Secure Command Line / Scurl
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Frequently users will want to download files from the Internet. Secure downloading of files is a complex subject and the potential security implications are often poorly understood.
Downloads with scurl - SSL Command Line Downloader[edit]
Introduction[edit]
Note: This is for advanced users. In all cases avoid downloading files over plain HTTP.
When using the command line to download files or webpages, resorting to the simple wget command is ill-advised because it is buggy
. For example, if users do not force a request to use SSL encryption, wget can fail silently. Even when SSL is enforced with a command line option, this can break interoperability with some sites that use self-signed, expired or invalid certificates. Users could potentially ignore certificate verification warnings and proceed with downloads where the site's authenticity is in question.
To provide greater security when downloading, scurl comes pre-installed in Kicksecure and provides a simple wrapper around curl:
/usr/bin/scurl simply adds --tlsv1.3to allcurlinstances to enforce strong encryption.scurl-download additionally adds --locationto follow redirects as to use extract the file name from the URL. [1]
scurl is not vulnerable to SSLstrip. This is a man-in-the-middle attack which forces a user's browser to communicate with the adversary in plain-text over HTTP (poisoning the download). At present, scurl is available in Kicksecure and the command will generally not work in other distributions.
How-to: Invoke scurl-download[edit]
Note: In the examples below, the file will be saved in the user's current working directory. If the file should be saved elsewhere, change the current working directory before running scurl.
To invoke scurl-download to download a file, simply run (replace the https:// example with the actual file location).
scurl https://example.com/browser.tar.xz
This will download browser.tar.xz to the current working directory.
To invoke scurl-download to download a web page, run (replace the https:// example with the actual webpage).
scurl-download https://example.com
All other curl/Linux features continue to work, such as storing the input inside of a file (change index.html to the desired file name).
scurl https://example.com > index.html
scurl Errors[edit]
As expected, attempting scurl with plain HTTP will fail.
scurl http://example.com
This will result in the following output.
curl: (1) Protocol http not supported or disabled in libcurl
Similarly, scurl fails with the following attempt.
scurl example.com
Returning the following output.
curl: (1) Protocol http not supported or disabled in libcurl
Running scurl against a self-signed or invalid SSL certificate also fails.
scurl https://www.example.com
This results in an error, for example.
curl: (60) SSL certificate problem: self signed certificate More details here: https://curl.se/docs/sslcerts.html curl performs SSL certificate verification by default, using a "bundle" of Certificate Authority (CA) public keys (CA certs). If the default bundle file is not adequate, you can specify an alternate file using the --cacert option. If this HTTPS server uses a certificate signed by a CA represented in the bundle, the certificate verification probably failed due to a problem with the certificate (it might be expired, or the name might not match the domain name in the URL). If you'd like to turn off curl's verification of the certificate, use the -k (or --insecure) option.
Secure Downloads with Web Browsers[edit]
Preventing SSLStrip Attacks[edit]
If clicking or pasting a download link, make sure it is https://. The s in https:// stands for "secure".
Users often mistakenly believe that a secure, green padlock and a https:// URL makes any download from that particular website secure. This is not the case because the website might be redirecting to http. In fact, an SSLstrip attack might succeed if a link is pasted or typed into the address bar without the https:// component (e.g. example.com instead of https://example.com). [2]
In this instance, it is impossible to confirm if the file is being downloaded over https://. Potentially, a SSLstrip attack might have made the download take place over plain http. The reason is a padlock is not visible; it just appears empty.
To avoid this risk and similar threats, always explicitly type or paste https:// in the URL / address bar. The SSL certificate button or padlock will not appear in this instance, but that is nothing to be concerned about. Unfortunately, few users follow this sage advice; instead most mistakenly believe pasting or typing www.example.com into the address bar is safe.
Secure Operating System Updates and Software Installation[edit]
See also:
Other Precautions[edit]
For even greater safety, download files from onion services (.onion addresses) whenever possible. Improved security is provided by onion service downloads, since the connection is encrypted end-to-end (with PFS), targeting of individuals is difficult, and adversaries cannot easily determine where the user is connecting to or from.
Also, if files are already available in repositories, then prefer mechanisms which simplify and automate software upgrades and installations (like APT functions), rather than download Internet resources. Avoid installing unsigned software and be sure to always verify key fingerprints and digital signatures of signed software from the Internet, before importing keys or completing installations. For more on this topic, see: Installing Software Best Practices.
Finally, consider using Multiple Kicksecure when downloading and installing additional software, to better compartmentalize user activities and minimize the threat of misbehaving applications.
Outside of Kicksecure[edit]
curl[edit]
curl --tlsv1.3
Attribution[edit]
Kicksecure is an Implementation of the Securing Debian Manual. This chapter has been inspired by:
Securing Debian Manual
Footnotes[edit]
- ↑
write output to a local file with the same name as the remote file retrieved. Only the file part of the remote file is used and the path is cut off.
- ↑ And that website does not:
- Use HTTP Strict Transport Security (HSTS). See also: https://security.stackexchange.com/questions/91092/how-does-bypassing-hsts-with-sslstrip-work-exactly. Without HSTS, sites with non-encrypted resources or sub-domains are vulnerable to SSLstrip.
- Have a HTTPS Everywhere rule in effect.
- Use HSTS preloading.
- Use HTTP Public Key Pinning. See also: https://news.netcraft.com/archives/2016/03/22/secure-websites-shun-http-public-key-pinning.html. HPKP limits trust to a handful of Certificate Authorities, but is not used by many websites due to the risk of site breakage if keys are not managed vigilantly.
- Use HTTP Strict Transport Security (HSTS)
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