Unicode + Best-Fit Mapping = Evasions
Impedance mismatches between a security inspection system (IDS, IPS or WAF) and the target web application is a major concern with regards to security inspection. The process of data normalization or canonicalization and how the destination web application handles best-fit mappings can cause issues with bypasses. Here is a great recent references for this issue - Lost In Translation - Giorgio Maone (of the NoScript FF extension fame) outlines how ASP classic web apps attempt to do best-fit mappings of non-ASCII Unicode characters. One example issue is the following XSS payload:
%u3008scr%u0131pt%u3009%u212fval(%uFF07al%u212Frt(%22XSS%22)%u02C8)%u2329/scr%u0131pt%u232A
This payload should be correctly decoded to this -
〈scrıpt〉ℯval(＇alℯrt("XSS")ˈ)〈/scrıpt〉
This is simply a set of text and would not be treated as executable code by a web browser. ASP classic, however, will try and do best-fit mapping. Here is a short example of some of the mappings ASP will make:
〈(0x2329) ~= <(0x3c)〈(0x3008) ~= <(0x3c)＜(0xff1c) ~= <(0x3c)ʹ(0x2b9) ~= '(0x27)ʼ(0x2bc) ~= '(0x27)ˈ(0x2c8) ~= '(0x27)′(0x2032) ~= '(0x27)＇(0xff07) ~= '(0x27)
With this mapping, the payload listed previous would actually be normalized the into working JS code:
<script>eval('alert("XSS")')</script>
The issue that this raises, for security inspection, is that the inbound payload will most likely not match most XSS regular expression payloads however the application itself will modify it into executable code! Yikes...
ModSecurity Evasion Issue
Here is an example rule that simply looks for a "<" character so that we can test out this issue.
SecRule ARGS "@contains <" "phase:1,t:none,t:urlDecodeUni,log,pass"
When I send the example payload above, this is how ModSecurity normalizes the data:
Recipe: Invoking rule 1009ac868; [file "/usr/local/apache/conf/crs/activated_rules/modsecurity_crs_15_customrules.conf"] [line "4"].Rule 1009ac868: SecRule "ARGS" "@contains <" "phase:1,t:none,t:urlDecodeUni,log,pass"T (0) urlDecodeUni: "\bscr1pt\t/val('al/rt("XSS")\xc8))/scr1pt*"Transformation completed in 46 usec.Executing operator "contains" with param "<" against ARGS:foo.Target value: "\bscr1pt\t/val('al/rt("XSS")\xc8))/scr1pt*"Operator completed in 1 usec.Rule returned 0.
As you can see, the current urlDecodeUni transformation function did not fully complete and there caused a false negative evasion issue.
ModSecurity's Unicode Mapping Enhancement
In order to combat these types of Unicode evasions, we have just updated the current urlDecodeUni transformation function to allow for inclusion of a unicode mapping file and code point declaration. ModSecurity v2.6.1 in trunk includes the following enhancements.
Unicode Mapping File
We include a new unicode.mapping file that allows for specific mapping of characters depending on various code points. Here is an example section from the US-ASCII 20127 code point:
20127 (US-ASCII)00a0:20 00a1:21 00a2:63 00a4:24 00a5:59 00a6:7c 00a9:43 00aa:61 00ab:3c 00ad:2d 00ae:52 00b2:32 00b3:33 00b7:2e 00b8:2c 00b9:31 00ba:6f 00bb:3e 00c0:41 00c1:41 00c2:41 00c3:41 00c4:41 00c5:41 00c6:41 00c7:43 00c8:45 00c9:45 00ca:45 00cb:45 00cc:49 00cd:49 00ce:49 00cf:49 00d0:44 00d1:4e 00d2:4f 00d3:4f 00d4:4f 00d5:4f 00d6:4f 00d8:4f 00d9:55 00da:5500db:55 00dc:55 00dd:59 00e0:61 00e1:61 00e2:61 00e3:61 00e4:61 00e5:61 00e6:61 00e7:63 00e8:65 00e9:65 00ea:65 00eb:65 00ec:69 00ed:69 00ee:69 00ef:69 00f1:6e 00f2:6f 00f3:6f 00f4:6f 00f5:6f 00f6:6f 00f8:6f 00f9:75 00fa:75 00fb:75 00fc:75 00fd:79 00ff:79 0100:41 0101:61 0102:41 