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As I write this yet another ransomware attack is underway. This time it’s called Petya and it again uses SMB to spread but here’s the thing. It uses an EXE to get its work done. That’s important because there are countless ways to infect systems, with old ones being patched and new ones being discovered all the time. And you definitely want to reduce your attack surface by disabling/uninstalling unneeded features. And you want to patch systems as soon as possible. 

Those are preventive controls and they are irreplaceable in terms of defense in depth. But no layer of defense is ever a silver bullet. Patching and surface area management will never stop everything.

So we need an effective detective control that tells us as soon as something like Petya gets past our frontline preventive layers of defense. The cool thing is you can do that using nothing more than the Windows security log – or even better – Sysmon. Event ID 4688, activated by enabling Audit Process Creation for success, is a Security log event produced every time and EXE loads as a new process. 

If we simply keep a running baseline of known EXE names and compare each 4688 against that list, BAM!, you’ll know as soon as something new like Petya’s EXE’s run on your network. Of course you need to be collecting 4688s from your workstations and your SIEM needs to be able to do this kind of constant learning whitelist analysis. And you are going to get events when you install new software or patch old software. But only when new EXE names show up.

The only problem with using 4688 is it’s based on EXE name (including path). Bad guys can – but don’t usually bother to use replace known EXEs to stay below the radar. That would defeat the above scheme. So what can you do? Implement Sysmon which logs the hash of each EXE. Sysmon is a free element of Microsoft Sysinternals written by Mark Russonovich and friends. Sysmon event ID 1 (shown below) is logged the same time as 4688 (if you have both process creation auditing and Sysmon configured) but it also proves the hash of the EXE. So even if the attacker does replace a known EXE, the hash will difference, and your comparison against known hashes will fail – thus detecting a new EXE executing for the first time in your environment.

Log Name: Microsoft-Windows-Sysmon/Operational
Source: Microsoft-Windows-Sysmon
Date: 4/28/2017 3:08:22 PM
Event ID: 1
Task Category: Process Create (rule: ProcessCreate)
Level: Information
Keywords:
User: SYSTEM
Computer: rfsH.lab.local
Description:
Process Create:
UtcTime: 2017-04-28 22:08:22.025
ProcessGuid: {a23eae89-bd56-5903-0000-0010e9d95e00}
ProcessId: 6228
Image: C:\Program Files (x86)\Google\Chrome\Application\chrome.exe
CommandLine: "C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --type=utility --lang=en-US --no-sandbox --service-request-channel-token=F47498BBA884E523FA93E623C4569B94 --mojo-platform-channel-handle=3432 /prefetch:8
CurrentDirectory: C:\Program Files (x86)\Google\Chrome\Application\58.0.3029.81\
User: LAB\rsmith
LogonGuid: {a23eae89-b357-5903-0000-002005eb0700}
LogonId: 0x7EB05
TerminalSessionId: 1
IntegrityLevel: Medium
Hashes: SHA256=6055A20CF7EC81843310AD37700FF67B2CF8CDE3DCE68D54BA42934177C10B57
ParentProcessGuid: {a23eae89-bd28-5903-0000-00102f345d00}
ParentProcessId: 13220
ParentImage: C:\Program Files (x86)\Google\Chrome\Application\chrome.exe
ParentCommandLine: "C:\Program Files (x86)\Google\Chrome\Application\chrome.exe"  

Tracking by hash will generate more false positives because anytime a known EXE is updated by the vendor, the first time the new version runs, a new hash will be generated and trip a new alarm or entry on your dashboard. But this tells you that patches are rolling out and confirms that your detection is working. And you are only notified the first time the EXE runs provided you automatically add new hashes to your whitelist. 

Whether you track new EXEs in your environment by name using the Security Log or by hash using Sysmon – do it! New process tracking is one of those highly effective, reliable and long lived, strategic controls that will alert you against other attacks that rely on EXE still beyond the horizon.

“This article by Randy Smith was originally published by EventTracker” https://www.eventtracker.com/newsletters/yet-another-ransomware-that-can-be-immediately-detected-with-process-tracking-on-workstations/

The Domain Name System (DNS), if not properly secured, may be susceptible to abuse by malicious actors. Cybercriminals recognize the value of DNS availability and look for ways to compromise DNS uptime and the DNS servers that support it. As such, DNS becomes an important point of security enforcement and a potential point in the […]

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On 23 November, 2017, we reported two vulnerabilities to Exim. These bugs exist in the SMTP daemon and attackers do not need to be authenticated, including CVE-2017-16943 for a use-after-free (UAF) vulnerability, which leads to Remote Code Execution (RCE); and CVE-2017-16944 for a Denial-of-Service (DoS) vulnerability.

Exim is a message transfer agent (MTA) used on Unix systems. Exim is an open source project and is the default MTA on Debian GNU/Linux systems. According to our survey, there are about 600k SMTP servers running exim on 21st November, 2017 (data collected from scans.io). Also, a mail server survey by E-Soft Inc. shows over half of the mail servers identified are running exim.

• Exim version 4.88 & 4.89 with chunking option enabled.
• According to our survey, about 150k servers affected on 21st November, 2017 (data collected from scans.io).

