openssh-5.0p1-blacklist.patch

   1 --- openssh-5.0p1/Makefile.in
   2 +++ openssh-5.0p1/Makefile.in
   3 @@ -62,7 +62,7 @@
   4
   5  TARGETS=ssh$(EXEEXT) sshd$(EXEEXT) ssh-add$(EXEEXT) ssh-keygen$(EXEEXT) ssh-keyscan${EXEEXT} ssh-keysign${EXEEXT} ssh-agent$(EXEEXT) scp$(EXEEXT) ssh-rand-helper${EXEEXT} sftp-server$(EXEEXT) sftp$(EXEEXT)
   6
   7 -LIBSSH_OBJS=acss.o authfd.o authfile.o bufaux.o bufbn.o buffer.o \
   8 +LIBSSH_OBJS=acss.o authfd.o authfile.o blacklist.o bufaux.o bufbn.o buffer.o \
   9         canohost.o channels.o cipher.o cipher-acss.o cipher-aes.o \
  10         cipher-bf1.o cipher-ctr.o cipher-3des1.o cleanup.o \
  11         compat.o compress.o crc32.o deattack.o fatal.o hostfile.o \
  12 --- openssh-5.0p1/auth-rh-rsa.c
  13 +++ openssh-5.0p1/auth-rh-rsa.c
  14 @@ -34,6 +34,7 @@
  15  #include "ssh-gss.h"
  16  #endif
  17  #include "monitor_wrap.h"
  18 +#include "blacklist.h"
  19
  20  /* import */
  21  extern ServerOptions options;
  22 @@ -48,6 +49,9 @@
  23         if (!auth_rhosts(pw, cuser))
  24                 return 0;
  25
  26 +       if (blacklisted_key(client_host_key, 0))
  27 +               return 0;
  28 +
  29         host_status = check_key_in_hostfiles(pw, client_host_key,
  30             chost, _PATH_SSH_SYSTEM_HOSTFILE,
  31             options.ignore_user_known_hosts ? NULL : _PATH_SSH_USER_HOSTFILE);
  32 --- openssh-5.0p1/auth-rsa.c
  33 +++ openssh-5.0p1/auth-rsa.c
  34 @@ -47,6 +47,7 @@
  35  #include "monitor_wrap.h"
  36  #include "ssh.h"
  37  #include "misc.h"
  38 +#include "blacklist.h"
  39
  40  /* import */
  41  extern ServerOptions options;
  42 @@ -265,6 +272,9 @@
  43                             "actual %d vs. announced %d.",
  44                             file, linenum, BN_num_bits(key->rsa->n), bits);
  45
  46 +               if (blacklisted_key(key, 0))
  47 +                       continue;
  48 +
  49                 /* We have found the desired key. */
  50                 /*
  51                  * If our options do not allow this key to be used,
  52 --- openssh-5.0p1/auth2-hostbased.c
  53 +++ openssh-5.0p1/auth2-hostbased.c
  54 @@ -47,6 +47,7 @@
  55  #endif
  56  #include "monitor_wrap.h"
  57  #include "pathnames.h"
  58 +#include "blacklist.h"
  59
  60  /* import */
  61  extern ServerOptions options;
  62 @@ -145,6 +146,9 @@
  63         HostStatus host_status;
  64         int len;
  65
  66 +       if (blacklisted_key(key, 0))
  67 +               return 0;
  68 +
  69         resolvedname = get_canonical_hostname(options.use_dns);
  70         ipaddr = get_remote_ipaddr();
  71
  72 --- openssh-5.0p1/auth2-pubkey.c
  73 +++ openssh-5.0p1/auth2-pubkey.c
  74 @@ -52,6 +52,7 @@
  75  #endif
  76  #include "monitor_wrap.h"
  77  #include "misc.h"
  78 +#include "blacklist.h"
  79
  80  /* import */
  81  extern ServerOptions options;
  82 @@ -272,6 +273,9 @@
  83         int success;
  84         char *file;
  85
  86 +       if (blacklisted_key(key, 0))
  87 +               return 0;
  88 +
  89         file = authorized_keys_file(pw);
  90         success = user_key_allowed2(pw, key, file);
  91         xfree(file);
  92 new file mode 100644
  93 --- /dev/null
  94 +++ openssh-5.0p1/blacklist.c
  95 @@ -0,0 +1,267 @@
  96 +/*
  97 + * Support for RSA/DSA key blacklisting based on partial fingerprints,
  98 + * developed under Openwall Project for Owl - http://www.openwall.com/Owl/
  99 + *
 100 + * Copyright (c) 2008 Dmitry V. Levin <ldv at cvs.openwall.com>
 101 + *
 102 + * Permission to use, copy, modify, and distribute this software for any
 103 + * purpose with or without fee is hereby granted, provided that the above
 104 + * copyright notice and this permission notice appear in all copies.
