ModSecurity CRS RuleID-960911

Rule
SecRule REQUEST_LINE "!^(?:(?:[a-z]{3,10}\s+(?:\w{3,7}?://[\w\-\./]*(?::\d+)?)?/[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)?|connect (?:\d{1,3}\.){3}\d{1,3}\.?(?::\d+)?|options \*)\s+[\w\./]+|get /[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)?)$" \ "t:none,t:lowercase,phase:2,rev:'2.0.8',pass,nolog,auditlog,msg:'Invalid HTTP Request Line',id:'960911',severity:'4',tag:'http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.2.1',setvar:'tx.msg=%{rule.msg}',setvar:tx.anomaly_score=+%{tx.notice_anomaly_score},setvar:tx.protocol_violation_score=+%{tx.notice_anomaly_score},setvar:'tx.%{rule.id}-PROTOCOL_VIOLATION/INVALID_REQ-%{matched_var_name}=%{matched_var}'"

Rule Summary

 * Validate request line against the format specified in the HTTP RFC

Impact
4- Warning.

Detailed Information
"http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
 * Uses rule negation against the regex for positive security. The regex specifies the proper construction of URI request lines such as:
 * It also outlines proper construction for CONNECT, OPTIONS and GET requests.

/// A description of the regular expression: ///  ///  Beginning of line or string /// Match expression but don't capture it. [(?:[a-z]{3,10}\s+(?:\w{3,7}?://[\w\-\./]*(?::\d+)?)?/[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)?|connect (?:\d{1,3}\.){3}\d{1,3}\.?(?::\d+)?|options \*)\s+[\w\./]+|get /[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)?] ///     Select from 2 alternatives ///         (?:[a-z]{3,10}\s+(?:\w{3,7}?://[\w\-\./]*(?::\d+)?)?/[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)?|connect (?:\d{1,3}\.){3}\d{1,3}\.?(?::\d+)?|options \*)\s+[\w\./]+ ///             Match expression but don't capture it. [[a-z]{3,10}\s+(?:\w{3,7}?://[\w\-\./]*(?::\d+)?)?/[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)?|connect (?:\d{1,3}\.){3}\d{1,3}\.?(?::\d+)?|options \*] ///                 Select from 3 alternatives ///                      [a-z]{3,10}\s+(?:\w{3,7}?://[\w\-\./]*(?::\d+)?)?/[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)? ///                          Any character in this class: [a-z], between 3 and 10 repetitions ///                          Whitespace, one or more repetitions ///                          Match expression but don't capture it. [\w{3,7}?://[\w\-\./]*(?::\d+)?], zero or one repetitions ///                              \w{3,7}?://[\w\-\./]*(?::\d+)? ///                                  Alphanumeric, between 3 and 7 repetitions, as few as possible ///                                  :// ///                                  Any character in this class: [\w\-\./], any number of repetitions ///                                  Match expression but don't capture it. [:\d+], zero or one repetitions ///                                      :\d+ ///                                          : ///                                          Any digit, one or more repetitions ///                          / ///                          Any character that is NOT in this class: [?#], any number of repetitions ///                          Match expression but don't capture it. [\?[^#\s]*], zero or one repetitions ///                              \?[^#\s]* ///                                  Literal ? ///                                  Any character that is NOT in this class: [#\s], any number of repetitions ///                          Match expression but don't capture it. [#[\S]*], zero or one repetitions ///                              #[\S]* ///                                  # ///                                  Any character in this class: [\S], any number of repetitions ///                      connect (?:\d{1,3}\.){3}\d{1,3}\.?(?::\d+)? ///                          connect ///                          Space ///                          Match expression but don't capture it. [\d{1,3}\.], exactly 3 repetitions ///                              \d{1,3}\. ///                                  Any digit, between 1 and 3 repetitions ///                                  Literal . ///                          Any digit, between 1 and 3 repetitions ///                          Literal ., zero or one repetitions ///                          Match expression but don't capture it. [:\d+], zero or one repetitions ///                              :\d+ ///                                  : ///                                  Any digit, one or more repetitions ///                      options \* ///                          options ///                          Space ///                          Literal * ///              Whitespace, one or more repetitions ///              Any character in this class: [\w\./], one or more repetitions ///          get /[^?#]*(?:\?[^#\s]*)?(?:#[\S]*)? ///              get ///              Space ///              / ///              Any character that is NOT in this class: [?#], any number of repetitions ///              Match expression but don't capture it. [\?[^#\s]*], zero or one repetitions ///                  \?[^#\s]* ///                      Literal ? ///                      Any character that is NOT in this class: [#\s], any number of repetitions ///              Match expression but don't capture it. [#[\S]*], zero or one repetitions ///                  #[\S]* ///                      # ///                      Any character in this class: [\S], any number of repetitions ///  End of line or string

Example Payload
Here is an example payloads taken from HTTPrint (http://net-square.com/httprint/) that will trigger this rule.

127.0.0.1 - - [14/Sep/2010:11:51:43 -0400] "\x16\x03" 501 214 TI@aD8CoAWYAAAOFHNMAAACA

Example Audit Log Entry
Include an example ModSecurity Audit Log Entry for when this rule matchs.

--1167a167-A-- [14/Sep/2010:11:51:43 --0400] TI@aD8CoAWYAAAOFHNMAAACA 127.0.0.1 51285 127.0.0.1 80 --1167a167-B-- ^V^C --1167a167-F-- --1167a167-H-- Message: Match of "rx ^(?:(?:[a-z]{3,10}\\s+(?:\\w{3,7}?://[\\w\\-\\./]*(?::\\d+)?)?/[^?#]*(?:\\?[^#\\s]*)?(?:#[\\S]*)?|connect (?:\\d{1,3}\\.){3}\\d{1,3}\\.?(?::\\d+)?|options \\*)\\s+[\\w\\./]+|get /[^?#]*(?:\\?[^#\\s]*)?(?:#[\\S]*)?)$" against "REQUEST_LINE" required. [file "/usr/local/apache/conf/modsec_current/base_rules/modsecurity_crs_20_protocol_violations.conf"] [line "34"] [id "960911"] [rev "2.0.8"] [msg "Invalid HTTP Request Line"] [severity "WARNING"] [tag "http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.2.1"] Apache-Error: [file "core.c"] [line 3773] [level 3] Invalid method in request \\x16\\x03 Stopwatch: 1284479503453580 43606 (4888 42377 -) Producer: ModSecurity for Apache/2.5.12 (http://www.modsecurity.org/); core ruleset/2.0.8. Server: Apache/2.2.12 (Unix) mod_ssl/2.2.12 OpenSSL/0.9.8l DAV/2 --1167a167-Z--

Affected Software
Different web server software may react differently to malformed URIs.

Attack Scenarios
Some malformed URIs are created on purpose as part of HTTP fingerprinting scans - http://projects.webappsec.org/Fingerprinting

Other times, these are caused by poorly written web clients.

Ease of Attack
Easy.

Ease of Detection
Easy with either regular expressions or by monitoring Apache error logging in phase:5.

False Positives
None known.

False Negatives
None known.

Contributors
Ryan Barnett

Additional References
http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.2.1