Through our research, the following vulnerabilies were discovered in Exim. Both vulnerabilies involve in BDAT command. BDAT is an extension in SMTP protocol, which is used to transfer large and binary data. A BDAT command is like BDAT 1024 or BDAT 1024 LAST. With the SIZE and LAST declared, mail servers do not need to scan for the end dot anymore. This command was introduced to exim in version 4.88, and also brought some bugs.

• Use-after-free in receive_msg leads to RCE (CVE-2017-16943)
• Incorrect BDAT data handling leads to DoS (CVE-2017-16944)

To explain this bug, we need to start with the memory management of exim. There is a series of functions starts with store_ such as store_get, store_release, store_reset. These functions are used to manage dynamically allocated memory and improve performance. Its architecture is like the illustration below:

Initially, exim allocates a big storeblock (default 0x2000) and then cut it into stores when store_get is called, using global pointers to record the size of unused memory and where to cut in next allocation. Once the current_block is insufficient, it allocates a new block and appends it to the end of the chain, which is a linked list, and then makes current_block point to it. Exim maintains three store_pool, that is, there are three chains like the illustration above and every global variables are actually arrays. This vulnerability is in receive_msg where exim reads headers: receive.c: 1817 receive_msg

It seems normal if the store functions are just like realloc, malloc and free. However, they are different and cannot be used in this way. When exim tries to extend store, the function store_extend checks whether the old store is the latest store allocated in current_block. It returns False immediately if the check is failed. store.c: 276 store_extend

Once store_extend fails, exim tries to get a new store and release the old one. After we look into store_get and store_release, we found that store_get returns a store, but store_release releases a block if the store is at the head of it. That is to say, if next->text points to the start the current_block and store_get cuts store inside it for newtext, then store_release(next->text) frees next->text, which is equal to current_block, and leaves newtext and current_block pointing to a freed memory area. Any further usage of these pointers leads to a use-after-free vulnerability. To trigger this bug, we need to make exim call store_get after next->text is allocated. This was impossible until BDAT command was introduced into exim. BDAT makes store_get reachable and finally leads to an RCE. Exim uses function pointers to switch between different input sources, such as receive_getc, receive_getbuf. When receiving BDAT data, receive_getc is set to bdat_getc in order to check left chunking data size and to handle following command of BDAT. In receive_msg, exim also uses receive_getc. It loops to read data, and stores data into next->text, extends if insufficient. receive.c: 1817 receive_msg

In bdat_getc, once the SIZE is reached, it tries to read the next BDAT command and raises error message if the following command is incorrect. smtp_in.c: 628 bdat_getc

In exim, it usually calls synprot_error to raise error message, which also logs at the same time. smtp_in.c: 628 bdat_getc

The log messages are printed by string_printing. This function ensures a string is printable. For this reason, it extends the string to transfer characters if any unprintable character exists, such as '\n'->'\\n'. Therefore, it asks store_get for memory to store strings. This store makes if (!store_extend(next->text, oldsize, header_size)) in receive_msg failed when next extension occurs and then triggers use-after-free.

The following is the Proof-of-Concept(PoC) python script of this vulnerability. This PoC controls the control flow of SMTP server and sets instruction pointer to 0xdeadbeef. For fuzzing issue, we did change the runtime configuration of exim. As a result, this PoC works only when dkim is enabled. We use it as an example because the situation is less complicated. The version with default configuration is also exploitable, and we will discuss it at the end of this section.

1. Running out of current_block In order to achieve code execution, we need to make the next->text get the first store of a block. That is, running out of current_block and making store_get allocate a new block. Therefore, we send a long message 'a'*0x1250+'\x7f' with an unprintable character to cut current_block, making yield_length less than 0x100.
2. Starts BDAT data transfer After that, we send BDAT command to start data transfer. At the beginning, next and next->text are allocated by store_get. The function dkim_exim_verify_init is called sequentially and it also calls store_get. Notice that this function uses ANOTHER store_pool, so it allocates from heap without changing current_block which next->text also points to. receive.c: 1734 receive_msg if (smtp_input && !smtp_batched_input && !dkim_disable_verify) dkim_exim_verify_init(chunking_state <= CHUNKING_OFFERED);
3. Call store_getc inside bdat_getc Then, we send a BDAT command without SIZE. Exim complains about the incorrect command and cuts the current_block with store_get in string_printing.
4. Keep sending msg until extension and bug triggered In this way, while we keep sending huge messages, current_block gets freed after the extension. In the malloc.c of glibc (so called ptmalloc2), system manages a linked list of freed memory chunks, which is called unsorted bin. Freed chunks are put into unsorted bin if it is not the last chunk on the heap. In step 2, dkim_exim_verify_init allocated chunks after next->text. Therefore, this chunk is put into unsorted bin and the pointers of linked list are stored into the first 16 bytes of chunk (on x86-64). The location written is exactly current_block->next, and therefore current_block->next is overwritten to unsorted bin inside main_arena of libc (linked list pointer fd points back to unsorted bin if no other freed chunk exists).
5. Keep sending msg for the next extension When the next extension occurs, store_get tries to cut from main_arena, which makes attackers able to overwrite all global variables below main_arena.
6. Overwrite global variables in libc
7. Finish sending message and trigger free() In the PoC, we simply modified __free_hook and ended the line. Exim calls store_reset to reset the buffer and calls __free_hook in free(). At this stage, we successfully controlled instruction pointer $rip. However, this is not enough for an RCE because the arguments are uncontrollable. As a result, we improved this PoC to modify both __free_hook and _IO_2_1_stdout_. We forged the vtable of stdout and set __free_hook to any call of fflush(stdout) inside exim. When the program calls fflush, it sets the first argument to stdout and jumps to a function pointer on the vtable of stdout. Hence, we can control both $rip and the content of first argument. We consulted past CVE exploits and decided to call expand_string, which executes command with execv if we set the first argument to ${run{cmd}}, and finally we got our RCE.