0103:61 0104:41 0105:61 0106:43 0107:63 0108:43 0109:63 010a:43 010b:63 010c:43 010d:63 010e:44 010f:64 0110:44 0111:64 0112:45 0113:65 0114:45 0115:65 0116:45 0117:65 0118:45 0119:65 011a:45 011b:65 011c:47 011d:67 011e:47 011f:67 0120:47 0121:67 0122:47 0123:67 0124:48 0125:68 0126:48 0127:68 0128:49 0129:69 012a:49 012b:69 012c:49 012d:69 012e:49 012f:69 0130:49 0131:69 0134:4a 0135:6a 0136:4b 0137:6b 0139:4c 013a:6c 013b:4c 013c:6c 013d:4c 013e:6c 0141:4c 0142:6c 0143:4e 0144:6e 0145:4e 0146:6e 0147:4e 0148:6e 014c:4f 014d:6f 014e:4f 014f:6f 0150:4f 0151:6f 0152:4f 0153:6f 0154:52 0155:72 0156:52 0157:72 0158:52 0159:72 015a:53 015b:73 015c:53 015d:73 015e:53 015f:73 0160:53 0161:73 0162:54 0163:74 0164:54 0165:74 0166:54 0167:74 0168:55 0169:75 016a:55 016b:75 016c:55 016d:75 016e:55 016f:75 0170:55 0171:75 0172:55 0173:75 0174:57 0175:77 0176:59 0177:79 0178:59 0179:5a 017b:5a 017c:7a 017d:5a 017e:7a 0180:62 0189:44 0191:46 0192:66 0197:49 019a:6c 019f:4f 01a0:4f 01a1:6f 01ab:74 01ae:54 01af:55 01b0:75 01b6:7a 01cd:41 01ce:61 01cf:49 01d0:69 01d1:4f 01d2:6f 01d3:55 01d4:75 01d5:55 01d6:75 01d7:55 01d8:75 01d9:55 01da:75 01db:55 01dc:75 01de:41 01df:61 01e4:47 01e5:67 01e6:47 01e7:67 01e8:4b 01e9:6b 01ea:4f 01eb:6f 01ec:4f 01ed:6f 01f0:6a 0261:67 02b9:27 02ba:22 02bc:27 02c4:5e 02c6:5e 02c8:27 02cb:60 02cd:5f 02dc:7e 0300:60 0302:5e 0303:7e 030e:22 0331:5f 0332:5f 2000:20 2001:20 2002:20 2003:20 2004:20 2005:20 2006:20 2010:2d 2011:2d 2013:2d 2014:2d 2018:27 2019:27 201a:2c 201c:22 201d:22 201e:22 2022:2e 2026:2e 2032:27 2035:60 2039:3c 203a:3e 2122:54 ff01:21 ff02:22 ff03:23 ff04:24 ff05:25 ff06:26 ff07:27 ff08:28 ff09:29 ff0a:2a ff0b:2b ff0c:2c ff0d:2d ff0e:2e ff0f:2f ff10:30 ff11:31 ff12:32 ff13:33 ff14:34 ff15:35 ff16:36 ff17:37 ff18:38 ff19:39 ff1a:3a ff1b:3b ff1c:3c ff1d:3d ff1e:3e ff20:40 ff21:41 ff22:42 ff23:43 ff24:44 ff25:45 ff26:46 ff27:47 ff28:48 ff29:49 ff2a:4a ff2b:4b ff2c:4c ff2d:4d ff2e:4e ff2f:4f ff30:50 ff31:51 ff32:52 ff33:53 ff34:54 ff35:55 ff36:56 ff37:57 ff38:58 ff39:59 ff3a:5a ff3b:5b ff3c:5c ff3d:5d ff3e:5e ff3f:5f ff40:60 ff41:61 ff42:62 ff43:63 ff44:64 ff45:65 ff46:66 ff47:67 ff48:68 ff49:69 ff4a:6a ff4b:6b ff4c:6c ff4d:6d ff4e:6e ff4f:6f ff50:70 ff51:71 ff52:72 ff53:73 ff54:74 ff55:75 ff56:76 ff57:77 ff58:78 ff59:79 ff5a:7a ff5b:7b ff5c:7c ff5d:7d ff5e:7e
SecUnicodeMapFile
Added a new directive called SecUnicodeMapFile which tells ModSecurity which file to use for mapping.
SecUnicodeCodePage
Added a new directive called SecUnicodeCodePage which tells ModSecurity which code point is being used on the site and then it will normalize Unicode data to that specific setting.
Example Usage
Let's look now at an updated rule set that includes the new directives.
SecUnicodeMapFile /usr/local/apache/conf/crs/unicode.mappingSecUnicodeCodePage 20127SecRule ARGS "@contains <" "phase:1,t:none,t:urlDecodeUni,log,pass"
With this in place, we can now resend the attack payload and look at the debug log processing:
Recipe: Invoking rule 1009b9bd8; [file "/usr/local/apache/conf/crs/activated_rules/modsecurity_crs_15_customrules.conf"] [line "4"].Rule 1009b9bd8: SecRule "ARGS" "@contains <" "phase:1,t:none,t:urlDecodeUni,log,pass"T (0) urlDecodeUni: "<script>eval('alert("XSS")')</script>"Transformation completed in 32 usec.Executing operator "contains" with param "<" against ARGS:foo.Target value: "<script>eval('alert("XSS")')</script>"Operator completed in 8 usec.Warning. String match "<" at ARGS:foo. [file "/usr/local/apache/conf/crs/activated_rules/modsecurity_crs_15_customrules.conf"] [line "4"]Rule returned 1.
