NEWS
X41 D-Sec GmbH Security Advisory: X41-2017-001
Multiple Vulnerabilities in X.org
Overview
Vendor: X.org/Freedesktop.org
Vendor URL: https://www.x.org/wiki/
Credit: X41 D-Sec GmbH, Eric Sesterhenn
Advisory-URL: https://www.x41-dsec.de/lab/advisories/x41-2017-001-xorg/
Status: Public
Timing attack against MIT Cookie
Vulnerability Type: Other
Affected Products: Xorg Server
Attack Type: Local
Impact: Escalation of Privileges
Severity Rating: low
Confirmed Affected Version: 1.19.0 and lower
Confirmed Patched Version:
Vector: local
CVE: CVE-2017-2624
CVSS Score: 5.9
CVSS Vector: CVSS:3.0/AV:L/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N
Summary and Impact
The xorg-server uses memcmp() to check the received MIT cookie against a series of valid cookies. If the cookie is correct, it is allowed to attach to the Xorg session:
XID
MitCheckCookie(unsigned short data_length,
const char *data, ClientPtr client, const char **reason)
{
struct auth *auth;
for (auth = mit_auth; auth; auth = auth->next) {
if (data_length == auth->len &&
memcmp(data, auth->data, (int) data_length) == 0)
return auth->id;
}
*reason = "Invalid MIT-MAGIC-COOKIE-1 key";
return (XID) -1;
}
Since most memcmp() implementations return after an invalid byte is seen, this causes a time difference between a valid and invalid byte, which in theory could allow an efficient brute force attack.
Analysis
X41 was not able to measure a significant difference using the optimised memcmp() version of a standard Linux system, but for a naive implementation consisting of a loop comparing the bytes. Since timing attacks against memcmp() have been successful in the past and fixed elsewhere X41 would consider this an issue. If this would be exploited, it would allow a local attacker to run code in the Xorg session of another user.
In order to prevent this, MIT-COOKIES should be removed or a memcmp() similar to timingsafe_memcmp() used. Other projects (e.g. openssl) use timing safe memcmp() implementations to compare cookies retrieved via the network.
Workaround
None
Potential Use after Free in Xorg Server
Vulnerability Type: Other
Affected Products: Xorg Server
Attack Type: Local
Impact: -
Severity Rating: none
Confirmed Affected Version: 1.19.0 and lower
Confirmed Patched Version: -
Vector: local
CVE: -
CVSS Score: -
CVSS Vector: -
Summary and Impact
In XDM is a (currently non security) issue, regarding a potential use after free.
The ToID() function in os/auth.c is not used anywhere, just defined in the struct and filled by the protocols, but there are no users.
AuthToIDFunc ToID; /* convert cookie to ID */
X41 noticed that, XdmToID() frees the cookie argument in case it can resolve the ID or on failure, but not if it can’t allocate memory for plain:
XdmToID(unsigned short cookie_length, char *cookie)
{
XdmAuthorizationPtr auth;
XdmClientAuthPtr client;
unsigned char *plain;
plain = malloc(cookie_length);
if (!plain)
return (XID) -1;
for (auth = xdmAuth; auth; auth = auth->next) {
XdmcpUnwrap((unsigned char *) cookie, (unsigned char *) &auth->key,
plain, cookie_length);
if ((client =
XdmAuthorizationValidate(plain, cookie_length, &auth->rho, NULL,
NULL)) != NULL) {
free(client);
free(cookie);
free(plain);
return auth->id;
}
}
free(cookie);
free(plain);
return (XID) -1;
}
The same return value is given, whether no memory could be allocated or it just failed to lookup the ID, so the caller cannot distinguish whether this memory is freed or not, which might lead to double-free or memory leaks. The other ToID functions do not free this parameter.
Weak entropy usage for session keys in libxdm
Vulnerability Type: Other
Affected Products: libXdmcp
Attack Type: Local
Impact: Escalation of Privileges
Severity Rating: medium
Confirmed Affected Version: 1.1.2 and lower
Confirmed Patched Version: -
Vector: local
CVE: CVE-2017-2625
CVSS Score: 7.1
CVSS Vector: CVSS:3.0/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
Summary and Impact
To further explore the auth mechanism libXdmcp-1.1.2 was checked as well.
