# CVE-2026-109584: HTTP/2 cookie crumb join expands a small HPACK header block into an unbounded allocation in Bandit


- **Severity:** High 8.7 (`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`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 26 Oct 2026
- **Updated:** 26 Oct 2026
- **Also known as:** EEF-CVE-2026-109584, GHSA-7gfj-7gvq-3xqj, UBUNTU-CVE-2026-109584

## What is CVE-2026-109584?

CVE-2026-109584 is a high-severity vulnerability in mtrudel bandit, published on 26 Oct 2026. 

### Official description

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.

Weakness: CWE-409 (Improper Handling of Highly Compressed Data (Data Amplification))

## Which versions are affected by CVE-2026-109584?

1 open source package ship an affected version, including bandit.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bandit` | Hex | From 0.3.3 | 1.14.0 |

## How do I fix CVE-2026-109584?

Upgrade bandit to 1.14.0 or later.

## Is CVE-2026-109584 being exploited?

There is no known exploitation of CVE-2026-109584 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-109584 and fix it if it is affected.

What it is: CVE-2026-109584 is a high-severity vulnerability in mtrudel bandit, published on 26 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-109584 in the wild and no public exploit code.

Affected packages and fixed versions:
- bandit (Hex): affected from 0.3.3; fixed in 1.14.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-109584/ (Markdown: https://insights.sbomify.com/CVE-2026-109584/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-109584
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109584
- OSV: https://osv.dev/vulnerability/CVE-2026-109584

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-109584/
Summaries are generated and may contain mistakes; the official description above is the record.