Mesop: Unauthenticated ANSI Escape Sequence Injection in CSP Reporting Endpoint

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.3 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N · score source

EPSS

• CVE-2026-93421: 0.53% estimated exploitation activity in the next 30 days; percentile 44.74 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The /csp endpoint accepts unauthenticated JSON reports and logs user-controlled values directly to stdout using print() without escaping control characters. A remote attacker can include ANSI/VT100 escape sequences in fields such as blocked-uri or document-uri. When the logs are viewed in an ANSI-capable terminal, these sequences can manipulate the displayed output (e.g., clear the screen, hide text, or inject misleading messages), affecting the integrity of operator-facing logs. Details The CSP reporting endpoint accepts arbitrary JSON and prints several request fields directly: mesop/server/staticfileserving.py Since these values are written to stdout without sanitization, ANSI escape sequences supplied by a remote client are preserved and interpreted by ANSI-compatible terminals. PoC Send the following request: The request is accepted (HTTP 204), and the injected escape sequences are written to stdout unchanged. When the captured output is rendered in a VT100-compatible…

Published by the source

23 Sep 2026, 19:01 UTC

Last modified by the source

1 Oct 2026, 17:55 UTC

Source last checked

26 Oct 2026, 13:54 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-g7f6-rxc4-qhph

2. Revision 3 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-g7f6-rxc4-qhph

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-g7f6-rxc4-qhph

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# Patch Next advisory brief: CVE-2026-93421

Advisory: Mesop: Unauthenticated ANSI Escape Sequence Injection in CSP Reporting Endpoint
IDs: CVE-2026-93421, GHSA-g7f6-rxc4-qhph
Record: https://patchnext.com/advisories/cve-2026-93421 (revision 4, source checked 26 Oct 2026, 13:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mesop 1.3.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mesop: affected <= 1.3.3; fixed in 1.3.4

## Suggested inspection (read-only)
- `python -m pip show mesop`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mesop.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-g7f6-rxc4-qhph (GitHub-reviewed advisory GHSA-g7f6-rxc4-qhph lists affected ranges: PyPI mesop >=0 <=1.3.3)
- Verified: https://osv.dev/vulnerability/GHSA-g7f6-rxc4-qhph (OSV record GHSA-g7f6-rxc4-qhph lists affected ranges: PyPI mesop >=0 <1.3.4 (fixed))

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