FTP daemon fails to set effective group ID

Credit: Tom Yu
Risk: Medium
Local: No
Remote: Yes
CWE: CWE-264

CVSS Base Score: 6.5/10
Impact Subscore: 6.4/10
Exploitability Subscore: 8/10
Exploit range: Remote
Attack complexity: Low
Authentication: Single time
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: Partial

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 MITKRB5-SA-2011-005 MIT krb5 Security Advisory 2011-005 Original release: 2011-07-05 Topic: FTP daemon fails to set effective group ID CVE-2011-1526 CVSSv2 Vector: AV:N/AC:L/Au:S/C:P/I:P/A:P/E:H/RL:O/RC:C CVSSv2 Base Score: 6.5 Access Vector: Network Access Complexity: Low Authentication: Single Confidentiality Impact: Partial Integrity Impact: Partial Availability Impact: Partial CVSSv2 Temporal Score: 5.7 Exploitability: High Remediation Level: Official Fix Report Confidence: Confirmed SUMMARY ======= A remote client of the GSS-API FTP daemon in the krb5-appl distribution can access files using the effective group ID that the FTP daemon process had when it started. IMPACT ====== An authenticated remote user can gain unauthorized read or write access to files whose group owner is the initial effective group ID of the FTP daemon process. This is often GID 0 ("root" or "wheel"). The severity of the consequences depends heavily on the details of an individual system's configuration. Prudent system administration practices concerning file permissions will largely prevent any malicious damage from this vulnerability. No exploit code is necessary; any user able to log into the FTP server will gain unauthorized access. AFFECTED SOFTWARE ================= * The GSS-API FTP daemon in all krb5-appl releases, up to and including krb5-appl-1.0.1, is vulnerable. FIXES ===== * An upcoming release of krb5-appl will contain a fix. * Apply the following patch. You will need to run ./autogen.sh to rebuild the configure script, and you should rebuild from a clean source tree to ensure that the correct macro definitions are compiled. diff --git a/configure.ac b/configure.ac index 86e23f1..2fe68ad 100644 - --- a/configure.ac +++ b/configure.ac @@ -107,6 +107,7 @@ AC_CHECK_FUNCS(_getpty cgetent getcwd getenv gettosbyname getusershell getutmp) AC_CHECK_FUNCS(getutmpx grantpt inet_aton initgroups isatty killpg killpg) AC_CHECK_FUNCS(line_push ptsname revoke rmufile rresvport_af) AC_CHECK_FUNCS(seteuid setlogin setpgid setpriority setresuid setreuid) +AC_CHECK_FUNCS(setegid setregid setresgid) AC_CHECK_FUNCS(setutent setutsent setutxent strsave tcgetpgrp tcsetpgrp) AC_CHECK_FUNCS(ttyname unsetenv updwtmp updwtmpx utimes utmpname utmpxname) AC_CHECK_FUNCS(vhangup vsnprintf waitpid) diff --git a/gssftp/ftpd/ftpd.c b/gssftp/ftpd/ftpd.c index fe62a9c..a150819 100644 - --- a/gssftp/ftpd/ftpd.c +++ b/gssftp/ftpd/ftpd.c @@ -994,9 +994,14 @@ login(passwd, logincode) #endif } - - (void) krb5_setegid((gid_t)pw->pw_gid); - - (void) initgroups(pw->pw_name, pw->pw_gid); - - + if (krb5_setegid((gid_t)pw->pw_gid) < 0) { + reply(550, "Can't set egid."); + goto bad; + } + if (geteuid() == 0 && initgroups(pw->pw_name, pw->pw_gid) < 0) { + reply(550, "Can't initgroups"); + goto bad; + } /* open wtmp before chroot */ (void) snprintf(ttyline, sizeof(ttyline), "ftp%ld", (long) getpid()); pty_logwtmp(ttyline, pw->pw_name, rhost_sane); diff --git a/k5-util.h b/k5-util.h index 7bb8cfb..64cd53d 100644 - --- a/k5-util.h +++ b/k5-util.h @@ -69,8 +69,7 @@ #elif defined(HAVE_SETREUID) # define krb5_seteuid(EUID) setreuid(geteuid(), (uid_t)(EUID)) #else - - /* You need to add a case to deal with this operating system.*/ - -# define krb5_seteuid(EUID) (errno = EPERM, -1) +# error "You need to add a case to deal with this operating system." #endif #ifdef HAVE_SETEGID @@ -80,8 +79,7 @@ #elif defined(HAVE_SETREGID) # define krb5_setegid(EGID) (setregid(getegid(), (gid_t)(EGID))) #else - - /* You need to add a case to deal with this operating system.*/ - -# define krb5_setegid(EGID) (errno = EPERM, -1) +# error "You need to add a case to deal with this operating system." #endif #endif This patch is also available at http://web.mit.edu/kerberos/advisories/2011-005-patch.txt A PGP-signed patch is available at http://web.mit.edu/kerberos/advisories/2011-005-patch.txt.asc REFERENCES ========== This announcement is posted at: http://web.mit.edu/kerberos/advisories/MITKRB5-SA-2011-005.txt 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 CVSSv2: http://www.first.org/cvss/cvss-guide.html http://nvd.nist.gov/cvss.cfm?calculator&adv&version=2 CVE: CVE-2011-1526 http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-1526 ACKNOWLEDGMENTS =============== We thank Tim Zingelman for discovering this vulnerability and providing a preliminary patch. CONTACT ======= The MIT Kerberos Team security contact address is <krbcore-security (at) mit (dot) edu [email concealed]>. When sending sensitive information, please PGP-encrypt it using the following key: pub 2048R/56CD8F76 2010-12-29 [expires: 2012-02-01] uid MIT Kerberos Team Security Contact <krbcore-security (at) mit (dot) edu [email concealed]> DETAILS ======= The vulnerability results from two interacting flaws: omission of required autoconf tests, causing krb5_setegid() to always fail, and the FTP daemon's failure to check for the successful execution of krb5_setegid(). The FTP daemon calls the portability macro krb5_setegid() from k5-util.h, which is intended to wrap or emulate the POSIX interface setegid(). The definition of the macro depends on macros that the autoconf configure script defines (based on tests of the target platform environment) when it runs. When the krb5 application programs moved out of the main krb5 source tree, the new configure script inadvertently omitted the necessary autoconf tests for setegid() and related legacy interfaces. If no setegid() equivalent appears to exist on the system, k5-util.h defines krb5_setegid() to always fail with errno EPERM. Since the relevant autoconf tests never execute, k5-util.h will always define krb5_setegid() to fail. The FTP daemon does not check the return value of krb5_setegid(), so it silently fails to set its effective GID, allowing users to gain unauthorized access using the effective GID that the daemon process started with. REVISION HISTORY ================ 2011-07-05 original release Copyright (C) 2011 Massachusetts Institute of Technology -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.8 (SunOS) iEYEARECAAYFAk4TT7IACgkQSO8fWy4vZo7ttQCglKWLu47znBVfeGXl3LlnZtF2 llIAoNgVuCYIdVWcMEJaZ0oBp9RxhkzJ =RUE/ -----END PGP SIGNATURE-----



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