{"id":220700,"date":"2020-04-14T10:01:38","date_gmt":"2020-04-14T08:01:38","guid":{"rendered":"https:\/\/www.intrinsec.com\/?p=220700"},"modified":"2020-04-14T10:01:38","modified_gmt":"2020-04-14T08:01:38","slug":"deobfuscating-hunting-ostap","status":"publish","type":"post","link":"https:\/\/www.intrinsec.com\/en\/deobfuscating-hunting-ostap\/","title":{"rendered":"Deobfuscating and hunting for OSTAP, Trickbot's dropper and best friend"},"content":{"rendered":"

[et_pb_section fb_built=\u00a0\u00bb1″ admin_label=\u00a0\u00bbsection\u00a0\u00bb _builder_version=\u00a0\u00bb3.22″][et_pb_row admin_label=\u00a0\u00bbrow\u00a0\u00bb _builder_version=\u00a0\u00bb3.25″ background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb custom_margin=\u00a0\u00bb|auto|-2px|auto||\u00a0\u00bb][et_pb_column type=\u00a0\u00bb4_4″ _builder_version=\u00a0\u00bb3.25″ custom_padding=\u00a0\u00bb|||\u00a0\u00bb custom_padding__hover=\u00a0\u00bb|||\u00a0\u00bb][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb custom_padding=\u00a0\u00bb||0px|||\u00a0\u00bb]<\/p>\n

During a recent investigation dealing with ransomware attack<\/strong>, CERT Intrinsec faced OSTAP loader. This loader is used to deliver other malwares<\/strong> (such as Trickbot) on an infected system. It uses high obfuscation techniques to prevent the code from being read and to bypass detection processes.<\/p>\n

<\/p>\n

<\/span><\/p>\n

Obfuscated loader<\/span><\/p>\n

[\/et_pb_text][et_pb_image src="https:\/\/www.intrinsec.com\/wp-content\/uploads\/2020\/04\/fig1-1.png" _builder_version="4.4.2"][\/et_pb_image][et_pb_text _builder_version="4.4.2"]<\/p>\n

Figure 1 : Extract of the loader code (Javascript)<\/span><\/em><\/p>\n

[\/et_pb_text][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb]<\/p>\n

 <\/p>\n

The ostap loader (Figure 1) we analysed was about 10 000 lines long. We started the <\/span>static analysis <\/strong>by going through the code by hand to understand its structure. We identified a key part of the code that helped us to deobfuscate the loader (Figure 2).<\/span><\/p>\n

<\/span><\/p>\n

[\/et_pb_text][et_pb_image src="https:\/\/www.intrinsec.com\/wp-content\/uploads\/2020\/04\/fig2-1.png" _builder_version="4.4.2"][\/et_pb_image][et_pb_text _builder_version="4.4.2"]<\/p>\n

Figure 2 : Key part of the loader code<\/span><\/em><\/p>\n

[\/et_pb_text][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb]<\/p>\n

 <\/p>\n

The instructions, on <\/span>Figure 2<\/em>, aim at executing <\/span>String.fromCharCode<\/em> function <\/span>ewnfBeth8<\/strong> parameter. There are lots of noise instructions in the program. For example, <\/span>ppfhair_3(ewnfBeth8)<\/strong> instruction in <\/span>try <\/em>statement will never be triggered because the function does not exist. It is done on purpose to always enter the <\/span>catch<\/em>. Besides, <\/span>etvulike2<\/strong> parameter is always equal to <\/span>\u2018f\u2019<\/strong>. A large part of the program consists of a concatenation of functions such as the one shown on <\/span>Figure 3<\/em>.<\/span><\/p>\n

<\/p>\n

 <\/p>\n

[\/et_pb_text][et_pb_image src="https:\/\/www.intrinsec.com\/wp-content\/uploads\/2020\/04\/fig3-1.png" _builder_version="4.4.2"][\/et_pb_image][et_pb_text _builder_version="4.4.2"]<\/p>\n

\u00a0Figure 3 : Repeated function model<\/span><\/em><\/p>\n

[\/et_pb_text][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb custom_padding=\u00a0\u00bb|||1px||\u00a0\u00bb]<\/p>\n

 <\/p>\n

The action of the function above (<\/span>Figure 3<\/em>) is to apply <\/span>String.fromCharCode <\/em>to 69, i.e. \u00ab\u00a0E\u00a0\u00bb. <\/span>The program uses this method to set all its instructions<\/strong>. Knowing that, we decided to write a script to extract each obfuscated character.<\/span><\/p>\n

[\/et_pb_text][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb custom_padding=\u00a0\u00bb|||1px||\u00a0\u00bb]<\/p>\n

 <\/p>\n

\u00a0Deobfuscation script<\/span><\/p>\n

\u00a0The main goal of the script is <\/span>to get indicators of compromise from the loader.<\/strong> It has been developed using Node JS. It first goes through the obfuscated loader, retrieves the targeted numbers and apply <\/span>String.fromCharCode<\/em> to decode them. Then, it collects the indicators of compromise in the decoded payload using regular expressions. Extracted IOCs are IPs, URLs and User-Agents. The figure below represents the output of the script using a sample hunted on VirusTotal. We can see, at the top of <\/span>Figure 4<\/em>, a list of file extensions that are targeted (their content will be replaced by the Ostap JS code).<\/span><\/p>\n

