oss-security - CVE Request: lighttpd/mod_auth out-of-bounds read due to signedness error

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Message-ID: <4ED4DD48.3010403@lighttpd.net>
Date: Tue, 29 Nov 2011 14:25:28 +0100
From: Stefan Bühler <stbuehler@...httpd.net>
To: oss-security@...ts.openwall.com
CC: security@...httpd.net, Xi Wang <xi.wang@...il.com>
Subject: CVE Request: lighttpd/mod_auth out-of-bounds read due to signedness
 error

Hi,

Xi Wang discovered the following issue in lighttpd:

for http auth we need to base64-decode user input; the allowed character 
range includes non ASCII characters above 0x7f.

The function to decode this string takes a "const char *in"; and reads
each character into an "int ch", which is used as offset in the table.

So characters above 0x7f lead to negative indices (as char is signed on 
most platforms).

Here the vulnerable code (src/http_auth.c:67)

---
static const short base64_reverse_table[256] = ...;
static unsigned char * base64_decode(buffer *out, const char *in) {
	...
	int ch, ...;
	size_t i;
	...

		ch = in[i];
		...
		ch = base64_reverse_table[ch];
	...
}
---

It doesn't matter if "broken" data is read - it just may allow more
encodings of the correct login information.

The only possible impact is a segfault, leading to DoS.

I had a look at some debian and openSUSE binaryies, and it looks like 
there is always enough data (>= 256 bytes) in the .rodata section 
before the base64_reverse_table table, so these binaries are not 
vulnerable afaict.

we plan to release 1.4.30 soon, including the fix for this issue.

regards,
stefan

bug tracked as:
   http://redmine.lighttpd.net/issues/2370
announcement (not complete yet):
   http://download.lighttpd.net/lighttpd/security/lighttpd_sa_2011_01.txt

proposed patch
===
diff --git a/src/http_auth.c b/src/http_auth.c
index f2f86dd..33adf71 100644
--- a/src/http_auth.c
+++ b/src/http_auth.c
@@ -99,7 +99,7 @@ static unsigned char * base64_decode(buffer *out, 
const char *in) {
  	ch = in[0];
  	/* run through the whole string, converting as we go */
  	for (i = 0; i < in_len; i++) {
-		ch = in[i];
+		ch = (unsigned char) in[i];

  		if (ch == '\0') break;

===

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