# CVE-2026-81638

## Summary

- **CVE ID:** CVE-2026-81638
- **Severity:** LOW
- **CVSS Score:** 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)
- **CWE:** CWE-173
- **Published:** Sep 7, 2026
- **Last Modified:** Sep 8, 2026

## 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.

## Affected Products

- ash-project — ash_double_entry (0.1.0)
- ash-project — ash_double_entry (1e5f7ce8ff25f519c904731a29eb1258324e561a)

## References

- [CNA](https://github.com/ash-project/ash_double_entry/security/advisories/GHSA-qxp2-vgp9-268q)
- [CNA](https://cna.erlef.org/cves/CVE-2026-81638.html)
- [CNA](https://osv.dev/vulnerability/EEF-CVE-2026-81638)
- [CNA](https://github.com/ash-project/ash_double_entry/commit/d3e688d300a581ae214b3ca7d95ef4de63fbb050)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.13%
- **EPSS Percentile:** 3.1

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._