Security readout for executives and security teams
Plain-English summary
CVE-2026-21857 affects REDAXO CMS before 5.20.2. A logged-in user with backup permissions could abuse the Backup addon to include readable files from the webroot in an exported archive. This could expose sensitive application files. The issue is fixed in REDAXO 5.20.2.
Executive priority
Prioritize remediation for REDAXO sites with multiple administrators, delegated backup access, or sensitive files under the webroot. The issue is high severity but requires authenticated backup privileges, making access control review and timely upgrade the key actions.
Technical view
The Backup addon’s file export functionality did not validate the EXPDIR POST parameter against the UI-generated allowlist. An authenticated attacker with backup permissions could use path traversal or absolute paths within the document root to cause arbitrary readable webroot files to be added to a .tar.gz export. The CVE maps to CWE-22 and CWE-24.
Likely exposure
Exposure is limited to REDAXO installations before 5.20.2 where untrusted or compromised authenticated users have backup permissions. Internet exposure increases concern, but exploitation still requires high privileges according to the CVSS vector.
Exploitation context
The public advisory describes the vulnerability mechanics and fixed version. The source bundle does not indicate CISA KEV listing or active exploitation in the wild. No weaponized exploit status is established from the provided sources.
Researcher notes
The key control failure is server-side trust in EXPDIR despite the UI allowlist. Scope appears to be readable files within the document root, not arbitrary filesystem-wide reads, based on the provided description. Validate only in authorized environments and avoid extracting sensitive data unnecessarily.
Mitigation direction
- Upgrade REDAXO to version 5.20.2 or later.
- Review which accounts have Backup addon permissions.
- Remove backup permissions from users who do not require them.
- Check vendor advisory and release notes for any additional guidance.
Validation and detection
- Inventory REDAXO instances and confirm version numbers.
- Identify installations running versions earlier than 5.20.2.
- Review REDAXO user roles for backup permissions.
- Confirm the 5.20.2 update is deployed successfully.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-22: File access and web shell behavior lookup
File traversal and upload weaknesses can lead teams to review file, web shell, execution, and collection telemetry. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCWE-24: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupFile access behavior lookup
The CVE wording references file access or upload behavior, so file telemetry and web shell review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2026-21857 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 8.3 (4.0)
- Known Exploited
- No
- Published
Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:N/VA:N/SC:H/SI:H/SA:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:N/VA:N/SC:H/SI:H/SA:N——Primary CVE scoreVulnerability scoring details
Base CVSS 4.0 score
8.3HighVector: CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:N/VA:N/SC:H/SI:H/SA:N
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/redaxo/redaxo/security/advisories/GHSA-824x-88xg-cwrvCVE reference · x_refsource_CONFIRM
- https://github.com/redaxo/redaxo/releases/tag/5.20.2CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Path Traversal: '../filedir'
Path Traversal: '../filedir' represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.