# CVE-2021-42756: Memory corruption in FortiWeb

> Unauthenticated remote attackers can achieve arbitrary code execution via stack-based buffer overflows in the FortiWeb proxy daemon.

- **Severity:** Critical 9.3 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 16 Feb 2023
- **Updated:** 23 Oct 2024
- **Also known as:** GHSA-wqmg-f43p-5ccf

## What is CVE-2021-42756?

CVE-2021-42756 is a critical memory corruption vulnerability in Fortinet Fortiweb, published on 16 Feb 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Multiple stack-based buffer overflow vulnerabilities [CWE-121] in the proxy daemon of FortiWeb 5.x all versions, 6.0.7 and below, 6.1.2 and below, 6.2.6 and below, 6.3.16 and below, 6.4 all versions may allow an unauthenticated remote attacker to achieve arbitrary code execution via specifically crafted HTTP requests.

Weakness: CWE-787 (Out-of-bounds Write), CWE-121 (Stack-based Buffer Overflow)

## Which versions are affected by CVE-2021-42756?

It affects 1 product.


Products: Fortinet Fortiweb.

## How do I fix CVE-2021-42756?

No fix has been published for CVE-2021-42756 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-42756 being exploited?

There is no known exploitation of CVE-2021-42756 in the wild and no public exploit code.

Public exploit code:

- https://github.com/3ndorph1n/CVE-2021-42756 (github_poc): https://github.com/3ndorph1n/CVE-2021-42756

## Other vulnerabilities in Fortinet Fortiweb

- [CVE-2025-25257](https://insights.sbomify.com/CVE-2025-25257/): Fortinet FortiWeb SQL Injection (critical 9.6, exploited in the wild)
- [CVE-2025-64446](https://insights.sbomify.com/CVE-2025-64446/): Fortinet FortiWeb Path Traversal (critical 9.4, exploited in the wild)
- [CVE-2025-58034](https://insights.sbomify.com/CVE-2025-58034/): Fortinet FortiWeb OS Command Injection (medium 6.7, exploited in the wild)
- [CVE-2020-29015](https://insights.sbomify.com/CVE-2020-29015/): SQL injection in Fortinet FortiWeb (critical 9.8)
- [CVE-2020-29016](https://insights.sbomify.com/CVE-2020-29016/): Memory corruption in Fortinet FortiWeb (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-42756 and fix it if it is affected.

What it is: CVE-2021-42756 is a critical memory corruption vulnerability in Fortinet Fortiweb, published on 16 Feb 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-42756 in the wild and no public exploit code.

Affected product: Fortinet Fortiweb. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-42756/ (Markdown: https://insights.sbomify.com/CVE-2021-42756/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-42756
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-42756
- OSV: https://osv.dev/vulnerability/CVE-2021-42756

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-42756/
Summaries are generated and may contain mistakes; the official description above is the record.