Addressable: Denial of service in URI path normalization

Impact

Addressable::URI.normalize_path takes time quadratic in the number of nested dot segments in a path (e.g., "/" + "a/" * n + "../" * n, which for n = 3 is /a/a/a/../../../), because it removes one dot segment per pass over the whole path. URI#normalize, URI#normalized_path, URI#==, URI#=== and URI#join all call it. Normalizing, comparing or joining a URI taken from user input can result in uncontrolled resource consumption, leading to denial of service. On 2.9.0, normalizing a 34 KB path takes about 3.5 seconds on MRI Ruby 4.0, and the cost grows with the square of the path length.

This affects every Ruby implementation, including MRI Ruby 3.2 and later. URI.parse by itself is unaffected, as is code that never normalizes, compares, or joins URIs from untrusted sources.

Affected Versions

This vulnerability affects all versions of Addressable prior to 2.9.1. It is fixed in 2.9.1.

Workarounds

• Limit the length of URIs taken from user input before calling normalize, == or join on them. Web servers often limit request lines to about 8 KB, which keeps the cost to about 150 milliseconds per call on typical hardware, but URIs from other sources (request bodies, headers such as Location, stored data) may not be limited.
• Reject paths with many dot segments before normalizing them.

Credits

Reported by Matthew Mongeau (@halogenandtoast), Ruby Central Gem Security, who also suggested the single-pass approach used in the fix.

For more information

If there are any questions or comments about this advisory:

• Open an issue