Security readout for executives and security teams
Plain-English summary
Certain FiberHome home internet routers (the AN5506-04-FA and HG6245D fiber ONTs) have a serious flaw in their built-in web interface. An attacker who can reach the device on the network can send an oversized cookie that crashes the router or potentially lets them run their own code on it. Because these are ISP-supplied units sitting at the edge of customer networks, a successful attack could expose home or business internet connections.
Executive priority
High urgency for any organization or ISP deploying these FiberHome ONT routers; replace or isolate affected units and confirm with the vendor whether fixed firmware is available.
Technical view
CVE-2021-4464 is a stack-based buffer overflow (CWE-121) in the 'webs' HTTP daemon on FiberHome AN5506-04-FA (≤ RP2631) and HG6245D (< RP2602). The service does not bound-check Cookie header values; cookies longer than 511 bytes overflow a fixed stack buffer, causing a crash and creating the conditions for control-flow hijack. CVSS 4.0 base score is 9.3 (AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H).
Likely exposure
FiberHome AN5506-04-FA fiber ONTs running firmware up to RP2631 and HG6245D models on firmware before RP2602. These devices are widely deployed by ISPs as customer premise equipment, and the embedded HTTP service ('webs') is reachable wherever the management interface is accessible, including the LAN side.
Exploitation context
No public KEV listing and no confirmed in-the-wild exploitation cited in the bundle. Researcher Pierre Kim publicly documented the flaw in January 2021, and a third-party advisory from VulnCheck corroborates the technical details, so weaponization is plausible against exposed devices.
Researcher notes
Stack-based buffer overflow (CWE-121) in the 'webs' HTTP daemon: cookie header values exceeding 511 bytes overrun a fixed stack buffer, leading to crash or potential control-flow hijack. CVSS 4.0 vector indicates network-reachable, no privileges, no user interaction. Affected firmware: AN5506-04-FA ≤ RP2631 and HG6245D < RP2602. CVE record's 'affected' block lists 'defaultStatus: unaffected' with version '0', which appears inconsistent with the description; rely on the description and Pierre Kim's advisory for accurate version coverage. Validate exposure with passive banner/version checks rather than crash-inducing probes.
Mitigation direction
- Inventory FiberHome AN5506-04-FA and HG6245D devices and capture current firmware versions.
- Restrict the device web management interface to trusted management networks; block WAN-side access.
- Contact FiberHome or the ISP supplying the ONT for fixed firmware (post-RP2631 / RP2602) and apply when available.
- Where no patch is available, plan replacement of end-of-support ONT units or place them behind a separate firewall.
- Monitor device logs and uplinks for crashes, reboots, or anomalous traffic indicating exploit attempts.
Validation and detection
- Identify devices via network discovery or ISP CPE inventory; capture model and firmware string.
- Confirm whether the web admin interface is reachable from untrusted networks using non-disruptive probes.
- Compare firmware version against AN5506-04-FA RP2631 and HG6245D RP2602 thresholds documented by Pierre Kim and VulnCheck.
- Cross-check the device model against vendor or ISP advisories for fixed releases or end-of-life status.
- Review network logs for spontaneous reboots of affected ONTs that could indicate exploit attempts.
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-121: 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 lookupCVE-2021-4464 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
- Critical
- CVSS
- 9.3 (4.0)
- Known Exploited
- No
- Published
Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/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:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N——Primary CVE scoreVulnerability scoring details
Base CVSS 4.0 score
9.3CriticalVector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Source materials
- CVE List V5 sourceCVE List V5
- https://pierrekim.github.io/blog/2021-01-12-fiberhome-ont-0day-vulnerabilities.html#misc-remote-stack-overflow-an5506CVE reference · technical-description, exploit
- https://pierrekim.github.io/advisories/2021-fiberhome-0x00-ont.txtCVE reference · technical-description, exploit
- https://www.vulncheck.com/advisories/fiberhome-routers-remote-stack-overflowCVE reference · third-party-advisory
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.
Stack-based Buffer Overflow
Stack-based Buffer Overflow represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.