# CVE-2019-12521: Memory corruption in n/a

> An off-by-one error in Squid's ESI parser causes a heap overflow, leading to a crash and denial of service.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/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:** Yes
- **Published:** 15 Apr 2020
- **Updated:** 4 Aug 2024
- **Also known as:** ALPINE-CVE-2019-12521, DEBIAN-CVE-2019-12521, GHSA-7mv4-9v7r-5gq3, OESA-2022-1851, UBUNTU-CVE-2019-12521

## What is CVE-2019-12521?

CVE-2019-12521 is a medium-severity memory corruption vulnerability in Canonical Ubuntu Linux, published on 15 Apr 2020. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An issue was discovered in Squid through 4.7. When Squid is parsing ESI, it keeps the ESI elements in ESIContext. ESIContext contains a buffer for holding a stack of ESIElements. When a new ESIElement is parsed, it is added via addStackElement. addStackElement has a check for the number of elements in this buffer, but it's off by 1, leading to a Heap Overflow of 1 element. The overflow is within the same structure so it can't affect adjacent memory blocks, and thus just leads to a crash while processing.

Weakness: CWE-787 (Out-of-bounds Write), CWE-193 (Off-by-one Error)

## Which versions are affected by CVE-2019-12521?

It affects 4 products.


Products: Canonical Ubuntu Linux, Debian Debian Linux, Opensuse Leap, Squid-Cache Squid.

## How do I fix CVE-2019-12521?

Upgrade to a fixed version.

## Is CVE-2019-12521 being exploited?

There is no known exploitation of CVE-2019-12521 in the wild and no public exploit code.

## Other vulnerabilities in Canonical Ubuntu Linux

- [CVE-2020-11651](https://insights.sbomify.com/CVE-2020-11651/): SaltStack Salt Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2020-7247](https://insights.sbomify.com/CVE-2020-7247/): OpenSMTPD Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2019-16928](https://insights.sbomify.com/CVE-2019-16928/): Exim Out-of-bounds Write (critical 9.8, exploited in the wild)
- [CVE-2018-6789](https://insights.sbomify.com/CVE-2018-6789/): Exim Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2015-2590](https://insights.sbomify.com/CVE-2015-2590/): Oracle Java SE and Java SE Embedded Remote Code Execution (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2019-12521 and fix it if it is affected.

What it is: CVE-2019-12521 is a medium-severity memory corruption vulnerability in Canonical Ubuntu Linux, published on 15 Apr 2020. 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-2019-12521 in the wild and no public exploit code.

Affected product: Canonical Ubuntu Linux. 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-2019-12521/ (Markdown: https://insights.sbomify.com/CVE-2019-12521/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2019-12521
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2019-12521
- OSV: https://osv.dev/vulnerability/CVE-2019-12521

---
From sbomify Insights: https://insights.sbomify.com/CVE-2019-12521/
Summaries are generated and may contain mistakes; the official description above is the record.