# CVE-2026-75870: Punk versions before 0.18 for Perl allow session cookie forgery via an empty default HMAC key when a session is declared without a secret.

> Attackers can forge session cookies to impersonate users or escalate privileges if the Punk Perl library is configured without a session secret.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 22 Aug 2026
- **Updated:** 25 Aug 2026
- **Also known as:** GHSA-3679-xr9w-9625

## What is CVE-2026-75870?

CVE-2026-75870 is a critical authentication bypass vulnerability, published on 22 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Punk versions before 0.18 for Perl allow session cookie forgery via an empty default HMAC key when a session is declared without a secret.

The session keyword freezes its options onto the application as given: it does not require a secret, warn, or refuse to start when one is absent. The cookie read and the write-back both default that key to the empty string, so a declaration with no secret option, or with an undefined or empty one, signs and verifies with a zero-length HMAC-SHA256 key.

An attacker who knows the cookie format can then mint one offline carrying any contents the session holds, such as a user identifier or a role. Nothing marks the misconfiguration at runtime: cookies are well formed and sessions round-trip as expected.

Weakness: CWE-1394 (Use of Default Cryptographic Key)

## Which versions are affected by CVE-2026-75870?


## How do I fix CVE-2026-75870?

No fix has been published for CVE-2026-75870 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-75870 being exploited?

There is no known exploitation of CVE-2026-75870 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-75870 and fix it if it is affected.

What it is: CVE-2026-75870 is a critical authentication bypass vulnerability, published on 22 Aug 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-2026-75870 in the wild and no public exploit code.

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-2026-75870/ (Markdown: https://insights.sbomify.com/CVE-2026-75870/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-75870
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-75870
- OSV: https://osv.dev/vulnerability/CVE-2026-75870

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-75870/
Summaries are generated and may contain mistakes; the official description above is the record.