CVE-2021-27853
CVE-2021-27853 — L2 network filtering can be bypassed using stacked VLAN0 and LLC/SNAP headers
Layer 2 network filtering capabilities such as IPv6 RA guard or ARP inspection can be bypassed using combinations of VLAN 0 headers and LLC/SNAP headers.
Published Updated Sources: cvelistV5, certcc
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.
Exploitation
Unreported
no source claims exploitation
EPSS
1%
chance of exploitation in 30 days
CVSS base
4.7
MEDIUM
CISA SSVC assessment
Three decision points CISA publishes for the CVEs it assesses · SSVC 2.0.3. A stakeholder decision, not a severity score.
Exploitation
None
none · proof-of-concept · active
Automatable
No
can an attacker script all four kill-chain steps
Technical impact
Partial
partial · total control of the vulnerable component
Affected scope
The catalog records vendors and products as separate lists, not pairs, so which product belongs to which vendor is not something this page can say.
Every base score collected
Sources score independently and disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Source |
|---|---|---|---|---|---|
| 4.7 | CVSS 3.1 | MEDIUM | — | — | cvelistV5 |
Weakness & attack patterns
- CWE-290
Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.
- T1134Access Token Manipulation
- T1528Steal Application Access Token
- T1539Steal Web Session Cookie
Detection
Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.
- Search application, proxy, and WAF logs for requests touching /SNAP.
References
7 on the record
- datatracker.ietf.org/doc/draft-ietf-v6ops-ra-guard/08/
reference
- standards.ieee.org/ieee/802.2/1048/
reference
- standards.ieee.org/ieee/802.1Q/10323/
reference
- tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-VU855201-J3z8CKTX
vendor-advisory
- kb.cert.org/vuls/id/855201
reference
- blog.champtar.fr/VLAN0_LLC_SNAP/
reference
- www.kb.cert.org/vuls/id/855201
reference
Elsewhere on this site
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.