch" is our attempt at correcting this issue:

--- modules/ssl/ssl_engine_kernel.c	2004-06-07 12:18:37.000000000 +0200
+++ modules/ssl/ssl_engine_kernel.c.ulf	2004-08-02 12:49:18.000000000 +0200
@@ -1372,7 +1372,7 @@
 
             BIO_free(bio);
 
-            ap_log_error(APLOG_MARK, APLOG_DEBUG, 0, s, buff);
+            ap_log_error(APLOG_MARK, APLOG_DEBUG, 0, s, "%s", buff);
         }
 
         /*