CVE-2026-45734

MEDIUMPre-NVD 5.35.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.3 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.6% (44th percentile of EPSS-scored CVEs). 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, secondary
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.6%CVSS: 5.3Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

MyBB is free and open source forum software. Prior to 1.8.40, the built-in CAPTCHA does not consistently enforce single-use semantics, allowing remote attackers to bypass CAPTCHA controls through challenge replay. The successful validation paths in contact.php, member.php?action=do_resendactivation, member.php?action=do_lostpw, member.php?action=do_emailuser, and sendthread.php?action=do_sendtofriend do not call captcha::invalidate_captcha() for the MyBB Default CAPTCHA selected by the captchaimage setting. A valid response can therefore be reused until a non-vulnerable endpoint invalidates it, an incorrect response is submitted, or the challenge expires. This issue is fixed in version 1.8.40.

CVSS v3
5.3
EG Score
5.3MEDIUMmedium confidence
EG Risk
39
EG Risk 39/100CISA SSVC

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity53% × 45%
Exploitation1% × 40%
Automatability100% × 15%
CISA SSVC: Track at low or medium mission impact; Attend at high (mission-essential systems).
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
0.6%
EPSS %ILE
44th
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation none (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

August 18, 2026

Last Modified

September 8, 2026

Advisory Details (4)

Auto-updated Aug 18, 2026
Patch available. Sources: github, github_commit, github_release.
generic

Version 1.8.40 - MyBB

https://mybb.com/versions/1.8.40
github_release Patch Available

mybb_1840

Patch available: mybb/mybb mybb_1840

https://github.com/mybb/mybb/releases/tag/mybb_1840
github_commit Patch Available

commit c2ed54f9259b (mybb/mybb)

Patch available: mybb/mybb mybb_1840 (contains commit c2ed54f9259b)

https://github.com/mybb/mybb/commit/c2ed54f9259b9ce05728a9e657169033fe4adffc
githubMentions a patch (unverified)

