Subject: [PATCH] Fix use after free on subsequenct erroneous calls

From: Tobias Stoeckmann <[email-masked]>
Date: Mon, 21 Nov 2016 20:39:38 +0100

The function IceAuthFileName is vulnerable to a use after free. The
flaw can be triggered by calling the function three times:

- First call succeeds and stores the path in buf, a dynamically
  allocated buffer with size bsize.
- Second call fails due to out of memory. It frees buf, but keeps
  the old size in bsize.
- Third call only checks if bsize is large enough. Then it uses
  buf without allocating it again -- the use after free happens.

In order to exploit this, an attacker must change environment variables
between each call, namely ICEAUTHORITY or HOME. It also takes subsequent
calls. Due to these limitations, I don't consider this to be of high
priority.
---
 src/authutil.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/authutil.c b/src/authutil.c
index 04c0791..ca0504a 100644
--- a/src/authutil.c
+++ b/src/authutil.c
@@ -114,8 +114,10 @@ IceAuthFileName (void)
 	if (buf)
 	    free (buf);
 	buf = malloc (size);
-	if (!buf)
+	if (!buf) {
+	    bsize = 0;
 	    return (NULL);
+	}
 	bsize = size;
     }
 
-- 
2.10.2