Weakness Type (CWE)

CWE-407 — CWE-407 Inefficient Algorithmic Complexity

CAPEC

CVSS 4.0 Score

8.7

HIGH

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Vulnerability description

Inefficient Algorithmic Complexity vulnerability in mtrudel bandit allows unauthenticated remote denial of service via CPU exhaustion during WebSocket fragment reassembly.

The size guard 'Elixir.Bandit.WebSocket.Connection':oversize_message?/2 called from handle_frame/3 in lib/bandit/websocket/connection.ex appends each non-final continuation frame to a left-nested iolist and then re-measures the entire accumulated buffer with IO.iodata_length/1 on every frame. Because the buffer grows by one element per frame and is fully re-traversed each time, reassembly work is quadratic (O(n^2)) in the number of continuation frames.

The max_fragmented_message_size limit (default 8 MB) bounds total bytes but not frame count, and each frame can carry as little as one payload byte, so an attacker can send millions of tiny continuation frames using modest bandwidth to pin a CPU core for minutes to hours. Many concurrent connections can starve the whole server of CPU, denying service to legitimate users. The WebSocket read timeout does not help, because it is an idle timeout evaluated between reads and cannot preempt the synchronous reassembly work spent inside a single callback.

This issue affects bandit: from 1.11.0 before 1.12.1.

Affected

pkg:hex/bandit

Module Source File Routine
Elixir.Bandit.WebSocket.Connection lib/bandit/websocket/connection.ex Bandit.WebSocket.Connection.handle_frame/3
Bandit.WebSocket.Connection.oversize_message?/2
Status Type Version Changes / Fixed in
affected semver ⓘ 1.11.0 < 1.12.1

pkg:github/mtrudel/bandit

Module Source File Routine
Elixir.Bandit.WebSocket.Connection lib/bandit/websocket/connection.ex Bandit.WebSocket.Connection.handle_frame/3
Bandit.WebSocket.Connection.oversize_message?/2
Status Type Version Changes / Fixed in
affected git ⓘ 21612c7c7b < 418ef7e906

Configurations

This vulnerability only affects applications that serve WebSocket endpoints through Bandit (including Phoenix applications that use Bandit as the HTTP adapter). Applications that do not upgrade any connections to WebSocket are not affected.

References

Credits

  • Finder: Peter Ullrich
  • Remediation developer: Mat Trudel

CVE record as JSON:  GET /cves/CVE-2026-65623.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-65623.json