Fulcio allocates excessive memory during token parsing

Exploitation

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

CVSS

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

EPSS

• CVE-2025-66506: 0.21% estimated exploitation activity in the next 30 days; percentile 11.40 (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.

Function identity.extractIssuerURL currently splits (via a call to strings.Split) its argument (which is untrusted data) on periods. As a result, in the face of a malicious request with an (invalid) OIDC identity token in the payload containing many period characters, a call to extractIssuerURL incurs allocations to the tune of O(n) bytes (where n stands for the length of the function's argument), with a constant factor of about 16. Relevant weakness: CWE-405: Asymmetric Resource Consumption (Amplification) Details See identity.extractIssuerURL Impact Excessive memory allocation

Published by the source

5 Dec 2025, 18:18 UTC

Last modified by the source

5 Dec 2025, 18:18 UTC

Source last checked

26 Oct 2026, 15:13 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-f83f-xpx7-ffpw

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# Patch Next advisory brief: CVE-2025-66506

Advisory: Fulcio allocates excessive memory during token parsing
IDs: CVE-2025-66506, GHSA-f83f-xpx7-ffpw
Record: https://patchnext.com/advisories/cve-2025-66506 (revision 1, source checked 26 Oct 2026, 15:13 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/sigstore/fulcio 1.8.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/sigstore/fulcio: affected <= 1.8.2; fixed in 1.8.3

## Suggested inspection (read-only)
- Go github.com/sigstore/fulcio: check the installed version with your package manager.

## 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 github.com/sigstore/fulcio.
- 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-f83f-xpx7-ffpw (GitHub-reviewed advisory GHSA-f83f-xpx7-ffpw lists affected ranges: Go github.com/sigstore/fulcio >=0 <=1.8.2)

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