Impact

A specially crafted value of the Sec-Websocket-Protocol header can be used to significantly slow down a ws server.

Proof of concept

for (const length of [1000, 2000, 4000, 8000, 16000, 32000]) {
  const value = 'b' + ' '.repeat(length) + 'x';
  const start = process.hrtime.bigint();

  value.trim().split(/ *, */);

  const end = process.hrtime.bigint();

  console.log('length = %d, time = %f ns', length, end - start);
}

Patches

The vulnerability was fixed in ws@7.4.6 (websockets/ws@00c425e) and backported to ws@6.2.2 (websockets/ws@78c676d) and ws@5.2.3 (websockets/ws@76d47c1).

Workarounds

In vulnerable versions of ws, the issue can be mitigated by reducing the maximum allowed length of the request headers using the --max-http-header-size=size and/or the maxHeaderSize options.

Credits

The vulnerability was responsibly disclosed along with a fix in private by Robert McLaughlin from University of California, Santa Barbara.

References

• GHSA-6fc8-4gx4-v693
• https://nvd.nist.gov/vuln/detail/CVE-2021-32640
• websockets/ws@00c425e
• websockets/ws#1895
• https://lists.apache.org/thread.html/rdfa7b6253c4d6271e31566ecd5f30b7ce1b8fb2c89d52b8c4e0f4e30@%3Ccommits.tinkerpop.apache.org%3E
• https://security.netapp.com/advisory/ntap-20210706-0005/