<\/p>\n

 <\/p>\n

[\/et_pb_text][et_pb_image src="https:\/\/www.intrinsec.com\/wp-content\/uploads\/2020\/04\/fig4.png" _builder_version="4.4.2"][\/et_pb_image][et_pb_text _builder_version="4.4.2"]<\/p>\n

Figure 4 : Script execution output<\/span><\/em><\/p>\n

[\/et_pb_text][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb]<\/p>\n

 <\/p>\n

Hunting\u00a0<\/h2>\n

<\/p>\n

\u00a0After deobfuscating the loader as a part of our investigation, <\/span>we decided to hunt recent and similar files on VirusTotal, using searches on static code patterns<\/strong> (<\/span>content: \u00ab\u00a0‘String’)[‘slice’]\u00a0\u00bb<\/em>, for instance). We found lots of samples (<\/span>Figure 5<\/em>) and process them so as to extract as many IOCs as possible.<\/span><\/p>\n

[\/et_pb_text][et_pb_image src="https:\/\/www.intrinsec.com\/wp-content\/uploads\/2020\/04\/fig5-1.png" _builder_version="4.4.2"][\/et_pb_image][et_pb_text _builder_version="4.4.2"]<\/p>\n

\u00a0Figure 5 : Ostap samples from VirusTotal<\/span><\/em><\/p>\n

[\/et_pb_text][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb]<\/p>\n

 <\/p>\n

We collected about <\/span>140 samples from VirusTotal using the script<\/strong>. We analysed them and extracted the indicators of compromise presented in the table below. We can say that at least one of the IP addresses (185[.]234[.]73[.]125) <\/span>is related to the Trickbot campaign happening since the Coronavirus appeared such as in Italy, as reported by Sophos<\/strong> (1).<\/span><\/p>\n

[\/et_pb_text][et_pb_text _builder_version=\u00a0\u00bb4.4.2″ hover_enabled=\u00a0\u00bb0″]<\/p>\n

 <\/p>\n\n\n\n\n
\n

IP<\/strong><\/p>\n<\/td>\n

\n

URL<\/strong><\/p>\n<\/td>\n

\n

User-Agent<\/strong><\/p>\n<\/td>\n<\/tr>\n

\n

141[.]98[.]214[.]14<\/p>\n

185[.]159[.]82[.]205<\/p>\n

185[.]216[.]35[.]10<\/p>\n

185[.]234[.]73[.]125<\/p>\n

194[.]87[.]96[.]100<\/p>\n

45[.]128[.]133[.]41<\/p>\n

91[.]196[.]70[.]126<\/p>\n<\/td>\n

\n

hxxps[:\/\/]141[.]98[.]214[.]14\/6BcsTO\/AGVV5r[.]php<\/p>\n

hxxps[:\/\/]185[.]159[.]82[.]205\/2\/1[.]php<\/p>\n

hxxps[:\/\/]185[.]216[.]35[.]10\/VYut68\/L2KSUN[.]php<\/p>\n

hxxps[:\/\/]185[.]234[.]73[.]125\/wMB03o\/Wx9u79[.]php<\/p>\n

hxxps[:\/\/]194[.]87[.]96[.]100\/2\/1[.]php<\/p>\n

hxxp[:\/\/]45[.]128[.]133[.]41\/jTlp8P\/3OXkud[.]php<\/p>\n

hxxps[:\/\/]91[.]196[.]70[.]126\/2\/zsQX9M[.]php<\/p>\n<\/td>\n

\n

Mozilla\/5.0<\/p>\n

(Windows NT 6.; Win64; x64; Trident\/7.0; rv:11.0) like Gecko<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

[\/et_pb_text][et_pb_text admin_label=\u00a0\u00bbText\u00a0\u00bb _builder_version=\u00a0\u00bb4.4.2″ text_font=\u00a0\u00bbNunito Sans||||||||\u00a0\u00bb text_font_size=\u00a0\u00bb16px\u00a0\u00bb header_2_font=\u00a0\u00bbPoppins||||||||\u00a0\u00bb header_2_text_color=\u00a0\u00bb#071793″ header_2_font_size=\u00a0\u00bb31px\u00a0\u00bb background_size=\u00a0\u00bbinitial\u00a0\u00bb background_position=\u00a0\u00bbtop_left\u00a0\u00bb background_repeat=\u00a0\u00bbrepeat\u00a0\u00bb]<\/p>\n

 <\/p>\n

References<\/span><\/p>\n

    \n
  1. https:\/\/news.sophos.com\/en-us\/2020\/03\/04\/trickbot-campaign-targets-coronavirus-fears-in-italy\/<\/a><\/li>\n
  2. https:\/\/www.esentire.com\/blog\/oh-snap-new-ostap-variant-observed-in-the-wild<\/a><\/li>\n
  3. https:\/\/www.bromium.com\/deobfuscating-ostap-trickbots-javascript-downloader\/<\/a><\/li>\n
  4. https:\/\/blog.trendmicro.com\/trendlabs-security-intelligence\/latest-trickbot-campaign-delivered-via-highly-obfuscated-js-file\/<\/a><\/li>\n
  5. https:\/\/github.com\/cryptogramfan\/Malware-Analysis-Scripts<\/a><\/li>\n
  6. https:\/\/www.cert.pl\/en\/news\/single\/ostap-malware-analysis-backswap-dropper\/<\/a><\/li>\n
  7. Link to the script on Intrinsec Github : https:\/\/github.com\/Intrinsec\/CERT\/tree\/master\/Scripts\/ostap_deobfuscator<\/a><\/li>\n<\/ol>\n

     <\/p>\n

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