 105 + *
 106 + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 107 + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 108 + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 109 + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 110 + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 111 + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 112 + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 113 + *
 114 + * The blacklist encoding was designed by Solar Designer and Dmitry V. Levin.
 115 + * No intellectual property rights to the encoding scheme are claimed.
 116 + *
 117 + * This effort was supported by CivicActions - http://www.civicactions.com
 118 + *
 119 + * The file size to encode 294,903 of 48-bit fingerprints is just 1.3 MB,
 120 + * which corresponds to less than 4.5 bytes per fingerprint.
 121 + */
 122 +
 123 +#include "includes.h"
 124 +#include <string.h>
 125 +#include <unistd.h>
 126 +#include <errno.h>
 127 +#include <fcntl.h>
 128 +
 129 +#include "atomicio.h"
 130 +#include "blacklist.h"
 131 +#include "canohost.h"
 132 +#include "log.h"
 133 +#include "pathnames.h"
 134 +#include "servconf.h"
 135 +#include "xmalloc.h"
 136 +
 137 +extern ServerOptions options;
 138 +
 139 +typedef struct
 140 +{
 141 +       /* format version identifier */
 142 +       char    version[8];
 143 +       /* index size, in bits */
 144 +       uint8_t index_size;
 145 +       /* offset size, in bits */
 146 +       uint8_t offset_size;
 147 +       /* record size, in bits */
 148 +       uint8_t record_bits;
 149 +       /* number of records */
 150 +       uint8_t records[3];
 151 +       /* offset shift */
 152 +       uint8_t shift[2];
 153 +
 154 +} __attribute__((packed)) blacklist_header;
 155 +
 156 +static unsigned
 157 +c2u(uint8_t c)
 158 +{
 159 +       return (c >= 'a') ? (c - 'a' + 10) : (c - '0');
 160 +}
 161 +
 162 +static blacklist_error_t
 163 +validate_blacklist(const char *fname, int fd, unsigned *bytes,
 164 +                  unsigned *records, unsigned *shift)
 165 +{
 166 +       unsigned expected;
 167 +       struct stat st;
 168 +       blacklist_header header;
 169 +
 170 +       if (fstat(fd, &st)) {
 171 +               error("fstat for blacklist file %s failed: %m", fname);
 172 +               return BLACKLIST_ERROR_ACCESS;
 173 +       }
 174 +
 175 +       if (atomicio(read, fd, &header, sizeof(header)) != sizeof(header)) {
 176 +               error("read blacklist file %s header failed: %m", fname);
 177 +               return BLACKLIST_ERROR_ACCESS;
 178 +       }
 179 +
 180 +       if (memcmp(header.version, "SSH-FP", 6)) {
 181 +               error("blacklist file %s has unrecognized format", fname);
 182 +               return BLACKLIST_ERROR_FORMAT;
 183 +       }
 184 +
 185 +       if (header.index_size != 16 || header.offset_size != 16 ||
 186 +           memcmp(header.version, "SSH-FP00", 8)) {
 187 +               error("blacklist file %s has unsupported format", fname);
 188 +               return BLACKLIST_ERROR_VERSION;
 189 +       }
 190 +
 191 +       *bytes = (header.record_bits >> 3) - 2;
 192 +       *records =
 193 +               (((header.records[0] << 8) +
 194 +                 header.records[1]) << 8) + header.records[2];
 195 +       *shift = (header.shift[0] << 8) + header.shift[1];
 196 +
 197 +       expected = sizeof(header) + 0x20000 + (*records) * (*bytes);
 198 +       if (st.st_size != expected) {
 199 +               error("blacklist file %s size mismatch: "
 200 +                     "expected size %u, found size %lu",
