[UNIX] EFingerd Remote Buffer Overflow
From: support@securiteam.comDate: 03/09/02
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From: support@securiteam.com To: list@securiteam.com Date: Sat, 9 Mar 2002 15:16:04 +0100 (CET)
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EFingerd Remote Buffer Overflow
------------------------------------------------------------------------
SUMMARY
Efingerd is a "finger daemon, giving you complete control over what are
you going to display about your computer" as is written in the man page.
However this is not completely true, as any local user can (even
unintentionally) expose more info than was originally intended by an
administrator. And more problematic is the fact that the product contains
an exploitable buffer overflow.
DETAILS
Vulnerable systems:
Debian Linux distributes versions 1.3 (stable) and 1.6.1 (unstable).
Remote buffer overflow
In the stable version it is possible to remotely cause a buffer overflow
condition through an exploitation of a reverse-lookup part of the code:
static char *lookup_addr (struct in_addr in)
{
static char addr[100];
struct hostent *he;
if (resolve_addr) {
he = gethostbyaddr ((char *)&in, sizeof(struct
in_addr),AF_INET);
if (he == NULL)
strcpy(addr, inet_ntoa(in));
else
strcpy(addr, he->h_name);
}
else
strcpy (addr, inet_ntoa (in));
return addr;
}
Usually efingerd runs as 'nobody'.
Dangerous feature
But there is another security issue with efingerd (in both versions). When
some existing user is fingered, efingerd looks for a ".efingerd" file in
that user's home directory and if it does exist and it is executable it
tries to execute it - as 'nobody'. The .efingerd's output is sent back to
the fingerer.
So whatever a local user puts in his .efingerd file can be executed under
nobody UID/GID simply by fingering himself. So getting a user nobody and
group nobody shell is straightforward. This can be very interesting for a
potential evildoer going to hide his identity during some nasty actions,
for example local DoS attacks. As the log file is writable by the UID of
efingerd, it can be easily manipulated.
This feature can be turned off with the -u option.
ADDITIONAL INFORMATION
The information has been provided by <mailto:spybreak@host.sk> Spybreak.
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