# CVE-2026-59248: Unbounded HPACK/QPACK prefixed-integer decoding in Cowlib causes memory-exhaustion DoS

> Unbounded HPACK/QPACK decoding in Cowlib allows remote attackers to exhaust server memory via crafted HTTP/2 or HTTP/3 headers, causing denial of service.

- **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
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 28 Jul 2026
- **Updated:** 29 Jul 2026
- **Also known as:** AZL-94305, DEBIAN-CVE-2026-59248, EEF-CVE-2026-59248, GHSA-f37p-pc7r-rghm, UBUNTU-CVE-2026-59248, USN-8869-1

## What is CVE-2026-59248?

CVE-2026-59248 is a high-severity denial of service vulnerability in ninenines cowlib, published on 28 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Allocation of resources without limits vulnerability in ninenines cowlib allows an unauthenticated remote HTTP/2 or HTTP/3 peer to exhaust memory on the vulnerable server (or client) and cause a denial of service.

The HPACK and QPACK prefixed-integer decoder cow_hpack_common:dec_big_int/3 in src/cow_hpack_common.hrl (invoked from cow_hpack:decode/2 in src/cow_hpack.erl and from cow_qpack:decode_field_section/3 in src/cow_qpack.erl) reads continuation octets until it sees one whose high bit is clear, evaluating Int + (Value bsl M) at each step with the shift M growing by seven per octet. No limit is enforced on the number of continuation octets, on the resulting bit width, or on the value; the decoder consumes whatever encoded length the peer supplies.

Because Erlang integers are immutable, each intermediate Value bsl M and each accumulator update allocates a fresh bignum whose digit width grows linearly with the number of octets processed so far. Summed across the whole decode, the transient bignum digit materialization is on the order of the square of the encoded length. A single maximal HPACK indexed representation carried inside one HTTP/2 HEADERS plus one CONTINUATION frame at Cowboy's default max_frame_size_received can force hundreds of megabytes of transient allocation and garbage-collection churn before the resulting header-table index is rejected as invalid. Repeated or concurrent connections multiply the pressure and can drive the Erlang VM to memory exhaustion.

Cowlib is the HTTP parser used by Cowboy, RabbitMQ's management plugin, and other Erlang and Elixir HTTP/2 and HTTP/3 servers and clients, so any exposed endpoint that accepts HPACK or QPACK from an untrusted peer is reachable.

This issue affects cowlib: from 2.0.0 before 2.19.0.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-59248?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `cowlib` | Hex | From 2.0.0-pre.1 | 2.19.0 |

## How do I fix CVE-2026-59248?

Upgrade cowlib to 2.19.0 or later.

## Is CVE-2026-59248 being exploited?

There is no known exploitation of CVE-2026-59248 in the wild and no public exploit code.

Public exploit code:

- CVE-2026-59248 (github_poc): https://github.com/DarynOngera/bug-free-broccoli

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-59248 and fix it if it is affected.

What it is: CVE-2026-59248 is a high-severity denial of service vulnerability in ninenines cowlib, published on 28 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-59248 in the wild and no public exploit code.

Affected packages and fixed versions:
- cowlib (Hex): affected from 2.0.0-pre.1; fixed in 2.19.0

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-2026-59248/ (Markdown: https://insights.sbomify.com/CVE-2026-59248/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-59248
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-59248
- OSV: https://osv.dev/vulnerability/CVE-2026-59248

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-59248/
Summaries are generated and may contain mistakes; the official description above is the record.