 201 +                     fname, expected, (unsigned long) st.st_size);
 202 +               return BLACKLIST_ERROR_ACCESS;
 203 +       }
 204 +
 205 +       return BLACKLIST_ERROR_NONE;
 206 +}
 207 +
 208 +static int
 209 +expected_offset(uint16_t index, uint16_t shift, unsigned records)
 210 +{
 211 +       return ((index * (long long) records) >> 16) - shift;
 212 +}
 213 +
 214 +static int
 215 +xlseek(const char *fname, int fd, unsigned seek)
 216 +{
 217 +       if (lseek(fd, seek, SEEK_SET) != seek) {
 218 +               error("lseek for blacklist file %s failed: %m", fname);
 219 +               return BLACKLIST_ERROR_ACCESS;
 220 +       }
 221 +       return BLACKLIST_ERROR_NONE;
 222 +}
 223 +
 224 +static blacklist_error_t
 225 +check(const char *fname, int fd, const char *s)
 226 +{
 227 +       unsigned bytes, records, shift;
 228 +       unsigned num, i, j;
 229 +       int     off_start, off_end;
 230 +       blacklist_error_t rc;
 231 +       uint16_t index;
 232 +       /* max number of bytes stored in record_bits, minus two bytes used for index */
 233 +       uint8_t buf[(0xff >> 3) - 2];
 234 +
 235 +       if ((rc = validate_blacklist(fname, fd, &bytes, &records, &shift)))
 236 +               return rc;
 237 +
 238 +       index = (((((c2u(s[0]) << 4) | c2u(s[1])) << 4) |
 239 +                 c2u(s[2])) << 4) | c2u(s[3]);
 240 +       if (xlseek(fname, fd, sizeof(blacklist_header) + index * 2))
 241 +               return BLACKLIST_ERROR_ACCESS;
 242 +
 243 +       if (atomicio(read, fd, buf, 4) != 4) {
 244 +               error("read blacklist file %s offsets failed: %m", fname);
 245 +               return BLACKLIST_ERROR_ACCESS;
 246 +       }
 247 +
 248 +       off_start = (buf[0] << 8) + buf[1] +
 249 +               expected_offset(index, shift, records);
 250 +       if (off_start < 0 || (unsigned) off_start > records) {
 251 +               error("blacklist file %s off_start overflow [%d] for index %#x",
 252 +                     fname, off_start, index);
 253 +               return BLACKLIST_ERROR_ACCESS;
 254 +       }
 255 +       if (index < 0xffff) {
 256 +               off_end = (buf[2] << 8) + buf[3] +
 257 +                       expected_offset(index + 1, shift, records);
 258 +               if (off_end < off_start || (unsigned) off_end > records) {
 259 +                       error("blacklist file %s off_end overflow [%d] for index %#x",
 260 +                             fname, off_end, index);
 261 +                       return BLACKLIST_ERROR_ACCESS;
 262 +               }
 263 +       } else
 264 +               off_end = records;
 265 +
 266 +       if (xlseek(fname, fd,
 267 +                  sizeof(blacklist_header) + 0x20000 + off_start * bytes))
 268 +               return BLACKLIST_ERROR_ACCESS;
 269 +
 270 +       num = off_end - off_start;
 271 +       for (i = 0; i < num; ++i) {
 272 +               if (atomicio(read, fd, buf, bytes) != bytes) {
 273 +                       error("read blacklist file %s fingerprints failed: %m",
 274 +                             fname);
 275 +                       return BLACKLIST_ERROR_ACCESS;
 276 +               }
 277 +
 278 +               for (j = 0; j < bytes; ++j)
 279 +                       if (((c2u(s[4 + j * 2]) << 4) | c2u(s[5 + j * 2])) !=
 280 +                           buf[j])
 281 +                               break;
 282 +               if (j >= bytes) {
 283 +                       debug("blacklisted fingerprint: %s offset=%u, number=%u",
 284 +                             s, off_start, i);
