CVE-2021-40556

HIGHNVD 8.88.8—
EchelonGraph scoreMEDIUM confidence

Score 8.8 from GitHub Security Advisory (severity: HIGH) published 2022-10-06. NVD baseline CVSS 8.8; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
8.8
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 1%CVSS: 8.8Exploit: NoneExposed: 0

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

A stack overflow vulnerability exists in the httpd service in ASUS RT-AX56U Router Version 3.0.0.4.386.44266. This vulnerability is caused by the strcat function called by "caupload" input handle function allowing the user to enter 0xFFFF bytes into the stack. This vulnerability allows an attacker to execute commands remotely. The vulnerability requires authentication.

CVSS v3
8.8
EG Score
8.8(medium)
EPSS
68.2%
KEV
Not listed

Published

October 6, 2022

Last Modified

November 21, 2024

References (4)

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 12× in last 7d / 49× 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.

  1. 2026-07-16 17:00 UTCEPSS rescore
  2. 2026-07-16 17:00 UTCEPSS rescore
  3. 2026-07-15 16:55 UTCEPSS rescore
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  13. 2026-07-09 19:07 UTCEPSS rescore
  14. 2026-07-08 15:12 UTCEPSS rescore
  15. 2026-07-06 16:25 UTCEPSS rescore
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  17. 2026-07-06 02:21 UTCEPSS rescore
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  23. 2026-07-02 16:57 UTCEPSS rescore
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  1. 2026-06-30 23:20 UTCEPSS rescore
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  3. 2026-06-28 14:05 UTCEPSS rescore
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  6. 2026-06-27 03:06 UTCEPSS rescore
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  8. 2026-06-25 13:48 UTCEPSS rescore
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  10. 2026-06-24 14:03 UTCEPSS rescore
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  12. 2026-06-23 21:31 UTCEPSS rescore
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  14. 2026-06-22 14:24 UTCEPSS rescore
  15. 2026-06-21 14:55 UTCEPSS rescore
  16. 2026-06-21 01:58 UTCEPSS rescore
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  18. 2026-06-19 19:24 UTCEPSS rescore
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  20. 2026-06-18 17:51 UTCEPSS rescore
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  22. 2026-06-17 17:51 UTCEPSS rescore
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  25. 2026-06-16 17:51 UTCEPSS rescore
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  27. 2026-06-15 17:46 UTCEPSS rescore
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  30. 2026-06-13 22:58 UTCEPSS rescore
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  32. 2026-06-12 23:10 UTCEPSS rescore
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  35. 2026-06-10 22:16 UTCEPSS rescore
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  37. 2026-06-10 13:20 UTCEPSS rescore
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  40. 2026-06-07 15:23 UTCEPSS rescore
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  42. 2026-06-06 13:46 UTCEPSS rescore
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  44. 2026-06-05 22:45 UTCEPSS rescore
  45. 2026-06-05 06:09 UTCEPSS rescore
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  47. 2026-06-04 13:10 UTCEPSS rescore
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  49. 2026-06-02 20:11 UTCEPSS rescore
  50. 2026-06-02 20:11 UTCEPSS rescore
  51. 2026-06-01 13:50 UTCEPSS rescore
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  53. 2026-05-31 22:29 UTCEPSS rescore
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  55. 2026-05-31 00:15 UTCEPSS rescore
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  57. 2026-05-29 13:42 UTCEPSS rescore
  58. 2026-05-28 13:43 UTCEPSS rescore
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  60. 2026-05-27 13:39 UTCEPSS rescore
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  62. 2026-05-26 13:42 UTCEPSS rescore
  63. 2026-05-26 13:42 UTCEPSS rescore
  64. 2026-05-25 14:21 UTCEG score recompute
  65. 2026-05-25 14:21 UTCGHSA enrichment

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Frequently asked(5)

What is CVE-2021-40556?
CVE-2021-40556 is a high vulnerability published on October 6, 2022. A stack overflow vulnerability exists in the httpd service in ASUS RT-AX56U Router Version 3.0.0.4.386.44266. This vulnerability is caused by the strcat function called by "caupload" input handle function allowing the user to enter 0xFFFF bytes into the stack. This vulnerability allows an attacker…
When was CVE-2021-40556 disclosed?
CVE-2021-40556 was first published in the National Vulnerability Database on October 6, 2022, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-40556 actively exploited?
CVE-2021-40556 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 68.2% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-40556?
CVE-2021-40556 has a CVSS v3 base score of 8.8 (NVD).
How do I remediate CVE-2021-40556?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-40556, 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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