CVE-2026-9739

CRITICALNVD 9.49.4—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.4 under NVD CVSS v3. EPSS exploit probability: 0.3%, top 80% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
9.4
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 0%CVSS: 9.4Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

Vulnerable to DNS rebinding attacks when using SSE (http://b/499408790). During the beta phase, we implemented allowed-origins and allowed-hosts flags to align with MCP security guidelines. However, the hardcoded Access-Control-Allow-Origin: * header in the SSE initialization handler was inadvertently retained. This vulnerability specifically impacts users connecting via Toolbox using SSE under specification v2024-11-05.

CVSS v3
9.4
EG Score
9.4(medium)
EPSS
19.9%
KEV
Not listed

Published

May 27, 2026

Last Modified

May 29, 2026

Advisory Details (2)

Auto-updated May 28, 2026
Patch available. Sources: github_pr.
github_pr Patch Available

fix: remove hardcoded * allowed origin for sse

Patch available: googleapis/mcp-toolbox v1.2.0 (PR #3054 merged 2026-04-16)

https://github.com/googleapis/mcp-toolbox/pull/3054
generic

[SECURITY] Hardcoded CORS bypass on SSE endpoint in internal/server/mcp.go · Issue #3053 · googleapis/mcp-toolbox · GitHub

https://github.com/googleapis/mcp-toolbox/issues/3053

Vendor Advisories for CVE-2026-9739(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 9× in last 7d / 112× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 299 total refreshes for this CVE.

