# CVE-2026-81638: Non-canonical ULID spellings are accepted and alias to the same record in ash_double_entry

> Attackers can bypass string-based security checks like idempotency or deny-lists by using non-canonical ULID spellings that resolve to the same record.

- **Severity:** Low 2.1 (`CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 7 Sep 2026
- **Updated:** 8 Sep 2026
- **Also known as:** EEF-CVE-2026-81638, GHSA-qxp2-vgp9-268q

## What is CVE-2026-81638?

CVE-2026-81638 is a low-severity vulnerability in ash-project ash_double_entry, published on 7 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Improper Handling of Alternate Encoding vulnerability in ash-project ash_double_entry allows an attacker to submit several distinct string spellings of the same identifier.

AshDoubleEntry.ULID renders a 128-bit ULID as 26 Crockford base-32 characters, but the first character encodes only 3 bits, so canonical values are 0 to 7. decode/1 in lib/ulid.ex masks the first character to its low 3 bits and valid?/1 accepts all 32 characters in that position, so 0..., 8..., G... and R... decode to the identical 16-byte value and resolve to the same row. When the type is exposed as a public ID over an HTTP or API boundary, an attacker-supplied ID can be spelled differently from the record it actually reads or writes, desynchronizing or bypassing string-level checks such as idempotency and deduplication keys, deny-lists, audit correlation, or signatures computed over the submitted ID.

This issue affects ash_double_entry: from 0.1.0 before 1.0.19.

Weakness: CWE-173 (Improper Handling of Alternate Encoding)

## Which versions are affected by CVE-2026-81638?

1 open source package ship an affected version, including ash_double_entry.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `ash_double_entry` | Hex | From 0.1.0 | 1.0.19 |

## How do I fix CVE-2026-81638?

Upgrade ash_double_entry to 1.0.19 or later.

## Is CVE-2026-81638 being exploited?

There is no known exploitation of CVE-2026-81638 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81638 and fix it if it is affected.

What it is: CVE-2026-81638 is a low-severity vulnerability in ash-project ash_double_entry, published on 7 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-81638 in the wild and no public exploit code.

Affected packages and fixed versions:
- ash_double_entry (Hex): affected from 0.1.0; fixed in 1.0.19

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-2026-81638/ (Markdown: https://insights.sbomify.com/CVE-2026-81638/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81638
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81638
- OSV: https://osv.dev/vulnerability/CVE-2026-81638

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81638/
Summaries are generated and may contain mistakes; the official description above is the record.