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: 0.4%, top 66% 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, secondary
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 5.3Exploit: None knownExposed: 0

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

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
5.3(medium)
EG Risk
39(Track)
EG Risk 39/100SSVC: Track

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%
Exploitation0% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
34%
KEV
Not listed

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
github Patch Available

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

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 in the National Vulnerability Database 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% probability of exploitation in the next 30 days, which ranks it in the top 65.6% of all scored CVEs.
What is the CVSS score of CVE-2026-45734?
CVE-2026-45734 has a CVSS v3 base score of 5.3 (NVD).
How do I remediate CVE-2026-45734?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-45734, 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.

Dependency Blast Radius

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