express@3.0.6

Fast, unopinionated, minimalist web framework.
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high severity

Denial of Service (Memory Exhaustion)

  • Vulnerable module: qs
  • Introduced through: connect@2.7.2

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2 › qs@0.5.1
    Remediation: Upgrade to express@3.16.0.

Overview

qs is a querystring parser that supports nesting and arrays, with a depth limit.

Affected versions of this package are vulnerable to Denial of Service (Dos) attacks. During parsing, the qs module may create a sparse area (an array where no elements are filled), and grow that array to the necessary size based on the indices used on it. An attacker can specify a high index value in a query string, thus making the server allocate a respectively big array. Truly large values can cause the server to run out of memory and cause it to crash - thus enabling a Denial-of-Service attack.

Remediation

Upgrade qs to version 1.0.0 or greater. In these versions, qs introduced a low limit on the index value, preventing such an attack

References

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high severity

Prototype Override Protection Bypass

  • Vulnerable module: qs
  • Introduced through: connect@2.7.2

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2 › qs@0.5.1
    Remediation: Upgrade to express@4.0.0.

Overview

qs is a querystring parser that supports nesting and arrays, with a depth limit.

By default qs protects against attacks that attempt to overwrite an object's existing prototype properties, such as toString(), hasOwnProperty(),etc.

From qs documentation:

By default parameters that would overwrite properties on the object prototype are ignored, if you wish to keep the data from those fields either use plainObjects as mentioned above, or set allowPrototypes to true which will allow user input to overwrite those properties. WARNING It is generally a bad idea to enable this option as it can cause problems when attempting to use the properties that have been overwritten. Always be careful with this option.

Overwriting these properties can impact application logic, potentially allowing attackers to work around security controls, modify data, make the application unstable and more.

In versions of the package affected by this vulnerability, it is possible to circumvent this protection and overwrite prototype properties and functions by prefixing the name of the parameter with [ or ]. e.g. qs.parse("]=toString") will return {toString = true}, as a result, calling toString() on the object will throw an exception.

Example:

qs.parse('toString=foo', { allowPrototypes: false })
        // {}
        
        qs.parse("]=toString", { allowPrototypes: false })
        // {toString = true} <== prototype overwritten

For more information, you can check out our blog.

Disclosure Timeline

  • February 13th, 2017 - Reported the issue to package owner.
  • February 13th, 2017 - Issue acknowledged by package owner.
  • February 16th, 2017 - Partial fix released in versions 6.0.3, 6.1.1, 6.2.2, 6.3.1.
  • March 6th, 2017 - Final fix released in versions 6.4.0,6.3.2, 6.2.3, 6.1.2 and 6.0.4

Remediation

Upgrade qs to version 6.4.0 or higher. Note: The fix was backported to the following versions 6.3.2, 6.2.3, 6.1.2, 6.0.4.

References

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high severity

Regular Expression Denial of Service (ReDoS)

  • Vulnerable module: fresh
  • Introduced through: fresh@0.1.0, connect@2.7.2 and others

Detailed paths

  • Introduced through: express@3.0.6 › fresh@0.1.0
    Remediation: Upgrade to express@4.15.5.
  • Introduced through: express@3.0.6 › connect@2.7.2 › fresh@0.1.0
    Remediation: Upgrade to express@4.0.0.
  • Introduced through: express@3.0.6 › connect@2.7.2 › send@0.1.0 › fresh@0.1.0
    Remediation: Upgrade to express@4.0.0.
  • Introduced through: express@3.0.6 › send@0.1.0 › fresh@0.1.0
    Remediation: Upgrade to express@4.15.5.

Overview

fresh is HTTP response freshness testing.

Affected versions of this package are vulnerable to Regular expression Denial of Service (ReDoS) attacks. A Regular Expression (/ *, */) was used for parsing HTTP headers and take about 2 seconds matching time for 50k characters.

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.

String Number of C's Number of steps
ACCCX 3 38
ACCCCX 4 71
ACCCCCX 5 136
ACCCCCCCCCCCCCCX 14 65,553

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.

You can read more about Regular Expression Denial of Service (ReDoS) on our blog.

Remediation

Upgrade fresh to version 0.5.2 or higher.

References

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medium severity

Cross-site Scripting (XSS)

  • Vulnerable module: connect
  • Introduced through: connect@2.7.2

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2
    Remediation: Upgrade to express@3.3.1.

Overview

Connect is a stack of middleware that is executed in order in each request.

The "methodOverride" middleware allows the http post to override the method of the request with the value of the _method post key or with the header "x-http-method-override".

