CVE-2026-59163: Mnemosyne has JWT signature verification bypass sync server that allows authentication bypass

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.3%, percentile 24: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 24% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes mnemosyne-memory (PyPI) <= 3.10.0, fixed in 3.10.1. Check your lockfile, including transitive dependencies.

What it is

Mnemosyne is a memory layer for artificial intelligence agents. Prior to v3.10.1, the auth check in mnemosyne/core/syncserver.py parsed the JWT's header and payload using base64 decoding, then passed the token to a jwt library call with options that effectively disabled signature verification. The server accepted any well-formed token regardless of the signature, including tokens with alg: none and tokens signed with the wrong key. The fix in v3.10.1 replaces the broken decode with a from-scratch HS256 verifier using only the Python standard library. For users who cannot upgrade immediately, restrict network access to the sync server endpoint to trusted clients only. Firewall, reverse proxy with mTLS, or localhost bind with SSH tunnel are all viable. The vulnerability is not exploitable against an unreachable endpoint.

From the GitHub advisory

Summary

The Mnemosyne sync server's authentication check decoded JWT bearer tokens but never verified their HMAC-SHA256 signatures. Any well-formed token was accepted, allowing an unauthenticated attacker to impersonate any user and read or modify their sync data.

Severity: Critical

CVSS 3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N = 9.1

Assumes the sync server endpoint is network-reachable. If your deployment is localhost-only, the score drops substantially and severity becomes High or Medium depending on local exposure. Confirm your threat model.

Weakness type: CWE-347.

Product: mnemosyne-memory (PyPI) · Assigned by: GitHub_M · Published: 2026-09-18

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.33% probability of exploitation activity in the next 30 days; percentile 24.

Timeline

• 2026-09-18 — CVE record last updated
• 2026-09-18 — CVE record published by GitHub_M
• 2026-09-18 — GitHub-reviewed advisory GHSA-xcw4-53cc-hv32 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-xcw4-53cc-hv32 github.com
2. CVE.org record CVE-2026-59163 cve.org
3. NVD entry CVE-2026-59163 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com

Critical PyPI advisories · Today’s Security Digest