# CVE-2026-31963: HTSlib CRAM reader has heap buffer overflow due to improper validation of input

> An attacker can achieve arbitrary code execution or crash the application by tricking a user into opening a malicious CRAM file processed by HTSlib.

- **Severity:** High 8.8 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Mar 2026
- **Updated:** 18 Mar 2026
- **Also known as:** DEBIAN-CVE-2026-31963, GHSA-qgqh-h2q9-7w3c, UBUNTU-CVE-2026-31963

## What is CVE-2026-31963?

CVE-2026-31963 is a high-severity memory corruption vulnerability in samtools htslib, published on 18 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

HTSlib is a library for reading and writing bioinformatics file formats. CRAM is a compressed format which stores DNA sequence alignment data. As one method of removing redundant data, CRAM uses reference-based compression so that instead of storing the full sequence for each alignment record it stores a location in an external reference sequence along with a list of differences to the reference at that location as a sequence of "features". When decoding these features, an out-by-one error in a test for CRAM features that appear beyond the extent of the CRAM record sequence could result in an invalid write of one attacker-controlled byte beyond the end of a heap buffer. Exploiting this bug causes a heap buffer overflow. If a user opens a file crafted to exploit this issue, it could lead to the program crashing, or overwriting of data and heap structures in ways not expected by the program.  It may be possible to use this to obtain arbitrary code execution. Versions 1.23.1, 1.22.2 and 1.21.1 include fixes for this issue. There is no workaround for this issue.

Weakness: CWE-787 (Out-of-bounds Write), CWE-129 (Improper Validation of Array Index), CWE-122 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2026-31963?

It affects 1 product.


Products: Htslib Htslib.

## How do I fix CVE-2026-31963?

Upgrade to a fixed version.

## Is CVE-2026-31963 being exploited?

There is no known exploitation of CVE-2026-31963 in the wild and no public exploit code.

## Other vulnerabilities in samtools htslib

- [CVE-2018-13845](https://insights.sbomify.com/CVE-2018-13845/): Memory corruption in n/a (critical 9.8)
- [CVE-2017-1000206](https://insights.sbomify.com/CVE-2017-1000206/): Memory corruption in n/a (critical 9.8)
- [CVE-2026-31962](https://insights.sbomify.com/CVE-2026-31962/): HTSlib CRAM reader has heap buffer overflow due to improper validation of input (high 8.8)
- [CVE-2026-31968](https://insights.sbomify.com/CVE-2026-31968/): HTSlib CRAM decoder vulnerable to buffer overflow (high 8.8)
- [CVE-2020-36403](https://insights.sbomify.com/CVE-2020-36403/): Memory corruption in n/a (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31963 and fix it if it is affected.

What it is: CVE-2026-31963 is a high-severity memory corruption vulnerability in samtools htslib, published on 18 Mar 2026. 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-2026-31963 in the wild and no public exploit code.

Affected product: samtools htslib. 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-2026-31963/ (Markdown: https://insights.sbomify.com/CVE-2026-31963/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31963
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31963
- OSV: https://osv.dev/vulnerability/CVE-2026-31963

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31963/
Summaries are generated and may contain mistakes; the official description above is the record.