Because the user post input was not checked, req.method could contain any kind of value. Because the req.method did not match any common method VERB, connect answered with a 404 page containing the "Cannot [method] [url]" content. The method was not properly encoded for output in the browser.

Source: Node Security Project

Example

~ curl "localhost:3000" -d "_method=<script src=http://nodesecurity.io/xss.js></script>"
        Cannot <SCRIPT SRC=HTTP://NODESECURITY.IO/XSS.JS></SCRIPT> /

Mitigation factors

Update to the newest version of Connect or disable methodOverride. It is not possible to avoid the vulnerability if you have enabled this middleware in the top of your stack.

History

Details

Cross-Site Scripting (XSS) attacks occur when an attacker tricks a user’s browser to execute malicious JavaScript code in the context of a victim’s domain. Such scripts can steal the user’s session cookies for the domain, scrape or modify its content, and perform or modify actions on the user’s behalf, actions typically blocked by the browser’s Same Origin Policy.

These attacks are possible by escaping the context of the web application and injecting malicious scripts in an otherwise trusted website. These scripts can introduce additional attributes (say, a "new" option in a dropdown list or a new link to a malicious site) and can potentially execute code on the clients side, unbeknown to the victim. This occurs when characters like \< > \" \' are not escaped properly.

There are a few types of XSS:

  • Persistent XSS is an attack in which the malicious code persists into the web app’s database.
  • Reflected XSS is an which the website echoes back a portion of the request. The attacker needs to trick the user into clicking a malicious link (for instance through a phishing email or malicious JS on another page), which triggers the XSS attack.
  • DOM-based XSS is an that occurs purely in the browser when client-side JavaScript echoes back a portion of the URL onto the page. DOM-Based XSS is notoriously hard to detect, as the server never gets a chance to see the attack taking place.
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medium severity

Cross-site Scripting (XSS)

  • Vulnerable module: express
  • Introduced through: express@3.0.6

Detailed paths

  • Introduced through: express@3.0.6
    Remediation: Upgrade to express@3.11.0.

Overview

express is a minimalist web framework.

Affected versions of this package do not enforce the user's browser to set a specific charset in the content-type header while displaying 400 level response messages. This could be used by remote attackers to perform a cross-site scripting attack, by using non-standard encodings like UTF-7.

Details

Cross-Site Scripting (XSS) attacks occur when an attacker tricks a user’s browser to execute malicious JavaScript code in the context of a victim’s domain. Such scripts can steal the user’s session cookies for the domain, scrape or modify its content, and perform or modify actions on the user’s behalf, actions typically blocked by the browser’s Same Origin Policy.

These attacks are possible by escaping the context of the web application and injecting malicious scripts in an otherwise trusted website. These scripts can introduce additional attributes (say, a "new" option in a dropdown list or a new link to a malicious site) and can potentially execute code on the clients side, unbeknown to the victim. This occurs when characters like \< > \" \' are not escaped properly.

There are a few types of XSS:

  • Persistent XSS is an attack in which the malicious code persists into the web app’s database.
  • Reflected XSS is an which the website echoes back a portion of the request. The attacker needs to trick the user into clicking a malicious link (for instance through a phishing email or malicious JS on another page), which triggers the XSS attack.
  • DOM-based XSS is an that occurs purely in the browser when client-side JavaScript echoes back a portion of the URL onto the page. DOM-Based XSS is notoriously hard to detect, as the server never gets a chance to see the attack taking place.

Recommendations

Update express to 3.11.0, 4.5.0 or higher.

References

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medium severity

Denial of Service (Event Loop Blocking)

  • Vulnerable module: qs
  • Introduced through: connect@2.7.2

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2 › qs@0.5.1
    Remediation: Upgrade to express@3.16.0.

Overview

qs is a querystring parser that supports nesting and arrays, with a depth limit.

Affected versions of this package are vulnerable to Denial of Service (DoS). When parsing a string representing a deeply nested object, qs will block the event loop for long periods of time. Such a delay may hold up the server's resources, keeping it from processing other requests in the meantime, thus enabling a Denial-of-Service attack.

Remediation

Update qs to version 1.0.0 or higher. In these versions, qs enforces a max object depth (along with other limits), limiting the event loop length and thus preventing such an attack.

References

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medium severity

Directory Traversal

  • Vulnerable module: send
  • Introduced through: connect@2.7.2 and send@0.1.0

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2 › send@0.1.0
    Remediation: Upgrade to express@4.0.0.
  • Introduced through: express@3.0.6 › send@0.1.0
    Remediation: Upgrade to express@3.16.10.