 285 +                       return BLACKLIST_ERROR_ALL;
 286 +               }
 287 +       }
 288 +
 289 +       debug("non-blacklisted fingerprint: %s offset=%u, number=%u",
 290 +             s, off_start, num);
 291 +       return BLACKLIST_ERROR_NONE;
 292 +}
 293 +
 294 +static blacklist_error_t
 295 +blacklisted_fingerprint(const char *hex)
 296 +{
 297 +       int     fd = -1;
 298 +       blacklist_error_t rc = BLACKLIST_ERROR_ACCESS;
 299 +       const char *fname = _PATH_BLACKLIST;
 300 +       char   *s, *p;
 301 +
 302 +       debug("Checking fingerprint %s using blacklist file %s", hex, fname);
 303 +
 304 +       s = xstrdup(hex);
 305 +       for (p = s; *hex; ++hex)
 306 +               if (*hex != ':')
 307 +                       *p++ = *hex;
 308 +       *p = '\0';
 309 +
 310 +       if (strlen(s) != 32 || strlen(s) != strspn(s, "0123456789abcdef")) {
 311 +               error("%s: invalid fingerprint", s);
 312 +               goto out;
 313 +       }
 314 +
 315 +       if ((fd = open(fname, O_RDONLY)) < 0) {
 316 +               if (ENOENT == errno) {
 317 +                       rc = BLACKLIST_ERROR_MISSING;
 318 +                       verbose("open blacklist file %s failed: %m", fname);
 319 +               } else
 320 +                       logit("open blacklist file %s failed: %m", fname);
 321 +               goto out;
 322 +       }
 323 +
 324 +       rc = check(fname, fd, s);
 325 +
 326 +out:
 327 +       close(fd);
 328 +       xfree(s);
 329 +       return rc;
 330 +}
 331 +
 332 +int
 333 +blacklisted_key(Key *key, int hostkey)
 334 +{
 335 +       int     rc;
 336 +       const char *text;
 337 +       char   *fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
 338 +
 339 +       switch ((rc = blacklisted_fingerprint(fp))) {
 340 +               case BLACKLIST_ERROR_NONE:
 341 +                       break;
 342 +               case BLACKLIST_ERROR_ALL:
 343 +                       text = (options.ignore_blacklist_errors == rc) ?
 344 +                              "Permitted" : "Rejected";
 345 +                       if (hostkey)
 346 +                               logit("%s blacklisted host key %s", text, fp);
 347 +                       else
 348 +                               logit("%s blacklisted public key %s from %.100s",
 349 +                                     text, fp, get_remote_ipaddr());
 350 +                       break;
 351 +               default:
 352 +                       if (hostkey)
 353 +                               logit("Unable to check blacklist for host key %s",
 354 +                                     fp);
 355 +                       else
 356 +                               logit("Unable to check blacklist for public key %s from %.100s",
 357 +                                     fp, get_remote_ipaddr());
 358 +       }
 359 +
 360 +       xfree(fp);
 361 +       return (rc > options.ignore_blacklist_errors);
 362 +}
 363 new file mode 100644
 364 --- /dev/null
 365 +++ openssh-5.0p1/blacklist.h
 366 @@ -0,0 +1,37 @@
 367 +/*
 368 + * Support for RSA/DSA key blacklisting based on partial fingerprints,
 369 + * developed under Openwall Project for Owl - http://www.openwall.com/Owl/
 370 + *
 371 + * Copyright (c) 2008 Dmitry V. Levin <ldv at cvs.openwall.com>
 372 + *
 373 + * Permission to use, copy, modify, and distribute this software for any
 374 + * purpose with or without fee is hereby granted, provided that the above
 375 + * copyright notice and this permission notice appear in all copies.