Default CAPTCHA missing invalidation · Advisory · mybb/mybb · GitHub

https://github.com/mybb/mybb/security/advisories/GHSA-jrrr-f3jw-mjmc

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 11× in last 7d / 46× 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. 2026-10-09 20:07 UTCEPSS rescore
  2. 2026-10-08 17:12 UTCEPSS rescore
  3. 2026-10-07 23:48 UTCEPSS rescore
  4. 2026-10-07 11:13 UTCEG score recompute
  5. 2026-10-06 15:18 UTCEPSS rescore
  6. 2026-10-06 12:53 UTCEG score recompute
  7. 2026-10-05 17:43 UTCEPSS rescore
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  9. 2026-10-04 23:22 UTCEPSS rescore
  10. 2026-10-03 17:39 UTCEG score recompute
  11. 2026-10-03 14:25 UTCEPSS rescore
  12. 2026-10-02 06:53 UTCEG score recompute
  13. 2026-10-01 19:50 UTCEPSS rescore
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  15. 2026-09-30 15:03 UTCEPSS rescore
  16. 2026-09-30 09:49 UTCEG score recompute
  17. 2026-09-29 04:14 UTCEG score recompute
  18. 2026-09-28 06:05 UTCEG score recompute
  19. 2026-09-27 13:48 UTCEPSS rescore
  20. 2026-09-27 07:57 UTCEG score recompute
  21. 2026-09-26 15:58 UTCEPSS rescore
  22. 2026-09-25 01:28 UTCEG score recompute
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  24. 2026-09-23 17:53 UTCEPSS rescore
  25. 2026-09-22 20:26 UTCEG score recompute
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  1. 2026-09-20 21:12 UTCEG score recompute
  2. 2026-09-20 20:15 UTCEPSS rescore
  3. 2026-09-19 23:03 UTCEG score recompute
  4. 2026-09-19 00:55 UTCEG score recompute
  5. 2026-09-18 19:28 UTCEPSS rescore
  6. 2026-09-18 02:45 UTCEG score recompute
  7. 2026-09-17 19:31 UTCEPSS rescore
  8. 2026-09-17 04:36 UTCEG score recompute
  9. 2026-09-16 14:08 UTCEPSS rescore
  10. 2026-09-16 06:28 UTCEG score recompute
  11. 2026-09-16 05:15 UTCEPSS rescore
  12. 2026-09-15 08:19 UTCEG score recompute
  13. 2026-09-14 10:10 UTCEG score recompute
  14. 2026-09-13 16:47 UTCEPSS rescore
  15. 2026-09-13 12:02 UTCEG score recompute
  16. 2026-09-12 15:01 UTCEPSS rescore
  17. 2026-09-11 15:45 UTCEG score recompute
  18. 2026-09-11 14:53 UTCEPSS rescore
  19. 2026-09-11 09:37 UTCEPSS rescore
  20. 2026-09-10 17:34 UTCEG score recompute
  21. 2026-09-10 09:34 UTCEPSS rescore
  22. 2026-09-09 19:26 UTCEG score recompute
  23. 2026-09-08 22:00 UTCEPSS rescore
  24. 2026-09-07 16:08 UTCEG score recompute
  25. 2026-09-07 16:01 UTCEPSS rescore
  26. 2026-09-06 15:22 UTCEG score recompute
  27. 2026-09-06 13:47 UTCEPSS rescore
  28. 2026-09-05 17:11 UTCEG score recompute
  29. 2026-09-04 19:02 UTCEG score recompute
  30. 2026-09-04 05:06 UTCEPSS rescore
  31. 2026-09-02 21:57 UTCEG score recompute
  32. 2026-09-02 14:12 UTCEPSS rescore
  33. 2026-09-01 20:27 UTCEG score recompute
  34. 2026-09-01 13:54 UTCEPSS rescore
  35. 2026-08-31 00:10 UTCEG score recompute
  36. 2026-08-30 19:17 UTCEPSS rescore
  37. 2026-08-30 02:02 UTCEG score recompute
  38. 2026-08-30 01:22 UTCEPSS rescore
  39. 2026-08-29 03:53 UTCEG score recompute
  40. 2026-08-28 21:41 UTCEPSS rescore
  41. 2026-08-27 23:10 UTCEG score recompute
  42. 2026-08-27 14:25 UTCEPSS rescore
  43. 2026-08-27 01:00 UTCEG score recompute
  44. 2026-08-26 14:46 UTCEPSS rescore
  45. 2026-08-26 02:52 UTCEG score recompute
  46. 2026-08-25 13:49 UTCEPSS rescore
  47. 2026-08-25 04:43 UTCEG score recompute
  48. 2026-08-23 08:26 UTCEG score recompute
  49. 2026-08-23 00:19 UTCEPSS rescore
  50. 2026-08-22 10:18 UTCEG score recompute
  51. 2026-08-21 23:49 UTCEPSS rescore
  52. 2026-08-21 12:10 UTCEG score recompute
  53. 2026-08-20 22:55 UTCEPSS rescore
  54. 2026-08-20 14:01 UTCEG score recompute
  55. 2026-08-19 17:04 UTCEPSS rescore
  56. 2026-08-19 15:25 UTCEG score recompute
  57. 2026-08-19 14:58 UTCEG score recompute
  58. 2026-08-18 16:27 UTCEG score recompute
  59. 2026-08-18 15:59 UTCEG score recompute
  60. 2026-08-18 15:59 UTCMITRE cvelistV5first tracked

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Frequently asked(5)

What is CVE-2026-45734?
CVE-2026-45734 is a medium vulnerability published on August 18, 2026. MyBB is free and open source forum software. Prior to 1.8.40, the built-in CAPTCHA does not consistently enforce single-use semantics, allowing remote attackers to bypass CAPTCHA controls through challenge replay. The successful validation paths in contact.php, member.php?action=doresendactivation,…
When was CVE-2026-45734 disclosed?
CVE-2026-45734 was first published on August 18, 2026, with the most recent update on September 8, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-45734 actively exploited?
CVE-2026-45734 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.6% probability of exploitation in the next 30 days (44th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-45734?
CVE-2026-45734 has a CVSS base score of 5.3 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-45734?
A fix for CVE-2026-45734 is available: update to the fixed version the vendor names in its advisory.

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