# CVE-2021-0261: Denial of service in Junos OS

> Unauthenticated attackers can cause extended denial of service in Junos OS web services by sending a high volume of specific requests.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 22 Apr 2021
- **Updated:** 16 Sep 2024
- **Also known as:** GHSA-3pxh-pm6h-2cp2

## What is CVE-2021-0261?

CVE-2021-0261 is a high-severity denial of service vulnerability in Juniper Junos, published on 22 Apr 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A vulnerability in the HTTP/HTTPS service used by J-Web, Web Authentication, Dynamic-VPN (DVPN), Firewall Authentication Pass-Through with Web-Redirect, and Captive Portal allows an unauthenticated attacker to cause an extended Denial of Service (DoS) for these services by sending a high number of specific requests. This issue affects: Juniper Networks Junos OS 12.3 versions prior to 12.3R12-S17 on EX Series; 12.3X48 versions prior to 12.3X48-D105 on SRX Series; 15.1 versions prior to 15.1R7-S8; 15.1X49 versions prior to 15.1X49-D230 on SRX Series; 16.1 versions prior to 16.1R7-S8; 17.4 versions prior to 17.4R2-S12, 17.4R3-S3; 18.1 versions prior to 18.1R3-S11; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R2-S4, 18.3R3-S3; 18.4 versions prior to 18.4R2-S5, 18.4R3-S4; 19.1 versions prior to 19.1R2-S2, 19.1R3-S2; 19.2 versions prior to 19.2R1-S5, 19.2R3; 19.3 versions prior to 19.3R2-S4, 19.3R3; 19.4 versions prior to 19.4R1-S3, 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R1-S3, 20.1R2; 20.2 versions prior to 20.2R1-S1, 20.2R2.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2021-0261?

It affects 1 product.


Products: Juniper Junos.

## How do I fix CVE-2021-0261?

No fix has been published for CVE-2021-0261 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-0261 being exploited?

There is no known exploitation of CVE-2021-0261 in the wild and no public exploit code.

## Other vulnerabilities in Juniper Junos

- [CVE-2023-36845](https://insights.sbomify.com/CVE-2023-36845/): Juniper Junos OS EX Series and SRX Series PHP External Variable Modification (critical 9.8, exploited in the wild)
- [CVE-2020-1631](https://insights.sbomify.com/CVE-2020-1631/): Juniper Junos OS Path Traversal (high 8.8, exploited in the wild)
- [CVE-2025-21590](https://insights.sbomify.com/CVE-2025-21590/): Juniper Junos OS Improper Isolation or Compartmentalization (medium 6.7, exploited in the wild)
- [CVE-2023-36851](https://insights.sbomify.com/CVE-2023-36851/): Juniper Junos OS SRX Series Missing Authentication for Critical Function (medium 5.3, exploited in the wild)
- [CVE-2023-36844](https://insights.sbomify.com/CVE-2023-36844/): Juniper Junos OS EX Series PHP External Variable Modification (medium 5.3, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-0261 and fix it if it is affected.

What it is: CVE-2021-0261 is a high-severity denial of service vulnerability in Juniper Junos, published on 22 Apr 2021. 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-2021-0261 in the wild and no public exploit code.

Affected product: Juniper Junos. 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-2021-0261/ (Markdown: https://insights.sbomify.com/CVE-2021-0261/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-0261
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-0261
- OSV: https://osv.dev/vulnerability/CVE-2021-0261

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-0261/
Summaries are generated and may contain mistakes; the official description above is the record.