  1. 2026-07-15 16:57 UTCEPSS rescore
  2. 2026-07-15 02:00 UTCEPSS rescore
  3. 2026-07-13 22:31 UTCEPSS rescore
  4. 2026-07-13 06:13 UTCEPSS rescore
  5. 2026-07-12 05:46 UTCEPSS rescore
  6. 2026-07-12 02:57 UTCEG score recompute
  7. 2026-07-11 22:36 UTCEG score recompute
  8. 2026-07-11 08:27 UTCEPSS rescore
  9. 2026-07-09 19:10 UTCEPSS rescore
  10. 2026-07-08 22:50 UTCEG score recompute
  11. 2026-07-07 13:46 UTCEPSS rescore
  12. 2026-07-06 16:27 UTCEPSS rescore
  13. 2026-07-06 02:23 UTCEPSS rescore
  14. 2026-07-05 02:31 UTCEPSS rescore
  15. 2026-07-04 06:31 UTCEPSS rescore
  16. 2026-07-02 23:34 UTCEG score recompute
  17. 2026-07-01 20:32 UTCEG score recompute
  18. 2026-07-01 20:32 UTCGHSA enrichment
  19. 2026-07-01 15:07 UTCEPSS rescore
  20. 2026-07-01 14:01 UTCEG score recompute
  21. 2026-07-01 14:01 UTCGHSA enrichment
  22. 2026-07-01 09:15 UTCEG score recompute
  23. 2026-07-01 09:15 UTCGHSA enrichment
  24. 2026-07-01 03:49 UTCEG score recompute
  25. 2026-07-01 03:49 UTCGHSA enrichment
Show 75 more
  1. 2026-06-30 23:22 UTCEPSS rescore
  2. 2026-06-30 09:06 UTCEG score recompute
  3. 2026-06-30 09:06 UTCGHSA enrichment
  4. 2026-06-30 02:26 UTCEG score recompute
  5. 2026-06-30 02:26 UTCGHSA enrichment
  6. 2026-06-29 19:46 UTCEG score recompute
  7. 2026-06-29 19:46 UTCGHSA enrichment
  8. 2026-06-29 14:06 UTCEPSS rescore
  9. 2026-06-29 13:54 UTCEG score recompute
  10. 2026-06-29 13:54 UTCGHSA enrichment
  11. 2026-06-29 01:00 UTCEG score recompute
  12. 2026-06-29 01:00 UTCGHSA enrichment
  13. 2026-06-28 20:47 UTCEG score recompute
  14. 2026-06-28 20:47 UTCGHSA enrichment
  15. 2026-06-28 16:33 UTCEG score recompute
  16. 2026-06-28 16:33 UTCGHSA enrichment
  17. 2026-06-28 14:07 UTCEPSS rescore
  18. 2026-06-28 11:41 UTCEG score recompute
  19. 2026-06-28 11:41 UTCGHSA enrichment
  20. 2026-06-28 07:27 UTCEG score recompute
  21. 2026-06-28 07:27 UTCGHSA enrichment
  22. 2026-06-28 04:56 UTCEPSS rescore
  23. 2026-06-28 03:15 UTCEG score recompute
  24. 2026-06-28 03:15 UTCGHSA enrichment
  25. 2026-06-27 23:00 UTCEG score recompute
  26. 2026-06-27 23:00 UTCGHSA enrichment
  27. 2026-06-27 18:46 UTCEG score recompute
  28. 2026-06-27 18:46 UTCGHSA enrichment
  29. 2026-06-27 14:33 UTCEG score recompute
  30. 2026-06-27 14:33 UTCGHSA enrichment
  31. 2026-06-27 10:20 UTCEG score recompute
  32. 2026-06-27 10:20 UTCGHSA enrichment
  33. 2026-06-27 06:07 UTCEG score recompute
  34. 2026-06-27 06:07 UTCGHSA enrichment
  35. 2026-06-27 03:08 UTCEPSS rescore
  36. 2026-06-27 01:54 UTCEG score recompute
  37. 2026-06-27 01:54 UTCGHSA enrichment
  38. 2026-06-26 21:09 UTCEG score recompute
  39. 2026-06-26 21:09 UTCGHSA enrichment
  40. 2026-06-26 16:57 UTCEG score recompute
  41. 2026-06-26 16:57 UTCGHSA enrichment
  42. 2026-06-26 02:07 UTCEG score recompute
  43. 2026-06-26 02:07 UTCGHSA enrichment
  44. 2026-06-25 21:42 UTCEG score recompute
  45. 2026-06-25 21:42 UTCGHSA enrichment
  46. 2026-06-25 17:19 UTCEG score recompute
  47. 2026-06-25 17:19 UTCGHSA enrichment
  48. 2026-06-25 13:50 UTCEPSS rescore
  49. 2026-06-25 13:06 UTCEG score recompute
  50. 2026-06-25 13:05 UTCGHSA enrichment
  51. 2026-06-25 08:48 UTCEG score recompute
  52. 2026-06-25 08:48 UTCGHSA enrichment
  53. 2026-06-24 23:15 UTCEG score recompute
  54. 2026-06-24 23:15 UTCGHSA enrichment
  55. 2026-06-24 14:05 UTCEPSS rescore
  56. 2026-06-23 21:33 UTCEPSS rescore
  57. 2026-06-22 14:25 UTCEPSS rescore
  58. 2026-06-21 14:56 UTCEPSS rescore
  59. 2026-06-21 01:59 UTCEPSS rescore
  60. 2026-06-19 19:26 UTCEPSS rescore
  61. 2026-06-18 17:52 UTCEPSS rescore
  62. 2026-06-18 02:54 UTCEG score recompute
  63. 2026-06-18 02:54 UTCGHSA enrichment
  64. 2026-06-17 22:41 UTCEG score recompute
  65. 2026-06-17 22:41 UTCGHSA enrichment
  66. 2026-06-17 18:28 UTCEG score recompute
  67. 2026-06-17 18:28 UTCGHSA enrichment
  68. 2026-06-17 17:53 UTCEPSS rescore
  69. 2026-06-17 13:51 UTCEG score recompute
  70. 2026-06-17 13:51 UTCGHSA enrichment
  71. 2026-06-17 09:38 UTCEG score recompute
  72. 2026-06-17 09:38 UTCGHSA enrichment
  73. 2026-06-17 04:47 UTCEG score recompute
  74. 2026-06-17 04:47 UTCGHSA enrichment
  75. 2026-06-17 00:17 UTCEG score recompute

Frequently asked(5)

What is CVE-2026-9739?
CVE-2026-9739 is a critical vulnerability published on May 27, 2026. Vulnerable to DNS rebinding attacks when using SSE (http://b/499408790). During the beta phase, we implemented allowed-origins and allowed-hosts flags to align with MCP security guidelines. However, the hardcoded Access-Control-Allow-Origin: * header in the SSE initialization handler was…
When was CVE-2026-9739 disclosed?
CVE-2026-9739 was first published in the National Vulnerability Database on May 27, 2026, with the most recent update on May 29, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-9739 actively exploited?
CVE-2026-9739 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 19.9% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-9739?
CVE-2026-9739 has a CVSS v3 base score of 9.4 (NVD).
How do I remediate CVE-2026-9739?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-9739, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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