probe-image-size: Quadratic-time Denial of Service in the SVG Parser

Overview

probe-image-size scans the SVG header with a searching regular expression, /<[-_.:a-zA-Z0-9][^>]*>/. On input that contains many < characters but no >, the engine restarts the [^>]* scan at every < position and runs to end of input each time, giving quadratic time complexity.

Both the synchronous and the streaming parser are affected.

Impact

Every entry point that reaches the SVG parser is affected: probe.sync(), probe(stream) and probe(url). The URL form is the most exposed one — the input is fetched from a remote host, so an attacker only needs to supply a link.

Processing a crafted buffer blocks the Node.js event loop at 100% CPU for the whole duration. In production environments such as upload validators, image proxies or link unfurl services, a small number of concurrent requests is enough to deny service.

Root Cause Analysis

Two independent problems.

1. Absence of input size cap in the sync path. lib/parse_sync/svg.js copied the entire buffer into a string and matched against it. There was no size limit at all, so cost scaled with the size of the attacker-supplied buffer.

2. Repeated rescanning in the stream path. lib/parse_stream/svg.js did cap accumulated data at 64 KB, but called parseSvg(str) on the whole accumulated string on every chunk, giving O(chunks × N²). The cap does not help here: the more chunks the input is split into, the more times the quadratic scan is repeated.

   Chunk size is influenced by the sender. highWaterMark (16 KB) is a buffering threshold, not a lower bound — a socket read returns whatever has arrived. A server that writes one byte at a time produces one-byte chunks; this was confirmed against the real needle pipeline with default options.

The original report identified (1) only, and stated that the 64 KB cap mitigates the streaming path. It does not.

Proof of Concept (PoC)

Synchronous:

const probe = require('probe-image-size')

// ~200 KB of '<a' — contains '<' but never '>'
probe.sync(Buffer.from('<a'.repeat(100000), 'latin1'))

Streaming — the same payload split into chunks, slower per byte than the synchronous form:

const { Readable } = require('stream')
const probe = require('probe-image-size')

const payload = Buffer.from('<a'.repeat(32768), 'latin1')
const chunks = []
for (let i = 0; i < payload.length; i += 4096) chunks.push(payload.subarray(i, i + 4096))

await probe(Readable.from(chunks))

Measurements on the maintainer's machine: