Trustwave SpiderLabs Uncovers Ov3r_Stealer Malware Spread via Phishing and Facebook Advertising. Learn More

Trustwave SpiderLabs Uncovers Ov3r_Stealer Malware Spread via Phishing and Facebook Advertising. Learn More

Services
Capture
Managed Detection & Response

Eliminate active threats with 24/7 threat detection, investigation, and response.

twi-managed-portal-color
Co-Managed SOC (SIEM)

Maximize your SIEM investment, stop alert fatigue, and enhance your team with hybrid security operations support.

twi-briefcase-color-svg
Advisory & Diagnostics

Advance your cybersecurity program and get expert guidance where you need it most.

tw-laptop-data
Penetration Testing

Test your physical locations and IT infrastructure to shore up weaknesses before exploitation.

twi-database-color-svg
Database Security

Prevent unauthorized access and exceed compliance requirements.

twi-email-color-svg
Email Security

Stop email threats others miss and secure your organization against the #1 ransomware attack vector.

tw-officer
Digital Forensics & Incident Response

Prepare for the inevitable with 24/7 global breach response in-region and available on-site.

tw-network
Firewall & Technology Management

Mitigate risk of a cyberattack with 24/7 incident and health monitoring and the latest threat intelligence.

Solutions
BY TOPIC
Microsoft Exchange Server Attacks
Stay protected against emerging threats
Rapidly Secure New Environments
Security for rapid response situations
Securing the Cloud
Safely navigate and stay protected
Securing the IoT Landscape
Test, monitor and secure network objects
Why Trustwave
About Us
Awards and Accolades
Trustwave SpiderLabs Team
Trustwave Fusion Security Operations Platform
Trustwave Security Colony
Partners
Technology Alliance Partners
Key alliances who align and support our ecosystem of security offerings
Trustwave PartnerOne Program
Join forces with Trustwave to protect against the most advance cybersecurity threats
SpiderLabs Blog

ModSecurity Advanced Topic of the Week: Unicode Mapping Support

 

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.

Latest SpiderLabs Blogs

Trustwave SpiderLabs: Insights and Solutions to Defend Educational Institutions Against Cyber Threats

Security teams responsible for defending educational institutions at higher education and primary school levels often find themselves facing harsh lessons from threat actors who exploit the numerous...

Read More

Breakdown of Tycoon Phishing-as-a-Service System

Just weeks after Trustwave SpiderLabs reported on the Greatness phishing-as-a-service (PaaS) framework, SpiderLabs’ Email Security team is tracking another PaaS called Tycoon Group.

Read More

Trustwave SpiderLabs Uncovers Ov3r_Stealer Malware Spread via Phishing and Facebook Advertising

During an Advanced Continual Threat Hunt (ACTH) investigation that took place in early December 2023, Trustwave SpiderLabs discovered Ov3r_Stealer, an infostealer distributed using Facebook...

Read More