Published:
01 September 2004
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 =========================================================================== AUSCERT External Security Bulletin Redistribution ESB-2004.0551 -- MIT krb5 Security Advisory 2004-003 ASN.1 decoder denial of service 1 September 2004 =========================================================================== AusCERT Security Bulletin Summary --------------------------------- Product: MIT Kerberos 5 Publisher: MIT Impact: Denial of Service Access: Remote/Unauthenticated CVE Names: CAN-2004-0644 - --------------------------BEGIN INCLUDED TEXT-------------------- - -----BEGIN PGP SIGNED MESSAGE----- MIT krb5 Security Advisory 2004-003 Original release: 2004-08-31 Topic: ASN.1 decoder denial of service Severity: serious SUMMARY ======= The ASN.1 decoder library in the MIT Kerberos 5 distribution is vulnerable to a denial-of-service attack causing an infinite loop in the decoder. The KDC is vulnerable to this attack. IMPACT ====== * An unauthenticated remote attacker can cause a KDC or application server to hang inside an infinite loop. [CAN-2004-0644] * An attacker impersonating a legitimate KDC or application server may cause a client program to hang inside an infinite loop. [CAN-2004-0644] AFFECTED SOFTWARE ================= * KDC software and applications from MIT Kerberos 5 releases krb5-1.2.2 through krb5-1.3.4. * Applications using the MIT krb5 libraries from the above releases. FIXES ===== * The upcoming krb5-1.3.5 release will contain fixes for these problems. * Apply the appropriate patch referenced below, and rebuild the software. Patches available: * Patch against krb5-1.3.4 (should apply to earlier krb5-1.3.x releases) * Patch against krb5-1.2.8 (should apply to releases krb5-1.2.2 through krb5-1.2.7 as well) PATCH AGAINST krb5-1.3.4 ======================== * This patch was generated against krb5-1.3.4; it may apply, with some offset, to earlier krb5-1.3.x releases. This patch may also be found at: http://web.mit.edu/kerberos/advisories/2004-003-patch_1.3.4.txt The associated detached PGP signature is at: http://web.mit.edu/kerberos/advisories/2004-003-patch_1.3.4.txt.asc Index: src/lib/krb5/asn.1/asn1buf.c =================================================================== RCS file: /cvs/krbdev/krb5/src/lib/krb5/asn.1/asn1buf.c,v retrieving revision 5.24 *** src/lib/krb5/asn.1/asn1buf.c 12 Mar 2003 04:33:30 -0000 5.24 - - --- src/lib/krb5/asn.1/asn1buf.c 23 Aug 2004 03:43:47 -0000 *************** *** 122,127 **** - - --- 122,129 ---- return ASN1_OVERRUN; } while (nestlevel > 0) { + if (buf->bound - buf->next + 1 <= 0) + return ASN1_OVERRUN; retval = asn1_get_tag_2(buf, &t); if (retval) return retval; if (!t.indef) { PATCH AGAINST krb5-1.2.8 ======================== * This patch was generated against krb5-1.2.8; it may apply, with some offset, to releases krb5-1.2.2 through krb5-1.2.7. You are strongly encouraged to update to a release from the krb5-1.3.x series. This patch may also be found at: http://web.mit.edu/kerberos/advisories/2004-003-patch_1.2.8.txt The associated detached PGP signature is at: http://web.mit.edu/kerberos/advisories/2004-003-patch_1.2.8.txt.asc Index: src/lib/krb5/asn.1/asn1buf.c =================================================================== RCS file: /cvs/krbdev/krb5/src/lib/krb5/asn.1/asn1buf.c,v retrieving revision 5.19.2.1 diff -c -r5.19.2.1 asn1buf.c *** src/lib/krb5/asn.1/asn1buf.c 31 Jan 2001 18:00:12 -0000 5.19.2.1 - - --- src/lib/krb5/asn.1/asn1buf.c 23 Aug 2004 03:54:50 -0000 *************** *** 140,145 **** - - --- 140,147 ---- return ASN1_OVERRUN; } while (nestlevel > 0) { + if (buf->bound - buf->next + 1 <= 0) + return ASN1_OVERRUN; retval = asn1_get_tag_indef(buf, &class, &construction, &tagnum, &taglen, &tagindef); if (retval) return retval; REFERENCES ========== This announcement and related security advisories may be found on the MIT Kerberos security advisory page at: http://web.mit.edu/kerberos/advisories/index.html The main MIT Kerberos web page is at: http://web.mit.edu/kerberos/index.html CERT VU#550464 http://www.kb.cert.org/vuls/id/550464 CVE CAN-2004-0644 http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2004-0644 ASN.1 decoder bug in MIT Kerberos 5 releases krb5-1.2.2 through krb5-1.3.4 allows unauthenticated remote attackers to induce infinite loop, causing denial of service, including in KDC code ACKNOWLEDGMENTS =============== Thanks to Will Fiveash and Nico Williams at Sun for finding this vulnerability. DETAILS ======= The ASN.1 decoder in the MIT krb5 library handles indefinite-length BER encodings for the purpose of backwards compatibility with some non-conformant implementations. The ASN.1 decoders call asn1buf_sync() to skip any trailing unrecognized fields in the encoding of a SEQUENCE type. asn1buf_sync() calls asn1buf_skiptail() if the ASN.1 SEQUENCE type being decoded was encoded with an indefinite length. asn1buf_sync() is provided with a prefetched BER tag; a placeholder tag is provided by the prefetching code in the case where there is are no more octets in a sub-encoding. The loop in asn1buf_skiptail() which attempts to skip trailing sub-encodings of an indefinite-length SEQUENCE type does not properly check for end-of-subbuffer conditions or for the placeholder tag, leading to an infinite loop. Valid BER encodings cannot cause this condition; however, it is trivial to construct a corrupt encoding which will trigger the infinite loop. REVISION HISTORY ================ 2004-08-31 original release Copyright (C) 2004 Massachusetts Institute of Technology - -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.5 (SunOS) iQCVAwUBQTTAIKbDgE/zdoE9AQHyngP+OiwJxYxeHuhNjyXMyCr79mqJcsPP17DB tsDgQ9jZiD0m+I7rgu+PmPJQfl8qgfEZsEsW5QXppJoC0gIICSqdWbYypXjVzEfh N7g8ydTIOkKk5WP+ahisWyHiIWg/iX66dDLupzxufgb+1p/2CwoXgTszCBlQP67o 3LMSqXJGDfw= =RAVs - -----END PGP SIGNATURE----- - --------------------------END INCLUDED TEXT-------------------- You have received this e-mail bulletin as a result of your organisation's registration with AusCERT. The mailing list you are subscribed to is maintained within your organisation, so if you do not wish to continue receiving these bulletins you should contact your local IT manager. If you do not know who that is, please send an email to auscert@auscert.org.au and we will forward your request to the appropriate person. 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