CVE-2026-45121

MEDIUMPre-NVD 4.34.3
EchelonGraph scoreMEDIUM confidence

“—” beside the score: it is equal to the CVSS score beside it.

This medium-severity CVE scores 4.3 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.3% (25th percentile of EPSS-scored CVEs). No GitHub Security Advisory CVSS is on record for this CVE.

Triggered by: Headline CVSS from a secondary CVSS source
Sources: EPSS, a secondary CVSS source
4.3EG
EchelonGraph verdictMonitorRemediate within your standard update timelines (the SSVC decision on this page).
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.3%CVSS: 4.3Exploit: 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 calendar module does not check permissions consistently when listing calendars, allowing authenticated users to access titles of calendars that are otherwise inaccessible. The affected calendar-selection paths in calendar.php perform permission checks against an invalid calendar context before returning calendar titles. The uniquely identifying implementation details include titles of inaccessible calendars, and invalid calendar permission context. This issue is fixed in version 1.8.40.

CVSS v3
4.3
EG Score
4.3MEDIUMmedium confidence
EG Risk
19
EG Risk 19/100

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 (value × EchelonGraph’s weight)
Severity43% × 45%
The EG Score, 4.3 of 10.
Exploitation0.3% × 40%
FIRST EPSS: the 30-day exploitation probability when EchelonGraph scored this CVE.
Automatability0% × 15%
CISA's SSVC Automatable value: no (Vulnrichment).
SSVC (CISA's decision table, applied by EchelonGraph): Track at every mission impact level.
Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE.
Action: A fix is available. Apply it within your standard update timelines.
EPSS PROB
0.3%
EPSS %ILE
25th
KEV
Not listed

SSVC (CISA's decision table, applied by EchelonGraph)Track at every mission impact level.

A fix is available. Apply it within your standard update timelines.

Exploitation none (CISA Vulnrichment) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table

BOD 26-04 (CISA's remediation timeline, Table 1)60 days if the affected system is publicly exposed; Fix on system upgrade if it is not.

In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it

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.
Web page

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 78e07fea34a6 (mybb/mybb)

Patch available: mybb/mybb mybb_1840 (contains commit 78e07fea34a6)

https://github.com/mybb/mybb/commit/78e07fea34a6f6c326e668526bfb8e451c19e966
GitHub advisoryMentions a patch (unverified)

Insufficient permission check for calendar select · Advisory · mybb/mybb · GitHub

https://github.com/mybb/mybb/security/advisories/GHSA-r25v-7pcm-q34p

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 11× in last 7d / 43× in last 30d)

Each row is a time one of our pipelines fetched, updated or re-scored this CVE. 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. An arrow marks a change the record states ("CVSS v3 → 9.1"); EPSS and GHSA rows show the values recorded at that time, not what changed (EPSS values are rounded; hover one for the exact figures). Most recent first.