Overview

send is a library for streaming files from the file system.

Affected versions of this package are vulnerable to Directory-Traversal attacks due to insecure comparison. When relying on the root option to restrict file access a malicious user may escape out of the restricted directory and access files in a similarly named directory. For example, a path like /my-secret is consedered fine for the root /my.

Details

A Directory traversal attack (also known as path traversal) aims to access files and directories that are stored outside the intended folder. By manipulating variables that reference files with “dot-dot-slash (../)” sequences and its variations or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system including application source code, configuration and other critical system files.

Directory traversal vulnerabilities can be generally divided into two types:

  • Information Disclosure: Allows the attacker to gain information about the folder structure or read the contents of sensitive files on the system.

st is a module for serving static files on web pages, and contains a vulnerability of this type. In our example, we will serve files from the public route.

If an attacker requests the following URL from our server, it will in turn, leak the sensitive private key of the root user.

curl http://localhost:8080/public/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/root/.ssh/id_rsa

Note %2e is the URL encoded version of . (dot).

  • Writing arbitrary files: Allows the attacker to create or replace existing files.

This can be achieved using a specially crafted zip archive, that holds path traversal filenames. So when the filename gets concatenated to the target extraction folder, the final path ends up outside of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicous file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

2018-04-15 22:04:29 .....           19           19  good.txt
        2018-04-15 22:04:42 .....           20           20  ../../../../../../root/.ssh/authorized_keys

Remediation

Upgrade to a version greater than or equal to 0.8.4.

References

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medium severity

Non-Constant Time String Comparison

  • Vulnerable module: cookie-signature
  • Introduced through: connect@2.7.2 and cookie-signature@0.0.1

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2 › cookie-signature@0.0.1
    Remediation: Upgrade to express@3.12.1.
  • Introduced through: express@3.0.6 › cookie-signature@0.0.1
    Remediation: Upgrade to express@3.12.1.

Overview

'cookie-signature' is a library for signing cookies.

Versions before 1.0.4 of the library use the built-in string comparison mechanism, ===, and not a time constant string comparison. As a result, the comparison will fail faster when the first characters in the token are incorrect. An attacker can use this difference to perform a timing attack, essentially allowing them to guess the secret one character at a time.

You can read more about timing attacks in Node.js on the Snyk blog: https://snyk.io/blog/node-js-timing-attack-ccc-ctf/

Remediation

Upgrade to 1.0.4 or greater.

References

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medium severity

Root Path Disclosure

  • Vulnerable module: send
  • Introduced through: connect@2.7.2 and send@0.1.0

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2 › send@0.1.0
    Remediation: Upgrade to express@4.0.0.
  • Introduced through: express@3.0.6 › send@0.1.0
    Remediation: Upgrade to express@3.19.1.

Overview

Send is a library for streaming files from the file system as an http response. It supports partial responses (Ranges), conditional-GET negotiation, high test coverage, and granular events which may be leveraged to take appropriate actions in your application or framework.

Affected versions of this package are vulnerable to a Root Path Disclosure.

Remediation

Upgrade send to version 0.11.1 or higher. If a direct dependency update is not possible, use snyk wizard to patch this vulnerability.

References

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low severity

Regular Expression Denial of Service (ReDoS)

  • Vulnerable module: mime
  • Introduced through: connect@2.7.2 and send@0.1.0

Detailed paths

  • Introduced through: express@3.0.6 › connect@2.7.2 › send@0.1.0 › mime@1.2.6
    Remediation: Upgrade to express@4.0.0.
  • Introduced through: express@3.0.6 › send@0.1.0 › mime@1.2.6
    Remediation: Upgrade to express@4.16.0.

Overview

mime is a comprehensive, compact MIME type module.

Affected versions of this package are vulnerable to Regular expression Denial of Service (ReDoS). It uses regex the following regex /.*[\.\/\\]/ in its lookup, which can cause a slowdown of 2 seconds for 50k characters.

The Regular expression Denial of Service (ReDoS) is a type of Denial of Service attack. Many Regular Expression implementations may reach extreme situations that cause them to work very slowly (exponentially related to input size), allowing an attacker to exploit this and can cause the program to enter these extreme situations by using a specially crafted input and cause the service to excessively consume CPU, resulting in a Denial of Service.

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.

String Number of C's Number of steps
ACCCX 3 38
ACCCCX 4 71
ACCCCCX 5 136
ACCCCCCCCCCCCCCX 14 65,553

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 mime to versions 1.4.1, 2.0.3 or higher.

References

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