dynamically generate barcode in asp.net c# Banner-Grabbing Countermeasures in Software

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Banner-Grabbing Countermeasures
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As we ve already noted, the best defense against banner grabbing is to shut down unnecessary services Alternatively, restrict access to services using network access control Perhaps the widest avenue of entry into any environment is running vulnerable software services, so this restriction should be done to combat more than just banner grabbing Next, for those services that are business critical and can t simply be turned off, you ll need to research the correct way to disable the presentation of the vendor and version in banners Audit yourself regularly with port scans and raw netcat connects to active ports to make sure you aren t giving away inappropriate information to attackers
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ENUMERATING COMMON NETWORK SERVICES
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Let s use some of these basic enumeration techniques, and much more, to enumerate services commonly turned up by real-world port scans
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FTP Enumeration, TCP 21
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Popularity: Simplicity: Impact: Risk Rating: 1 10 1 4
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Although File Transfer Protocol (FTP) is becoming less common on the Internet, connecting to and examining the content of FTP repositories remains one of the simplest
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Hacking Exposed 6: Network Security Secrets & Solutions
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and potentially lucrative enumeration techniques We ve seen many public web servers that used FTP for uploading web content, providing an easy vector for uploading malicious executables (see 11 on web hacking for more details here) Typically, the availability of easily accessible file-sharing services quickly becomes widespread knowledge, and public FTP sites end up hosting sensitive and potentially embarrassing content Even worse, many such sites are configured for anonymous access Connecting to FTP is simple, using the client that is typically built into most modern operating systems The next example shows the Windows command-line FTP client Note that we use anonymous and a spurious e-mail address (not shown in the following output) to authenticate to this anonymous service:
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C:\>ftp ftpexamplecom Connected to ftpexamplecom 220 (vsFTPd 201) User (ftpexamplecom:(none)): anonymous 331 Please specify the password Password: 230 Login successful ftp> ls 200 PORT command successful Consider using PASV 150 Here comes the directory listing GO DROP hos2 hm1 LINK lib lost+found pub 226 Directory send OK ftp: 52 bytes received in 000Seconds 5200000Kbytes/sec ftp>
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Of course, graphical FTP clients are also available Most modern web browsers implement FTP and permit browsing of sites via the familiar file-and-folder metaphor An excellent open source graphical FTP client is FileZilla from http://filezilla-project org/ For a list of anonymous FTP sites see wwwftp-sitesorg Although this site hasn t been recently updated, it does contain many sites which are still available And, of course, the banner enumerated by FTP can indicate the presence of FTP server software with severe vulnerabilities Washington University s FTP server (wu-ftp), for example, is very popular and has a history of remotely exploitable buffer overflows that permit complete compromise of the system
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Enumeration
FTP Enumeration Countermeasures
FTP is one of those oldie-but-not-so-goodie-anymore services that should just be turned off Be especially skeptical of anonymous FTP, and don t allow unrestricted uploading of files under any circumstances
Enumerating Telnet, TCP 23
Popularity: Simplicity: Impact: Risk Rating: 4 9 3 5
Telnet was one of the most crucial services in use for many years In the early days of the Internet, telnet was so valuable because it provided one of the most essential services: remote access Telnet s major downfall is that it transmits data in cleartext This means that anyone with a sniffer can potentially view the entire conversation between the client and server including the username and password used to login With security becoming more of a necessity, this service was later replaced by a more secure, encrypted means of remote administration called secure shell, or SSH Even though telnet s insecurities are widely known, it is still very common to find this service available System Enumeration via Telnet Banners From an attacker s standpoint, telnet can be an easy way to obtain host information because telnet usually displays a system banner prior to login This banner will often contain the host s operating system and version With networking equipment such as routers and switches, you may not receive such an explicitly detailed banner Many times the system will display a unique prompt from which you can easily deduce what type of device it is through prior knowledge or a simple Google search For instance with Cisco equipment, you ll receive one of two prompts:
User Access Verification Password: Or User Access Verification Username:
If you receive either banner, it is pretty safe to assume that the host you re connecting to is a Cisco device The difference between the two prompts is that the Username prompt on Cisco telnet servers usually indicates that the device is using TACACS+ or some sort of AAA (authentication, authorization, and accounting) for authentication, which means it is likely that some set of lockout mechanisms are in place This can aid an attacker in choosing an attack plan when brute forcing In the case that only a password
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