SECURITY: DKIM DNS buffer overflow protection

CVE-2012-5671

malloc/heap overflow, with a 60kB window of overwrite.
Requires DNS under control of person sending email, leaves plenty of
evidence, but is very likely exploitable on OSes that have not been
well hardened.

3 files changed, 13 insertions, 2 deletions

diff --git a/doc/doc-txt/ChangeLog b/doc/doc-txt/ChangeLog
index 6c0554b5a..bc2fbc671 100644
--- a/doc/doc-txt/ChangeLog
+++ b/doc/doc-txt/ChangeLog
@@ -1,6 +1,14 @@
 Change log file for Exim from version 4.21
 -------------------------------------------
 
+Exim version 4.80.1
+-------------------
+
+PP/01 SECURITY: protect DKIM DNS decoding from remote exploit.
+      CVE-2012-5671
+      This, or similar/improved, will also be change PP/11 of 4.81.
+
+
 Exim version 4.80
 -----------------
 
diff --git a/src/src/dkim.c b/src/src/dkim.c
index 87e91dea2..05b5fec56 100644
--- a/src/src/dkim.c
+++ b/src/src/dkim.c
@@ -42,6 +42,9 @@ int dkim_exim_query_dns_txt(char *name, char *answer) {
                "%.*s", (int)len, (char *)((rr->data)+rr_offset));
       rr_offset+=len;
       answer_offset+=len;
+      if (answer_offset >= PDKIM_DNS_TXT_MAX_RECLEN) {
+        return PDKIM_FAIL;
+      }
     }
   }
   else return PDKIM_FAIL;
diff --git a/src/src/pdkim/pdkim.h b/src/src/pdkim/pdkim.h
index 764cc83be..1d364a3c9 100644
--- a/src/src/pdkim/pdkim.h
+++ b/src/src/pdkim/pdkim.h
@@ -27,8 +27,8 @@
 
 /* -------------------------------------------------------------------------- */
 /* Length of the preallocated buffer for the "answer" from the dns/txt
-   callback function. */
-#define PDKIM_DNS_TXT_MAX_RECLEN    4096
+   callback function. This should match the maximum RDLENGTH from DNS. */
+#define PDKIM_DNS_TXT_MAX_RECLEN    (1 << 16)
 
 /* -------------------------------------------------------------------------- */
 /* Function success / error codes */