CVE-2026-111058: Mercury: Unauthenticated Remote Code Execution via /api/kernels

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.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.4%, percentile 37: FIRST’s model estimates a 0.4% chance of exploitation activity in the next 30 days, higher than 37% 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 mercury (PyPI) <= 3.2.5, fixed in 3.2.6. Check your lockfile, including transitive dependencies.

What it is

Mercury is a Python framework for sharing Jupyter notebooks as web applications and dashboards. Prior to 3.2.6, default tokenless deployments allow anonymous clients to create Jupyter kernels and send executerequest messages containing arbitrary Python. The endpoint blocklist does not cover /api/kernels, and the websocket wrapper previously tracked idle activity without authorizing executable messages. This issue is fixed in version 3.2.6.

From the GitHub advisory

Mercury runs a JupyterLab server with anonymous access by default (mercuryapp/main.py sets --IdentityProvider.token='' when no --token/MERCURYTOKEN is given; when tokenless it also sets --ServerApp.disablecheckxsrf=True). It relies on a blocklist (mercuryapp/blockhandler.py BLOCKEDPATTERNS: /lab, /api/terminals, /api/shutdown, /files) to keep anonymous users off dangerous Jupyter endpoints.

The vulnerability /api/kernels is NOT in the blocklist (and cannot be — Mercury needs kernels to run notebooks), and patchkernelwebsockethandler() (mercuryapp/idletimeout.py) only tracks the idle-timeout: it wraps onmessage to call self.application.timeoutmanager.touch() and then invokes the original handler with no filtering of executerequest. So any unauthenticated network client can:

Distinct from GHSA-f8c7-rwf9-fmgm (incomplete fix) GHSA-f8c7 was "Unauthenticated RCE via /api/terminals" — a missing-comma blocklist bypass. Its fix (add the comma so /api/terminals is blocked) does NOT address /api/kernels, which remains exploitable. The blocklist approach is fundamentally unable to cover kernels, so this is not a variant that another blocklist entry fixes.

Runtime-proven (on the patched version) On mercury 3.2.4 — the release that fixed GHSA-f8c7 — with the default (tokenless) config, as an unauthenticated client: POST /api/terminals returns 403 (the f8c7 fix is present), but POST /api/kernels returns 201; opening the kernel websocket and sending an executerequest executed attacker-supplied Python (ran whoami as the server process user and wrote a marker file /tmp/PWNEDMERCURYRCE). Full unauthenticated remote code execution. Deterministic PoC (client script + notebook) included.

Impact Unauthenticated, network-reachable arbitrary code execution as the Mercury server process user on every default deployment — complete compromise of the host/container and any data/secrets it can reach.

Weakness type: CWE-862.

Product: mercury (PyPI) · Assigned by: GitHub_M · Published: 2026-10-19

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 none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.44% probability of exploitation activity in the next 30 days; percentile 37.

Timeline

• 2026-10-19 — CVE record last updated
• 2026-10-19 — CVE record published by GitHub_M
• 2026-10-19 — GitHub-reviewed advisory GHSA-rv4m-xm3f-3ccq 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-rv4m-xm3f-3ccq github.com
2. CVE.org record CVE-2026-111058 cve.org
3. NVD entry CVE-2026-111058 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

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