 376 + *
 377 + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 378 + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 379 + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 380 + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 381 + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 382 + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 383 + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 384 + */
 385 +
 386 +#ifndef BLACKLIST_H_
 387 +#define BLACKLIST_H_
 388 +
 389 +#include "key.h"
 390 +
 391 +int blacklisted_key(Key *, int);
 392 +
 393 +typedef enum
 394 +{
 395 +       BLACKLIST_ERROR_NONE = 0,
 396 +       BLACKLIST_ERROR_MISSING,
 397 +       BLACKLIST_ERROR_VERSION,
 398 +       BLACKLIST_ERROR_FORMAT,
 399 +       BLACKLIST_ERROR_ACCESS,
 400 +       BLACKLIST_ERROR_ALL
 401 +} blacklist_error_t;
 402 +
 403 +#endif /* BLACKLIST_H_ */
 404 --- openssh-5.0p1/pathnames.h
 405 +++ openssh-5.0p1/pathnames.h
 406 @@ -43,6 +43,8 @@
 407  /* Backwards compatibility */
 408  #define _PATH_DH_PRIMES                        SSHDIR "/primes"
 409
 410 +#define _PATH_BLACKLIST                        SSHDIR "/blacklist"
 411 +
 412  #ifndef _PATH_SSH_PROGRAM
 413  #define _PATH_SSH_PROGRAM              "/usr/bin/ssh"
 414  #endif
 415 --- openssh-5.0p1/servconf.c
 416 +++ openssh-5.0p1/servconf.c
 417 @@ -39,6 +39,7 @@
 418  #include "match.h"
 419  #include "channels.h"
 420  #include "groupaccess.h"
 421 +#include "blacklist.h"
 422
 423  static void add_listen_addr(ServerOptions *, char *, u_short);
 424  static void add_one_listen_addr(ServerOptions *, char *, u_short);
 425 @@ -94,6 +95,7 @@
 426         options->password_authentication = -1;
 427         options->kbd_interactive_authentication = -1;
 428         options->challenge_response_authentication = -1;
 429 +       options->ignore_blacklist_errors = -1;
 430         options->permit_empty_passwd = -1;
 431         options->permit_user_env = -1;
 432         options->use_login = -1;
 433 @@ -213,6 +217,8 @@
 434                 options->kbd_interactive_authentication = 0;
 435         if (options->challenge_response_authentication == -1)
 436                 options->challenge_response_authentication = 1;
 437 +       if (options->ignore_blacklist_errors == -1)
 438 +               options->ignore_blacklist_errors = BLACKLIST_ERROR_ALL; //VERSION;
 439         if (options->permit_empty_passwd == -1)
 440                 options->permit_empty_passwd = 0;
 441         if (options->permit_user_env == -1)
 442 @@ -282,7 +299,7 @@
 443         sListenAddress, sAddressFamily,
 444         sPrintMotd, sPrintLastLog, sIgnoreRhosts,
 445         sX11Forwarding, sX11DisplayOffset, sX11UseLocalhost,
 446 -       sStrictModes, sEmptyPasswd, sTCPKeepAlive,
 447 +       sStrictModes, sIgnoreBlacklistErrors, sEmptyPasswd, sTCPKeepAlive,
 448         sPermitUserEnvironment, sUseLogin, sAllowTcpForwarding, sCompression,
 449         sAllowUsers, sDenyUsers, sAllowGroups, sDenyGroups,
 450         sIgnoreUserKnownHosts, sCiphers, sMacs, sProtocol, sPidFile,
 451 @@ -372,6 +390,7 @@
 452         { "x11uselocalhost", sX11UseLocalhost, SSHCFG_ALL },
 453         { "xauthlocation", sXAuthLocation, SSHCFG_GLOBAL },
 454         { "strictmodes", sStrictModes, SSHCFG_GLOBAL },
 455 +       { "ignoreblacklisterrors", sIgnoreBlacklistErrors, SSHCFG_GLOBAL },
 456         { "permitemptypasswords", sEmptyPasswd, SSHCFG_GLOBAL },
 457         { "permituserenvironment", sPermitUserEnvironment, SSHCFG_GLOBAL },
 458         { "uselogin", sUseLogin, SSHCFG_GLOBAL },
 459 @@ -923,6 +944,32 @@
 460                 intptr = &options->tcp_keep_alive;
 461                 goto parse_flag;
 462
 463 +       case sIgnoreBlacklistErrors:
 464 +               intptr = &options->ignore_blacklist_errors;
 465 +               arg = strdelim(&cp);
 466 +               if (!arg || *arg == '\0')
 467 +                       fatal("%s line %d: missing none/missing/version/format/access/all argument.",
 468 +                           filename, linenum);
 469 +               value = 0;      /* silence compiler */
 470 +               if (strcmp(arg, "none") == 0)
