Security readout for executives and security teams
Plain-English summary
Fortinet FortiWeb versions in the listed ranges contain critical stack-based buffer overflows in the proxy daemon. An unauthenticated remote attacker could potentially run arbitrary code by sending crafted HTTP requests. Because FortiWeb commonly sits at the network edge, exposed appliances should be treated as urgent assets until Fortinet remediation guidance is confirmed and applied.
Executive priority
Treat this as a critical edge-appliance risk. Prioritize asset discovery and Fortinet-guided remediation for exposed FortiWeb systems before routine maintenance work.
Technical view
CVE-2021-42756 is a CWE-121 stack-based buffer overflow issue affecting FortiWeb 5.x, 6.0.7 and below, 6.1.2 and below, 6.2.6 and below, 6.3.16 and below, and all 6.4 versions. The CVSS 3.1 score is 9.3 with network attack vector, low complexity, no privileges, no user interaction, and high confidentiality, integrity, and availability impact.
Likely exposure
Exposure is likely where affected FortiWeb appliances process untrusted HTTP traffic, especially internet-facing deployments. The source bundle does not provide deployment prevalence, default exposure, or fixed-version details.
Exploitation context
The CVE record describes possible unauthenticated remote arbitrary code execution through specifically crafted HTTP requests. CISA KEV status is false in the bundle, and no cited source here confirms active exploitation.
Researcher notes
Evidence supports a critical unauthenticated remote code execution condition, but the provided sources do not include exploit maturity beyond CVSS E:P, exploit telemetry, fixed versions, or workaround specifics. Avoid assuming active exploitation without KEV or corroborating sources.
Mitigation direction
- Inventory FortiWeb appliances and record exact firmware versions.
- Compare versions against the affected ranges in Fortinet FG-IR-21-186.
- Check Fortinet PSIRT guidance for supported fixes or workarounds.
- Prioritize remediation for internet-facing or high-traffic FortiWeb deployments.
- Increase monitoring for proxy daemon crashes or unusual HTTP activity.
Validation and detection
- Confirm whether any FortiWeb device runs an affected version.
- Identify which appliances process untrusted external HTTP requests.
- Review external attack surface records for exposed FortiWeb services.
- Check logs for abnormal proxy daemon failures or suspicious HTTP request patterns.
- Verify remediation status through version evidence after changes.
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 lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-42756 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 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:P/RL:U/RC:C
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:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:P/RL:U/RC:C3.95.9Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
9.3CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:P/RL:U/RC:C
Source materials
- CVE List V5 sourceCVE List V5
- https://fortiguard.com/psirt/FG-IR-21-186CVE reference
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.