CVE-2026-81638

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CVE-2026-81638

Summary: Non-canonical ULID spellings are accepted and alias to the same record in ash_double_entry

Details: 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.

References: https://cna.erlef.org/cves/CVE-2026-81638.html, https://github.com, https://osv.dev/vulnerability/EEF-CVE-2026-81638, https://repo.hex.pm, https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/81xxx/CVE-2026-81638.json, https://github.com/ash-project/ash_double_entry/security/advisories/GHSA-qxp2-vgp9-268q, https://nvd.nist.gov/vuln/detail/CVE-2026-81638, https://github.com/ash-project/ash_double_entry/commit/d3e688d300a581ae214b3ca7d95ef4de63fbb050, https://github.com/ash-project/ash_double_entry