# CVE-2026-77528: Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

> Remote attackers can cause resource exhaustion in Autobahn Python by sending compressed WebSocket frames that bypass payload size limits after inflation.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Sep 2026
- **Updated:** 19 Sep 2026
- **Also known as:** DEBIAN-CVE-2026-77528, GHSA-hxp9-w8x3-p566, PYSEC-2026-4027, PYSEC-2026-4031, UBUNTU-CVE-2026-77528, openSUSE-SU-2026:11852-1

## What is CVE-2026-77528?

CVE-2026-77528 is a medium-severity denial of service vulnerability in crossbario autobahn-python, published on 18 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Autobahn Python is a WebSocket and WAMP implementation for Python that supports Twisted and asyncio. Prior to 26.7.1, WebSocket endpoints that accept permessage-deflate and rely on maxMessagePayloadSize enforce that limit against the compressed frame length before inflation but do not recheck the decompressed message size before delivery. A remote unauthenticated client can send a valid compressed frame below the configured wire-size limit that expands beyond the application message limit, causing oversized data to be allocated, joined, validated, and passed to application callbacks. This can create resource-exhaustion pressure, but the advisory does not establish confidentiality or integrity impact. This issue is fixed in version 26.7.1.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-409 (Improper Handling of Highly Compressed Data (Data Amplification))

## Which versions are affected by CVE-2026-77528?

6 open source packages ship an affected version, including autobahn and crossbar.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `autobahn` | PyPI | All versions | 26.7.1 |
| `autobahn` | PyPI | All versions | 26.7.1 |
| `autobahn` | PyPI | All versions | 26.7.1 |
| `crossbar` | PyPI | All versions | 26.7.1 |
| `crossbar` | PyPI | All versions | 26.7.1 |
| `crossbar` | PyPI | All versions | 26.7.1 |
| `python-autobahn` | openSUSE:Tumbleweed | All versions | 26.7.1-2.1 |

## How do I fix CVE-2026-77528?

Upgrade autobahn to 26.7.1 or later.

## Is CVE-2026-77528 being exploited?

There is no known exploitation of CVE-2026-77528 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77528 and fix it if it is affected.

What it is: CVE-2026-77528 is a medium-severity denial of service vulnerability in crossbario autobahn-python, published on 18 Sep 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-2026-77528 in the wild and no public exploit code.

Affected packages and fixed versions:
- autobahn (PyPI): affected all versions; fixed in 26.7.1
- autobahn (PyPI): affected all versions; fixed in 26.7.1
- autobahn (PyPI): affected all versions; fixed in 26.7.1
- crossbar (PyPI): affected all versions; fixed in 26.7.1
- crossbar (PyPI): affected all versions; fixed in 26.7.1
- crossbar (PyPI): affected all versions; fixed in 26.7.1

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-2026-77528/ (Markdown: https://insights.sbomify.com/CVE-2026-77528/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77528
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77528
- OSV: https://osv.dev/vulnerability/CVE-2026-77528

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77528/
Summaries are generated and may contain mistakes; the official description above is the record.