GHSA-9wv6-86v2-598j: path-to-regexp outputs backtracking regular expressions

Affected packages

path-to-regexpnpm

Affected: >=0.2.0 <1.9.0
Fixed in: 1.9.0

Update to the fixed version for your release line, for example:npm install path-to-regexp@1.9.0

path-to-regexpnpm

Affected: <0.1.10
Fixed in: 0.1.10

Update to the fixed version for your release line, for example:npm install path-to-regexp@0.1.10

path-to-regexpnpm

Affected: >=7.0.0 <8.0.0
Fixed in: 8.0.0

Update to the fixed version for your release line, for example:npm install path-to-regexp@8.0.0

path-to-regexpnpm

Affected: >=2.0.0 <3.3.0
Fixed in: 3.3.0

Update to the fixed version for your release line, for example:npm install path-to-regexp@3.3.0

path-to-regexpnpm

Affected: >=4.0.0 <6.3.0
Fixed in: 6.3.0

Update to the fixed version for your release line, for example:npm install path-to-regexp@6.3.0

Details

Impact

A bad regular expression is generated any time you have two parameters within a single segment, separated by something that is not a period (.). For example, /:a-:b.

Patches

For users of 0.1, upgrade to 0.1.10. All other users should upgrade to 8.0.0.

These versions add backtrack protection when a custom regex pattern is not provided:

They do not protect against vulnerable user supplied capture groups. Protecting against explicit user patterns is out of scope for old versions and not considered a vulnerability.

Version 7.1.0 can enable strict: true and get an error when the regular expression might be bad.

Version 8.0.0 removes the features that can cause a ReDoS.

Workarounds

All versions can be patched by providing a custom regular expression for parameters after the first in a single segment. As long as the custom regular expression does not match the text before the parameter, you will be safe. For example, change /:a-:b to /:a-:b([^-/]+).

If paths cannot be rewritten and versions cannot be upgraded, another alternative is to limit the URL length. For example, halving the attack string improves performance by 4x faster.

Details

Using /:a-:b will produce the regular expression /^\/([^\/]+?)-([^\/]+?)\/?$/. This can be exploited by a path such as /a${'-a'.repeat(8_000)}/a. OWASP has a good example of why this occurs, but the TL;DR is the /a at the end ensures this route would never match but due to naive backtracking it will still attempt every combination of the :a-:b on the repeated 8,000 -a.

Because JavaScript is single threaded and regex matching runs on the main thread, poor performance will block the event loop and can lead to a DoS. In local benchmarks, exploiting the unsafe regex will result in performance that is over 1000x worse than the safe regex. In a more realistic environment using Express v4 and 10 concurrent connections, this translated to average latency of ~600ms vs 1ms.

References

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