This vulnerability is only exploitable when AES-GCM is handled by the pure-Erlang jose_jwa_aes fallback. That fallback is used on deployments where the Erlang/OTP crypto NIF does not expose native AES-GCM (for example FIPS builds, or stripped or older OTP releases). Deployments with a native AES-GCM crypto provider dispatch GCM through crypto and are not affected.
The pure-Erlang AES-GCM fallback in jose_jwa_aes derives the number of verified authentication tag bits from the attacker-supplied tag itself. In the {aes_gcm, Bits} clause of block_decrypt/4, the incoming tag is guarded only by is_binary/1, with no check on its length. Verification in gcm_block_decrypt/6 then computes TBits = bit_size(T) and compares only those bits of the real 128-bit tag. An attacker who controls a JWE token can truncate the authentication tag to as little as 8 bits, so only 8 bits are ever checked. This reduces forgery resistance from 2^-128 to 2^-8, and a forged ciphertext is reliably accepted as authentic within 256 attempts without knowledge of the key (an authentication tag forgery and integrity bypass). Supplying a tag longer than 128 bits instead causes a badmatch that crashes the decrypting process. The fallback is reached through the public JWE decryption path whenever AES-GCM is not provided by a native crypto NIF, for example on FIPS or stripped or older OTP builds.
This issue affects jose: from 1.2.0 before 1.11.13.
"},{"base64":false,"type":"text/markdown","value":"The pure-Erlang AES-GCM fallback in `jose_jwa_aes` derives the number of verified authentication tag bits from the attacker-supplied tag itself. In the `{aes_gcm, Bits}` clause of `block_decrypt/4`, the incoming tag is guarded only by `is_binary/1`, with no check on its length. Verification in `gcm_block_decrypt/6` then computes `TBits = bit_size(T)` and compares only those bits of the real 128-bit tag. An attacker who controls a JWE token can truncate the authentication tag to as little as 8 bits, so only 8 bits are ever checked. This reduces forgery resistance from 2^-128 to 2^-8, and a forged ciphertext is reliably accepted as authentic within 256 attempts without knowledge of the key (an authentication tag forgery and integrity bypass). Supplying a tag longer than 128 bits instead causes a badmatch that crashes the decrypting process. The fallback is reached through the public JWE decryption path whenever AES-GCM is not provided by a native crypto NIF, for example on FIPS or stripped or older OTP builds.\n\nThis issue affects jose: from 1.2.0 before 1.11.13."}],"value":"The pure-Erlang AES-GCM fallback in jose_jwa_aes derives the number of verified authentication tag bits from the attacker-supplied tag itself. In the {aes_gcm, Bits} clause of block_decrypt/4, the incoming tag is guarded only by is_binary/1, with no check on its length. Verification in gcm_block_decrypt/6 then computes TBits = bit_size(T) and compares only those bits of the real 128-bit tag. An attacker who controls a JWE token can truncate the authentication tag to as little as 8 bits, so only 8 bits are ever checked. This reduces forgery resistance from 2^-128 to 2^-8, and a forged ciphertext is reliably accepted as authentic within 256 attempts without knowledge of the key (an authentication tag forgery and integrity bypass). Supplying a tag longer than 128 bits instead causes a badmatch that crashes the decrypting process. The fallback is reached through the public JWE decryption path whenever AES-GCM is not provided by a native crypto NIF, for example on FIPS or stripped or older OTP builds.\n\nThis issue affects jose: from 1.2.0 before 1.11.13."}],"impacts":[{"capecId":"CAPEC-475","descriptions":[{"lang":"en","value":"CAPEC-475 Signature Spoofing by Improper Validation"}]}],"metrics":[{"cvssV4_0":{"Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","Safety":"NOT_DEFINED","attackComplexity":"LOW","attackRequirements":"PRESENT","attackVector":"LOCAL","baseScore":5.9,"baseSeverity":"MEDIUM","privilegesRequired":"NONE","providerUrgency":"NOT_DEFINED","subAvailabilityImpact":"NONE","subConfidentialityImpact":"NONE","subIntegrityImpact":"NONE","userInteraction":"NONE","valueDensity":"NOT_DEFINED","vectorString":"CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N","version":"4.0","vulnAvailabilityImpact":"LOW","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"HIGH","vulnerabilityResponseEffort":"NOT_DEFINED"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"cweId":"CWE-354","description":"CWE-354 Improper Validation of Integrity Check Value","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-10-26T19:52:54.987Z","orgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","shortName":"EEF"},"references":[{"tags":["vendor-advisory","related"],"url":"https://github.com/potatosalad/erlang-jose/security/advisories/GHSA-6gxw-6f7r-cjr6"},{"name":"EEF CNA record for CVE-2026-65625","tags":["related"],"url":"https://cna.erlef.org/cves/CVE-2026-65625.html"},{"name":"OSV record EEF-CVE-2026-65625","tags":["related"],"url":"https://osv.dev/vulnerability/EEF-CVE-2026-65625"},{"name":"Introducing commit 8848b17 in potatosalad/erlang-jose","tags":["related"],"url":"https://github.com/potatosalad/erlang-jose/commit/8848b172575fb7d7e1e2977e8ae0a21678d2157a"},{"name":"Fix commit 3fa1f8e in potatosalad/erlang-jose","tags":["patch"],"url":"https://github.com/potatosalad/erlang-jose/commit/3fa1f8e3efda0da5ce6f4c9984789f979dc02740"}],"source":{"discovery":"EXTERNAL"},"title":"Attacker-controlled AES-GCM authentication tag length allows tag forgery in jose","workarounds":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"Ensure the runtime provides a crypto provider with native AES-GCM support (a standard Erlang/OTP build linked against OpenSSL) so that JOSE dispatches AES-GCM to the native crypto NIF instead of the jose_jwa_aes fallback.