Cyber Resilience

CVE-2025-66488

Discourse ≤ 3.5.4

Published
28 January 2026
Modified
30 January 2026
CVSS Score v3.1 4.6
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:L
EPSS Score 0.0017 7th percentile
Risk Priority 36 floored blend · peak EPSS

Summary

CVE-2025-66488 is a medium-severity Improper Encoding or Escaping of Output (CWE-116) vulnerability in Discourse Discourse. Its CVSS base score is 4.6 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Content Injection (T1659); ranked at the 7th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

OWASP Top 10 for Web (2025)

A05:2025 Injection

EU & UK References

Vulnerability Data

Discourse is an open source discussion platform. A vulnerability present in versions prior to 3.5.4, 2025.11.2, 2025.12.1, and 2026.1.0 affects anyone who uses S3 for uploads. While scripts may be executed, they will only be run in the context of…

more

the S3/CDN domain, with no site credentials. Versions 3.5.4, 2025.11.2, 2025.12.1, and 2026.1.0 fix the issue. As a workaround, disallow html or xml files for uploads in authorized_extensions. For existing html xml uploads, site owners can consider deleting them.

CWE(s)
CWE-116

Related Threats

MITRE ATT&CK Enterprise Techniques

T1659 Content Injection Initial Access
Adversaries may gain access and continuously communicate with victims by injecting malicious content into systems through online network traffic.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1059 Command and Scripting Interpreter Execution
Adversaries may abuse command and script interpreters to execute commands, scripts, or binaries.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2023-32301Same product: Discourse Discourse
CVE-2025-48062Same product: Discourse Discourse
CVE-2026-53963Same product: Discourse Discourse
CVE-2023-22454Same product: Discourse Discourse
CVE-2025-48954Same product: Discourse Discourse
CVE-2023-22468Same product: Discourse Discourse
CVE-2023-43659Same product: Discourse Discourse
CVE-2026-33411Same product: Discourse Discourse
CVE-2023-25172Same product: Discourse Discourse
CVE-2024-52794Same product: Discourse Discourse

Affected Assets

discourse
discourse
2025.12.0, 2026.1.0 · ≤ 3.5.4 · 2025.11.0 — 2025.11.2

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.1.2
  • V1.2.1
  • V1.2.3

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

SI-15 Information Output Filtering
addresses: CWE-116

Validating that output matches expected content directly mitigates failures to properly encode or escape data for its destination context.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require proper output encoding to prevent injection and message malformation.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.28 Secure coding mostly match
prevents

Secure coding standards explicitly require correct output encoding and escaping to preserve message structure.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect missing or incorrect encoding but does not itself implement the control.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates output encoding/escaping practices that directly prevent improper encoding.

A.8.26 Application security requirements partial match
prevents

Application security requirements include explicit rules for safe output handling and encoding.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles reduce the likelihood of missing encoding but do not prescribe the actual technique.

References