Headline
CVE-2020-5247
In Puma (RubyGem) before 4.3.2 and before 3.12.3, if an application using Puma allows untrusted input in a response header, an attacker can use newline characters (i.e. CR, LF or/r, /n) to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting. While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS). This is related to CVE-2019-16254, which fixed this vulnerability for the WEBrick Ruby web server. This has been fixed in versions 4.3.2 and 3.12.3 by checking all headers for line endings and rejecting headers with those characters.
Impact
If an application using Puma allows untrusted input in a response header, an attacker can use newline characters (i.e. CR, LF or/r, /n) to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting.
While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS).
This is related to CVE-2019-16254, which fixed this vulnerability for the WEBrick Ruby web server.
Patches
This has been fixed in 4.3.2 and 3.12.3 by checking all headers for line endings and rejecting headers with those characters.
Workarounds
Add a Rack middleware to your application which accomplishes the same header check:
class HTTPResponseSplittingMiddleware CRLF_REGEX = /[\r\n]/.freeze
def initialize(app) @app = app end
def call(env) status, headers, body = @app.call(env) headers.reject! { |_k, v| CRLF_REGEX =~ v.to_s } [status, headers, body] end end
References
- OWASP entry on HTTP Response Splitting
- The same issue fixed in WEBrick
For more information
If you have any questions or comments about this advisory:
- Open an issue in puma
- Email us a project maintainer. Email addresses are listed in our Code of Conduct.
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Ruby through 2.4.7, 2.5.x through 2.5.6, and 2.6.x through 2.6.4 allows HTTP Response Splitting. If a program using WEBrick inserts untrusted input into the response header, an attacker can exploit it to insert a newline character to split a header, and inject malicious content to deceive clients. NOTE: this issue exists because of an incomplete fix for CVE-2017-17742, which addressed the CRLF vector, but did not address an isolated CR or an isolated LF.