Fix base64d() buffer size (CVE-2018-6789)

Credits for discovering this bug: Meh Chang <meh@devco.re>

(cherry picked from commit 062990cc1b2f9e5d82a413b53c8f0569075de700)

1 files changed, 6 insertions, 2 deletions

diff --git a/src/src/base64.c b/src/src/base64.c
index ae6874b8a..1d84c1e5c 100644
--- a/src/src/base64.c
+++ b/src/src/base64.c
@@ -152,10 +152,14 @@ static uschar dec64table[] = {
 int
 b64decode(const uschar *code, uschar **ptr)
 {
+
 int x, y;
-uschar *result = store_get(3*(Ustrlen(code)/4) + 1);
+uschar *result;
 
-*ptr = result;
+{
+  int l = Ustrlen(code);
+  *ptr = result = store_get(1 + l/4 * 3 + l%4);
+}
 
 /* Each cycle of the loop handles a quantum of 4 input bytes. For the last
 quantum this may decode to 1, 2, or 3 output bytes. */