---
id: CVE-2026-109584
title: >-
  HTTP/2 cookie crumb join expands a small HPACK header block into an unbounded
  allocation in Bandit
summary: >-
  Improper Handling of Highly Compressed Data (Data Amplification) vulnerability
  in the HTTP/2 implementation of mtrudel bandit allows an unauthenticated
  remote attacker to exhaust server memory via a small HPACK-compressed HEADERS
  block. …
severity: high
cvss: 8.7
cvssVector: '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'
cvssSource: cna
cwe:
  - CWE-409
vendor: mtrudel
product: bandit
affected:
  - bandit >= 0.3.3 < 1.14.0
  - >-
    mtrudel/bandit >= 44bf969cd59851622fb6ee4d16227fbc39a77fb4 <
    220b9dfd27f9a21305be2afc75cbe585ddc24e7a
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-26T15:11:12.403348Z'
published: '2026-10-26'
updated: '2026-10-26'
sourceUpdated: '2026-10-26T15:35:51.081Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-109584'
references:
  - url: 'https://github.com/mtrudel/bandit/security/advisories/GHSA-7gfj-7gvq-3xqj'
    label: >-
      HTTP/2 cookie crumb join expands a small HPACK header block into an
      unbounded allocation in Bandit
  - url: 'https://cna.erlef.org/cves/CVE-2026-109584.html'
    label: EEF CNA record for CVE-2026-109584
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-109584'
    label: OSV record EEF-CVE-2026-109584
  - url: >-
      https://github.com/mtrudel/bandit/commit/44bf969cd59851622fb6ee4d16227fbc39a77fb4
    label: Introducing commit 44bf969 in mtrudel/bandit
  - url: >-
      https://github.com/mtrudel/bandit/commit/220b9dfd27f9a21305be2afc75cbe585ddc24e7a
    label: Fix commit 220b9df in mtrudel/bandit
tags:
  - cve.org
ingestedAt: '2026-10-26T14:27:31.821Z'
---

## Overview

Improper Handling of Highly Compressed Data (Data Amplification) vulnerability in the HTTP/2 implementation of mtrudel bandit allows an unauthenticated remote attacker to exhaust server memory via a small HPACK-compressed HEADERS block. The block stores one large cookie value in the HPACK dynamic table and then references it tens of thousands of times with 1-byte indexed fields. In the reporter's run, a 49 KB block made a server with default settings allocate a 183 MB cookie binary before any application code ran, and every concurrent stream can repeat the cost.

Bandit.HTTP2.Connection caps only the compressed block (max_header_block_size), and its check of the decoded list against max_header_list_size never fires because that setting defaults to :infinity. combine_cookie_crumbs/1 in lib/bandit/http2/stream.ex then joins every crumb into one binary.

This issue affects bandit: from 0.3.3 before 1.14.0.

## Affected

- `bandit >= 0.3.3 < 1.14.0`
- `mtrudel/bandit >= 44bf969cd59851622fb6ee4d16227fbc39a77fb4 < 220b9dfd27f9a21305be2afc75cbe585ddc24e7a`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

### Workarounds

On 1.12.2 and later, set a finite decoded header list limit, for example http_2_options: [default_local_settings: [max_header_list_size: 65_536]], which resets the stream before the cookie crumbs are joined. From 0.7.0, disabling HTTP/2 with http_2_options: [enabled: false] removes the vulnerable path.