 471 +                       value = BLACKLIST_ERROR_NONE;
 472 +               else if (strcmp(arg, "missing") == 0)
 473 +                       value = BLACKLIST_ERROR_MISSING;
 474 +               else if (strcmp(arg, "version") == 0)
 475 +                       value = BLACKLIST_ERROR_VERSION;
 476 +               else if (strcmp(arg, "format") == 0)
 477 +                       value = BLACKLIST_ERROR_FORMAT;
 478 +               else if (strcmp(arg, "access") == 0)
 479 +                       value = BLACKLIST_ERROR_ACCESS;
 480 +               else if (strcmp(arg, "all") == 0)
 481 +                       value = BLACKLIST_ERROR_ALL;
 482 +               else
 483 +                       fatal("%s line %d: Bad none/missing/version/format/access/all argument: %s",
 484 +                               filename, linenum, arg);
 485 +               if (*activep && *intptr == -1)
 486 +                       *intptr = value;
 487 +               break;
 488 +
 489         case sEmptyPasswd:
 490                 intptr = &options->permit_empty_passwd;
 491                 goto parse_flag;
 492 --- openssh-5.0p1/servconf.h
 493 +++ openssh-5.0p1/servconf.h
 494 @@ -95,6 +95,7 @@
 495                                                  * authentication. */
 496         int     kbd_interactive_authentication; /* If true, permit */
 497         int     challenge_response_authentication;
 498 +       int     ignore_blacklist_errors;        /* none/missing/version/format/access/all */
 499         int     permit_empty_passwd;    /* If false, do not permit empty
 500                                          * passwords. */
 501         int     permit_user_env;        /* If true, read ~/.ssh/environment */
 502 --- openssh-5.0p1/sshd.c
 503 +++ openssh-5.0p1/sshd.c
 504 @@ -118,6 +118,7 @@
 505  #include "monitor_wrap.h"
 506  #include "monitor_fdpass.h"
 507  #include "version.h"
 508 +#include "blacklist.h"
 509
 510  #ifdef LIBWRAP
 511  #include <tcpd.h>
 512 @@ -1484,6 +1494,11 @@
 513                         sensitive_data.host_keys[i] = NULL;
 514                         continue;
 515                 }
 516 +               if (blacklisted_key(key, 1)) {
 517 +                       sensitive_data.host_keys[i] = NULL;
 518 +                       key_free(key);
 519 +                       continue;
 520 +               }
 521                 switch (key->type) {
 522                 case KEY_RSA1:
 523                         sensitive_data.ssh1_host_key = key;
 524 --- openssh-5.0p1/sshd_config.5
 525 +++ openssh-5.0p1/sshd_config.5
 526 @@ -611,6 +611,39 @@
 527  Specifies whether password authentication is allowed.
 528  The default is
 529  .Dq yes .
 530 +.It Cm IgnoreBlacklistErrors
 531 +Specifies whether
 532 +.Xr sshd 8
 533 +should allow keys recorded in its blacklist of known-compromised keys.
 534 +If
 535 +.Dq all ,
 536 +then attempts to authenticate with compromised keys will be logged
 537 +but accepted.
 538 +If
 539 +.Dq access ,
 540 +then attempts to authenticate with compromised keys will be rejected,
 541 +but blacklist file access errors will be ignored.
 542 +If
 543 +.Dq format ,
 544 +then attempts to authenticate with compromised keys will be rejected, but
 545 +blacklist file access errors due to missing blacklist file or blacklist
 546 +file unrecognized format will be ignored.
 547 +If
 548 +.Dq version ,
 549 +then attempts to authenticate with compromised keys will be rejected, but
 550 +blacklist file access errors due to missing blacklist file or blacklist
 551 +file format version mismatch will be ignored.
 552 +If
 553 +.Dq missing ,
 554 +then attempts to authenticate with compromised keys will be rejected,
 555 +but blacklist file access errors due to missing blacklist file will
 556 +be ignored.
 557 +If
 558 +.Dq none ,
 559 +then attempts to authenticate with compromised keys, or in case of
 560 +any blacklist file access error, will be rejected.
 561 +The default is
 562 +.Dq version .
 563  .It Cm PermitEmptyPasswords
 564  When password authentication is allowed, it specifies whether the
 565  server allows login to accounts with empty password strings.