XDM uses weak entropy to generate the session keys on non BSD systems:
void
XdmcpGenerateKey (XdmAuthKeyPtr key)
{
#ifndef HAVE_ARC4RANDOM_BUF
long lowbits, highbits;
srandom ((int)getpid() ^ time((Time_t *)0));
lowbits = random ();
highbits = random ();
getbits (lowbits, key->data);
getbits (highbits, key->data + 4);
#else
arc4random_buf(key->data, 8);
#endif
}
On multi user systems it might possible to check the PID of the process and how long it is running to get an estimate of these values, which could allow an attacker to attach to the session of a different user. Several checked Linux distributions (debian, archlinux and Ubuntu) did not link against libbsd at the time this was found.
Workaround
Compile against libbsd.
Weak Entropy Usage in Session Keys in libICE
Vulnerability Type: Other
Affected Products: libICE
Attack Type: Local
Impact: Escalation of Privileges
Severity Rating: medium
Confirmed Affected Version: 1.0.9 and lower
Confirmed Patched Version: -
Vector: local
CVE: CVE-2017-2626
CVSS Score: 7.1
CVSS Vector: CVSS:3.0/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
Summary and Impact
libICE depends on arc4random() as well to generate the session cookies, thereby falling back to the same weak mechanism as libXdmcp:
IceGenerateMagicCookie (
int len
)
{
char *auth;
#ifndef HAVE_ARC4RANDOM_BUF
long ldata[2];
int seed;
int value;
int i;
#endif
if ((auth = malloc (len + 1)) == NULL)
return (NULL);
#ifdef HAVE_ARC4RANDOM_BUF
arc4random_buf(auth, len);
#else
#ifdef ITIMER_REAL
{
struct timeval now;
X_GETTIMEOFDAY (&now);
ldata[0] = now.tv_sec;
ldata[1] = now.tv_usec;
}
#else
{
long time ();
ldata[0] = time ((long *) 0);
ldata[1] = getpid ();
}
#endif
seed = (ldata[0]) + (ldata[1] << 16);
srand (seed);
for (i = 0; i < len; i++)
{
value = rand ();
auth[i] = value & 0xff;
}
#endif
auth[len] = '\0';
return (auth);
}
For this issue a PoC is available which takes 2-3 seconds to retrieve the key: icetest.c
Workaround
Compile against libbsd.
Weak Entropy Usage in xorg server
Vulnerability Type: Other
Affected Products: Xorg Server
Attack Type: Local
Impact: Unknown
Severity Rating: unclear
Confirmed Affected Version: 1.19.0 and lower
Confirmed Patched Version: -
Vector: local
CVE: -
CVSS Score: -
CVSS Vector: -
Summary and Impact
When looking at other places that use entropy, X41 also noticed os/auth.c and hw/xwin/winauth.c providing GenerateRandomData()
void
GenerateRandomData(int len, char *buf)
{
int fd;
fd = open("/dev/urandom", O_RDONLY);
read(fd, buf, len);
close(fd);
}
When the call to open or read fails, the contents of buf might be undefined. This should be handled more gracefully, it might fail in chrooted environments (unlikely) or when file descriptors are exhausted. Newer Linux Kernels provide getrandom() to protect against this and OpenBSD provides getentropy().
The security impact of these functions has not been further inspected.
About X41 D-Sec GmbH
X41 D-Sec GmbH is an expert provider for application security services. Having extensive industry experience and expertise in the area of information security, a strong core security team of world class security experts enables X41 to perform premium security services.
Fields of expertise in the area of application security are security centric code reviews, binary reverse engineering and vulnerability discovery. Custom research and a IT security consulting and support services are core competencies of X41.
Timeline
2017-01-17 to 01-23 Report to xorg-security@lists.x.org
2017-02-10 CVE Request at secalert@redhat.com
2017-02-13 CVE IDs assigned
2017-02-16 Information of distros@vs.openwall.org
2017-02-20 Patch for CVE-2017-2624 provided by Matthieu Herrb
2017-02-28 Advisory release