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CVE-2026-81900

HIGH 7.3 ALL-YEARS

Published 2026-09-14 · Last modified 2026-09-15 · ConcreteCMS · NVD: Analyzed

Concrete CMS before 9.5.3 applied only trim() to the YouTube block's stored width and height values and printed them into iframe HTML attributes without escaping or integer casting, resulting in stored cross-site scripting. A user with edit_block permission could inject an event handler that executed script for visitors rendering the page, acting with administrative privileges where the victim was an administrator. The Concrete CMS security team gave this vulnerability a CVSS v4.0 score of 7.3 with vector CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. Thanks sh4d0byss for reporting.

ELEVATED IMPACT

Severe if exploited (CVSS 7.3), but no confirmed exploitation. Patch on a normal cadence.

Exploitation likelihood

0.3%chance of exploitation in 30 days · 16th percentile

○ In CISA KEV○ Public exploit / PoC◆ SSVC: exploitation none, automatable no

Impact if exploited

7.3CVSS 4.0 · HIGH · CNA

  • ConfidentialityHigh
  • IntegrityHigh
  • AvailabilityHigh

What an attacker needs

  • ✓Access: Reachable over the network — no local access needed
  • ⚠Privileges: Requires an admin / high-privilege account
  • ⚠User interaction: Succeeds with passive user activity
  • ✓Complexity: No special conditions — reliably repeatable
  • ⚠Requirements: Specific conditions must be present

✓ lowers the bar for an attacker · ⚠ raises it

Proof of concept & exploit code

No public exploit or proof-of-concept code is catalogued for this CVE (no ExploitDB, Nuclei, or CISA-KEV entry). It may still exist — search GitHub or check the references below.

Affected

Vendors Concrete Cms Concretecms

Products Concrete Cms

Weakness (CWE)

  • CWE-79: Improper Neutralization of Input During Web Page Generation

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Known Affected Software Configurations

VendorProductVersion range
ConcretecmsConcrete Cms< 9.5.3

All CVSS metrics

  • HIGH 7.3 v4.0 · CNA Primary
    CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
  • HIGH 7.3 v4.0 · NVD Secondary
    CVSS:4.0/AV:N/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
  • MEDIUM 6.1 v3.1 · NVD Primary
    CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

References

Patches & mitigations

Sources: NVD · CVE.org · EPSS