# CVE-2026-55209: resdata insufficiently validates untrusted GRDECL files

> Attackers can trigger memory corruption or crashes in resdata by providing malformed GRDECL files, potentially leading to code execution or service termination.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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:** 14 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-pr85-w493-9w3x, PYSEC-2026-3694

## What is CVE-2026-55209?

CVE-2026-55209 is a critical memory corruption vulnerability in equinor resdata, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

resdata is software for reading and writing result files from the Eclipse reservoir simulator. Prior to 6.2.9, resdata insufficiently validates numeric fields, grid dimensions, keyword sizes, and array indexes while parsing untrusted GRDECL files in lib/resdata/rd_kw_grdecl.cpp and lib/resdata/rd_grid.cpp. Malformed COORD, ZCORN, CORSNUM, ACTNUM, or MAPAXES data can reach rd_grid_alloc_GRDECL_kw__ with inconsistent lengths, while unbounded floating-point conversion can exceed the intended parser buffer. In a network service that accepts untrusted GRDECL files, these conditions can cause a classic buffer overflow, out-of-bounds reads, invalid array access, NULL pointer dereference, memory corruption, or service termination. This issue is fixed in version 6.2.9.

Weakness: CWE-476 (NULL Pointer Dereference), CWE-129 (Improper Validation of Array Index), CWE-125 (Out-of-bounds Read), CWE-120 (Buffer Copy without Checking Size of Input)

## Which versions are affected by CVE-2026-55209?

3 open source packages ship an affected version, including resdata.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `resdata` | PyPI | All versions | 6.2.9 |
| `resdata` | PyPI | All versions | 6.2.9 |
| `resdata` | PyPI | All versions | 6.2.9 |

## How do I fix CVE-2026-55209?

Upgrade resdata to 6.2.9 or later.

## Is CVE-2026-55209 being exploited?

There is no known exploitation of CVE-2026-55209 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55209 and fix it if it is affected.

What it is: CVE-2026-55209 is a critical memory corruption vulnerability in equinor resdata, published on 14 Sep 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-2026-55209 in the wild and no public exploit code.

Affected packages and fixed versions:
- resdata (PyPI): affected all versions; fixed in 6.2.9
- resdata (PyPI): affected all versions; fixed in 6.2.9
- resdata (PyPI): affected all versions; fixed in 6.2.9

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-2026-55209/ (Markdown: https://insights.sbomify.com/CVE-2026-55209/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55209
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55209
- OSV: https://osv.dev/vulnerability/CVE-2026-55209

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55209/
Summaries are generated and may contain mistakes; the official description above is the record.