  1. 2026-10-13 16:34 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  2. 2026-10-12 16:34 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  3. 2026-10-12 16:04 UTCEG score recompute
  4. 2026-10-11 19:46 UTCEG score recompute
  5. 2026-10-11 17:01 UTCEG score recompute
  6. 2026-10-11 14:26 UTCEG score recompute
  7. 2026-10-11 13:48 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  8. 2026-10-10 20:00 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  9. 2026-10-09 20:07 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  10. 2026-10-08 17:12 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  11. 2026-10-07 23:48 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  12. 2026-10-07 08:49 UTCEG score recompute
  13. 2026-10-06 15:18 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  14. 2026-10-06 08:23 UTCEG score recompute
  15. 2026-10-05 17:43 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  16. 2026-10-04 23:22 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  17. 2026-10-03 17:39 UTCEG score recompute
  18. 2026-10-03 14:25 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  19. 2026-10-02 15:38 UTCEG score recompute
  20. 2026-10-01 19:50 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  21. 2026-10-01 10:12 UTCEG score recompute
  22. 2026-09-30 15:03 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  23. 2026-09-30 09:49 UTCEG score recompute
  24. 2026-09-28 08:31 UTCEG score recompute
  25. 2026-09-27 13:48 UTCEPSS rescoreEPSS 0.3% · 25th percentile
Show 68 more
  1. 2026-09-27 07:57 UTCEG score recompute
  2. 2026-09-26 15:58 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  3. 2026-09-25 07:42 UTCEG score recompute
  4. 2026-09-24 07:20 UTCEG score recompute
  5. 2026-09-23 17:53 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  6. 2026-09-23 06:49 UTCEG score recompute
  7. 2026-09-22 06:27 UTCEG score recompute
  8. 2026-09-21 06:04 UTCEG score recompute
  9. 2026-09-20 20:15 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  10. 2026-09-20 05:01 UTCEG score recompute
  11. 2026-09-19 04:37 UTCEG score recompute
  12. 2026-09-18 19:28 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  13. 2026-09-18 04:14 UTCEG score recompute
  14. 2026-09-17 19:31 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  15. 2026-09-17 03:51 UTCEG score recompute
  16. 2026-09-16 14:08 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  17. 2026-09-16 05:15 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  18. 2026-09-16 03:28 UTCEG score recompute
  19. 2026-09-14 02:39 UTCEG score recompute
  20. 2026-09-13 16:47 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  21. 2026-09-13 02:17 UTCEG score recompute
  22. 2026-09-12 15:01 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  23. 2026-09-12 01:53 UTCEG score recompute
  24. 2026-09-11 14:53 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  25. 2026-09-11 09:37 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  26. 2026-09-11 01:30 UTCEG score recompute
  27. 2026-09-10 09:34 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  28. 2026-09-10 01:07 UTCEG score recompute
  29. 2026-09-08 22:00 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  30. 2026-09-08 21:17 UTCEG score recompute
  31. 2026-09-07 16:01 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  32. 2026-09-07 10:26 UTCEG score recompute
  33. 2026-09-06 13:47 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  34. 2026-09-06 08:33 UTCEG score recompute
  35. 2026-09-05 08:11 UTCEG score recompute
  36. 2026-09-04 07:48 UTCEG score recompute
  37. 2026-09-04 05:06 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  38. 2026-09-03 06:24 UTCEG score recompute
  39. 2026-09-02 14:12 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  40. 2026-09-02 06:02 UTCEG score recompute
  41. 2026-09-01 13:54 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  42. 2026-09-01 05:12 UTCEG score recompute
  43. 2026-08-31 04:47 UTCEG score recompute
  44. 2026-08-30 19:17 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  45. 2026-08-30 04:24 UTCEG score recompute
  46. 2026-08-30 01:22 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  47. 2026-08-29 03:58 UTCEG score recompute
  48. 2026-08-28 21:41 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  49. 2026-08-27 22:47 UTCEG score recompute
  50. 2026-08-27 14:25 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  51. 2026-08-26 22:25 UTCEG score recompute
  52. 2026-08-26 14:46 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  53. 2026-08-25 22:02 UTCEG score recompute
  54. 2026-08-25 13:49 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  55. 2026-08-24 21:40 UTCEG score recompute
  56. 2026-08-23 21:17 UTCEG score recompute
  57. 2026-08-23 00:19 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  58. 2026-08-22 20:55 UTCEG score recompute
  59. 2026-08-21 23:49 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  60. 2026-08-21 20:32 UTCEG score recompute
  61. 2026-08-20 22:55 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  62. 2026-08-19 19:47 UTCEG score recompute
  63. 2026-08-19 17:04 UTCEPSS rescoreEPSS 0.3% · 16th percentile
  64. 2026-08-18 19:24 UTCEG score recompute
  65. 2026-08-18 19:05 UTCEG score recompute
  66. 2026-08-18 16:30 UTCEG score recompute
  67. 2026-08-18 15:59 UTCEG score recomputeFirst assessed: 4.3 MEDIUM
  68. 2026-08-18 15:59 UTCMITRE cvelistV5first tracked

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Frequently asked(5)

What is CVE-2026-45121?
CVE-2026-45121 is a medium-severity vulnerability; its CVE record was published on August 18, 2026. MyBB is free and open source forum software. Prior to 1.8.40, the calendar module does not check permissions consistently when listing calendars, allowing authenticated users to access titles of calendars that are otherwise inaccessible. The affected calendar-selection paths in calendar.php perform…
When was the CVE record for CVE-2026-45121 published?
The CVE record for CVE-2026-45121 was published on August 18, 2026. That is the record's publication date; the vulnerability itself may have been made public earlier. EchelonGraph's copy of the record carries a last-modified date of September 8, 2026; the record at its source may have been updated since.
Is CVE-2026-45121 actively exploited?
CVE-2026-45121 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.3% probability of exploitation in the next 30 days (25th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-45121?
CVE-2026-45121 has a CVSS base score of 4.3 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-45121?
A fix for CVE-2026-45121 is available: update to the fixed version the vendor names in its advisory.

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