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Overview

rexml is an An XML toolkit for Ruby.

Affected versions of this package are vulnerable to Denial of Service (DoS) via the REXML gem, when parsing an XML document that has many specific characters such as whitespace character,>] and ]>.

Details

Denial of Service (DoS) describes a family of attacks, all aimed at making a system inaccessible to its intended and legitimate users.

Unlike other vulnerabilities, DoS attacks usually do not aim at breaching security. Rather, they are focused on making websites and services unavailable to genuine users resulting in downtime.

One popular Denial of Service vulnerability is DDoS (a Distributed Denial of Service), an attack that attempts to clog network pipes to the system by generating a large volume of traffic from many machines.

When it comes to open source libraries, DoS vulnerabilities allow attackers to trigger such a crash or crippling of the service by using a flaw either in the application code or from the use of open source libraries.

Two common types of DoS vulnerabilities:

• High CPU/Memory Consumption- An attacker sending crafted requests that could cause the system to take a disproportionate amount of time to process. For example, commons-fileupload:commons-fileupload.

• Crash - An attacker sending crafted requests that could cause the system to crash. For Example, npm ws package

Remediation

Upgrade rexml to version 3.3.3 or higher.

References

• ruby-lang News

Overview

rexml is an An XML toolkit for Ruby.

Affected versions of this package are vulnerable to Regular Expression Denial of Service (ReDoS) due to the usage of insecure regular expressions in CHARACTER_REFERENCES. This vulnerability can be exploited when parsing XML content containing numerous digits between &# and x...; in a hex numeric character reference (&#x...;).

By supplying specially crafted XML documents, an attacker can cause the application to consume excessive resources.

Note:

This vulnerability doesn't affect Ruby 3.2 or later.

Details

Denial of Service (DoS) describes a family of attacks, all aimed at making a system inaccessible to its original and legitimate users. There are many types of DoS attacks, ranging from trying to clog the network pipes to the system by generating a large volume of traffic from many machines (a Distributed Denial of Service - DDoS - attack) to sending crafted requests that cause a system to crash or take a disproportional amount of time to process.

The Regular expression Denial of Service (ReDoS) is a type of Denial of Service attack. Regular expressions are incredibly powerful, but they aren't very intuitive and can ultimately end up making it easy for attackers to take your site down.

Let’s take the following regular expression as an example:

regex = /A(B|C+)+D/

This regular expression accomplishes the following:

• A The string must start with the letter 'A'
• (B|C+)+ The string must then follow the letter A with either the letter 'B' or some number of occurrences of the letter 'C' (the + matches one or more times). The + at the end of this section states that we can look for one or more matches of this section.
• D Finally, we ensure this section of the string ends with a 'D'

The expression would match inputs such as ABBD, ABCCCCD, ABCBCCCD and ACCCCCD

It most cases, it doesn't take very long for a regex engine to find a match:

$ time node -e '/A(B|C+)+D/.test("ACCCCCCCCCCCCCCCCCCCCCCCCCCCCD")'
0.04s user 0.01s system 95% cpu 0.052 total

$ time node -e '/A(B|C+)+D/.test("ACCCCCCCCCCCCCCCCCCCCCCCCCCCCX")'
1.79s user 0.02s system 99% cpu 1.812 total

The entire process of testing it against a 30 characters long string takes around ~52ms. But when given an invalid string, it takes nearly two seconds to complete the test, over ten times as long as it took to test a valid string. The dramatic difference is due to the way regular expressions get evaluated.

Most Regex engines will work very similarly (with minor differences). The engine will match the first possible way to accept the current character and proceed to the next one. If it then fails to match the next one, it will backtrack and see if there was another way to digest the previous character. If it goes too far down the rabbit hole only to find out the string doesn’t match in the end, and if many characters have multiple valid regex paths, the number of backtracking steps can become very large, resulting in what is known as catastrophic backtracking.

Let's look at how our expression runs into this problem, using a shorter string: "ACCCX". While it seems fairly straightforward, there are still four different ways that the engine could match those three C's:

1. CCC
2. CC+C
3. C+CC
4. C+C+C.

The engine has to try each of those combinations to see if any of them potentially match against the expression. When you combine that with the other steps the engine must take, we can use RegEx 101 debugger to see the engine has to take a total of 38 steps before it can determine the string doesn't match.

From there, the number of steps the engine must use to validate a string just continues to grow.

By the time the string includes 14 C's, the engine has to take over 65,000 steps just to see if the string is valid. These extreme situations can cause them to work very slowly (exponentially related to input size, as shown above), allowing an attacker to exploit this and can cause the service to excessively consume CPU, resulting in a Denial of Service.

Remediation

Upgrade rexml to version 3.3.9 or higher.

References

• GitHub Commit

Overview

rexml is an An XML toolkit for Ruby.

Affected versions of this package are vulnerable to Uncontrolled Resource Consumption ('Resource Exhaustion') via the SAX2 or pull parser API. An attacker can cause the application to consume excessive resources leading to a denial of service by submitting specially crafted XML documents that exploit entity expansions.

PoC

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE member [
  <!ENTITY a "&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;">
  <!ENTITY b "&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;">
  <!ENTITY c "&d;&d;&d;&d;&d;&d;&d;&d;&d;&d;">
  <!ENTITY d "&e;&e;&e;&e;&e;&e;&e;&e;&e;&e;">
  <!ENTITY e "&f;&f;&f;&f;&f;&f;&f;&f;&f;&f;">
  <!ENTITY f "&g;&g;&g;&g;&g;&g;&g;&g;&g;&g;">
  <!ENTITY g "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx">
]>
<member>
&a;
</member>

Remediation

Upgrade rexml to version 3.3.3 or higher.

References

• GitHub Commit
• GitHub Release Notes
• Security Advisory
• Security Advisory

Overview

rexml is an An XML toolkit for Ruby.

Affected versions of this package are vulnerable to Denial of Service (DoS) through the XML parsing process. An attacker can cause a denial of service by sending specially crafted XML documents that contain many specific characters such as <, 0, and %>.

This vulnerability is exploitable if the application is configured to parse untrusted XML documents.

Details

Denial of Service (DoS) describes a family of attacks, all aimed at making a system inaccessible to its intended and legitimate users.

Unlike other vulnerabilities, DoS attacks usually do not aim at breaching security. Rather, they are focused on making websites and services unavailable to genuine users resulting in downtime.

One popular Denial of Service vulnerability is DDoS (a Distributed Denial of Service), an attack that attempts to clog network pipes to the system by generating a large volume of traffic from many machines.

When it comes to open source libraries, DoS vulnerabilities allow attackers to trigger such a crash or crippling of the service by using a flaw either in the application code or from the use of open source libraries.

Two common types of DoS vulnerabilities:

• High CPU/Memory Consumption- An attacker sending crafted requests that could cause the system to take a disproportionate amount of time to process. For example, commons-fileupload:commons-fileupload.

• Crash - An attacker sending crafted requests that could cause the system to crash. For Example, npm ws package

Remediation

Upgrade rexml to version 3.3.2 or higher.

References

• GitHub Commit
• GitHub Release
• Security Advisory

Overview

rexml is an An XML toolkit for Ruby.

Affected versions of this package are vulnerable to Uncontrolled Resource Consumption when parsing an XML that has many <s in an attribute value. An attacker can cause a denial of service by exploiting this behavior.

Workaround

This vulnerability can be mitigated by not parsing untrusted XMLs.

Remediation

Upgrade rexml to version 3.2.7 or higher.

References

• Advisory
• GitHub Commit