Threats / Realtek / CVE-2021-35395
CVE-2021-35395
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Realtek AP-Router SDK vulnerability
Realtek AP-Router SDK contains a buffer overflow in the boa HTTP web server caused by unsafe handling of overly long form parameters, enabling denial-of-service attacks.
Verdict
Today item — known-exploited.
A remotely exploitable buffer overflow in the boa web server component of Realtek AP-Router SDK allows unauthenticated attackers to crash the service. The high EPSS score and active exploitation in the wild indicate significant risk to deployed router infrastructure.
CISA KEV Yes · 2021-11-034EPSS 0.97961 (verify live)5CVSS 9.8 (CISA)3CISA SSVC Exploitation: active · Automatable: yes · Impact: total3Exploit Public PoC / detection · nuclei template11
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
704 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03).
Probability (EPSS)
EPSS 0.97961 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 9.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) · Affected: Realtek, AP-Router SDK. Confirm exact fixed builds in the vendor advisory.CVSS — CISA-assigned via the ADP container (CVE.org), version 3.1; the CNA left no metrics, so CISA's own score is used ahead of NVD.
Weakness (CWE)
Mapped to CWE-20 Improper Input Validation, CWE-122 Heap-based Buffer Overflow — weakness family: Memory safety.CWE assignment from the public NVD record (may be stale); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: yes · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 79 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
Public exploit available
Public exploit tooling is catalogued for this CVE (a public proof-of-concept / detection template). Catalog entries: nuclei template. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft HTTP requests with oversized form parameters targeting the boa web server.
Business
Router availability is disrupted, interrupting network connectivity for end users.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I trigger the buffer overflow to cause the web server process to crash repeatedly.
Business
Management and configuration interfaces become inaccessible, preventing legitimate administration.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05