When dkim is disabled, the PoC above fails because current_block is the last chunk on heap. This makes the system merge it into a big chunk called top chunk rather than unsorted bin. The illustrations below describe the difference of heap layout:

To avoid this, we need to make exim allocate and free some memories before we actually start our exploitation. Therefore, we add some steps between step 1 and step 2.

After running out of current_block:

1. Use DATA command to send lots of data Send huge data, make the chunk big and extend many times. After several extension, it calls store_get to retrieve a bigger store and then releases the old one. This repeats many times if the data is long enough. Therefore, we have a big chunk in unsorted bin.
2. End DATA transfer and start a new email Restart to send an email with BDAT command after the heap chunk is prepared.
3. Adjust yield_length again Send invalid command with an unprintable charater again to cut the current_block.

Finally the heap layout is like:

And now we can go back to the step 2 at the beginning and create the same situation. When next->text is freed, it goes back to unsorted bin and we are able to overwrite libc global variables again. The following is the PoC for default configured exim:

A demo of our exploit is as below. Note that we have not found a way to leak memory address and therefore we use heap spray instead. It requires another information leakage vulnerability to overcome the PIE mitigation on x86-64.

When receiving data with BDAT command, SMTP server should not consider a single dot ‘.’ in a line to be the end of message. However, we found exim does in receive_msg when parsing header. Like the following output:

As we mentioned before, exim uses function pointers to switch input source. This bug makes exim go into an incorrect state because the function pointer receive_getc is not reset. If the next command is also a BDAT, receive_getc and lwr_receive_getc become the same and an infinite loop occurs inside bdat_getc. Program crashes due to stack exhaustion. smtp_in.c: 546 bdat_getc

This is not enough to pose a threat because exim runs a fork server. After a further analysis, we made exim go into an infinite loop without crashing, using the following commands.

This makes attackers able to launch a resource based DoS attack and then force the whole server down.

• Turn off Chunking option in config file: chunking_advertise_hosts =
• Update to 4.89.1 version
• Patch of CVE-2017-16943 released here
• Patch of CVE-2017-16944 released here

• 23 November, 2017 09:40 Report to Exim Bugzilla
• 25 November, 2017 16:27 CVE-2017-16943 Patch released
• 28 November, 2017 16:27 CVE-2017-16944 Patch released
• 3 December, 2017 13:15 Send an advisory release notification to Exim and wait for reply until now

While we were trying to report these bugs to exim, we could not find any method for security report. Therefore, we followed the link on the official site for bug report and found the security option. Unexpectedly, the Bugzilla posts all bugs publicly and therefore the PoC was leaked. Exim team responded rapidly and improved their security report process by adding a notification for security reports in reaction to this.

Vulnerabilities found by Meh, DEVCORE research team. meh [at] devco [dot] re

https://bugs.exim.org/show_bug.cgi?id=2199 https://bugs.exim.org/show_bug.cgi?id=2201 https://nvd.nist.gov/vuln/detail/CVE-2017-16943 https://nvd.nist.gov/vuln/detail/CVE-2017-16944 https://lists.exim.org/lurker/message/20171125.034842.d1d75cac.en.html

2017/11/23 我們發現 Unix 的開源軟體 EXIM 含有 Use-After-Free 弱點（CVE-2017-16943）以及 Denial-of-Service 弱點（CVE-2017-16944），當 EXIM 版本是 4.88 或 4.89 並且有開啟 chunking 選項（BDAT 指令）時，攻擊者可傳送特定字串給 EXIM 觸發弱點，可能造成郵件伺服器被遠端攻擊者入侵或是郵件伺服器無法繼續提供服務。

根據 E-Soft Inc. 在 11 月所做的調查，約有 57萬台（56%）的郵件伺服器使用 EXIM 軟體。建議 EXIM 的使用者檢查版本是否為 4.88 或 4.89，若是，則需修改 EXIM 的設定，將 chunking 選項關閉（在 config 裡將 chunking_advertise_hosts 選項留空），或是更新至 4.89.1 版，以避免遭受攻擊。

詳細的技術細節請參閱我們的 Advisory： https://devco.re/blog/2017/12/11/Exim-RCE-advisory-CVE-2017-16943-en/

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