# CVE-2026-9537: Mojo::JWT versions before 1.02 for Perl verify HMAC signatures with a non-constant-time string comparison

> Attackers can exploit timing side-channels in HMAC verification to forge JWT tokens, bypassing authentication in applications using this Perl library.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Jul 2026
- **Updated:** 20 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-9537, GHSA-3rp9-gq85-w2mf, UBUNTU-CVE-2026-9537, openSUSE-SU-2026:11450-1

## What is CVE-2026-9537?

CVE-2026-9537 is a medium-severity vulnerability in JBERGER Mojo::JWT, published on 17 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Mojo::JWT versions before 1.02 for Perl verify HMAC signatures with a non-constant-time string comparison.

The decode() method compares the supplied signature to the recomputed HMAC with Perl's eq operator, which stops at the first differing byte, so the comparison time varies with the number of matching leading bytes.

A caller that decodes attacker supplied tokens leaks the expected signature through this timing variation, which can be aggregated over many requests to recover the signature and forge a token.

Weakness: CWE-208 (Observable Timing Discrepancy)

## Which versions are affected by CVE-2026-9537?


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `perl-Mojo-JWT` | openSUSE:Tumbleweed | All versions | 1.20.0-2.1 |

## How do I fix CVE-2026-9537?

Upgrade to a fixed version.

## Is CVE-2026-9537 being exploited?

There is no known exploitation of CVE-2026-9537 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-9537 and fix it if it is affected.

What it is: CVE-2026-9537 is a medium-severity vulnerability in JBERGER Mojo::JWT, published on 17 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-2026-9537 in the wild and no public exploit code.

Affected product: JBERGER Mojo::JWT. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-9537/ (Markdown: https://insights.sbomify.com/CVE-2026-9537/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9537
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9537
- OSV: https://osv.dev/vulnerability/CVE-2026-9537

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-9537/
Summaries are generated and may contain mistakes; the official description above is the record.