The REXML gem before 3.2.5 in Ruby before 2.6.7, 2.7.x before 2.7.3, and 3.x before 3.0.1 does not properly address XML round-trip issues. An incorrect document can be produced after parsing and serializing.

An issue was discovered in Ruby through 2.5.8, 2.6.x through 2.6.6, and 2.7.x through 2.7.1. WEBrick, a simple HTTP server bundled with Ruby, had not checked the transfer-encoding header value rigorously. An attacker may potentially exploit this issue to bypass a reverse proxy (which also has a poor header check), which may lead to an HTTP Request Smuggling attack.

lib/oauth/consumer.rb in the oauth-ruby gem through 0.5.4 for Ruby does not verify server X.509 certificates if a certificate bundle cannot be found, which allows man-in-the-middle attackers to spoof servers and obtain sensitive information.

mruby through 2.1.2-rc has a heap-based buffer overflow in the mrb_yield_with_class function in vm.c because of incorrect VM stack handling. It can be triggered via the stack_copy function.

An issue was discovered in Ruby 2.5.x through 2.5.7, 2.6.x through 2.6.5, and 2.7.0. If a victim calls BasicSocket#read_nonblock(requested_size, buffer, exception: false), the method resizes the buffer to fit the requested size, but no data is copied. Thus, the buffer string provides the previous value of the heap. This may expose possibly sensitive data from the interpreter.

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.

BibTeX-ruby before 5.1.0 allows shell command injection due to unsanitized user input being passed directly to the built-in Ruby Kernel.open method through BibTeX.open.

In mruby 2.1.0, there is a use-after-free in hash_values_at in mrbgems/mruby-hash-ext/src/hash-ext.c.

In mruby 2.1.0, there is a use-after-free in hash_slice in mrbgems/mruby-hash-ext/src/hash-ext.c.

In mruby 2.1.0, there is a stack-based buffer overflow in mrb_str_len_to_dbl in string.c.