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- Vulnerable module: uglify-js
- Introduced through: email@example.com
Introduced through: firstname.lastname@example.org › email@example.comRemediation: Upgrade to firstname.lastname@example.org.
parse() function in the
uglify-js package prior to version 2.6.0 is vulnerable to regular expression denial of service (ReDoS) attacks when long inputs of certain patterns are processed.
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:
AThe 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.
DFinally, we ensure this section of the string ends with a 'D'
The expression would match inputs such as
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:
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|
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.
Upgrade to version
2.6.0 or greater.
If a direct dependency update is not possible, use
snyk wizard to patch this vulnerability.
- Vulnerable module: cli
- Introduced through: email@example.com
Introduced through: firstname.lastname@example.org › email@example.comRemediation: Upgrade to firstname.lastname@example.org.
cli is an npm package used for rapidly building command line apps.
When used in
daemon mode, the library makes insecure use of two files in the
/tmp/<app-name>.log. These allow an attacker to overwrite files they typically cannot access, but that are accessible by the user running the CLI-using app. This is possible since the
/tmp/ folder is (typically) writeable to all system users, and because the names of the files in question are easily predicted by an attacker.
Note that while this is a real vulnerability, it relies on functionality (
daemon mode) which is only supported in very old Node versions (0.8 or older), and so is unlikely to be used by most
cli users. To avoid any doubt, the fixed version (1.0.0) removes support for this feature entirely.
For example, assume user victim occasionally runs a CLI tool called
cli-tool, which uses the
If an attacker gains write access to the
/tmp/ folder of that machine (but not the higher permissions victim has), they can create the symbolic link
/tmp/cli-tool.pid -> /home/victim/important-file. When victim runs
important-file in victim's root directory will be nullified. If the CLI tool is run as root, the same can be done to nullify
/etc/passwd and make the system unbootable.
Note that popular CLI tools have no reason to mask their names, and so attackers can easily guess a long list of tools victims may run by checking the
cli package dependents.
cli to version
1.0.0 or greater, which disables the affected feature.
From the fix release notes:
This feature relies on a beta release (e.g. version 0.5.1) of a Node.js module on npm--one that was superseded by a stable (e.g. version 1.0) release published three years ago . Due to a build-time dependency on the long-since deprecated `node-waf` tool, the module at that version can only be built for Node.js versions 0.8 and below. Given this, actual usage of this feature is likely very limited. Remove it completely so the integrity of this module's core functionality can be verified.