CVE-2026-81638

Summary

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 Software

Weaknesses

• CWE-173: CWE-173 Improper Handling of Alternate Encoding

References

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

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