{"id":"CVE-2026-110952","kind":"vulnerability","aliases":["CVE-2026-110952"],"related":[],"title":"Kafka: OAUTHBEARER skips issuer and audience checks unless configured","description":"A Kafka broker that authenticates clients with SASL/OAUTHBEARER using the bundled JWKS-backed validator always verifies the JWT signature, but two further checks are only performed when explicitly configured:\n\n- The issuer (iss) claim is verified only when sasl.oauthbearer.expected.issuer is set. When it is unset, a validly-signed token carrying the expected audience is accepted regardless of which issuer minted it.\n- An audience (aud) claim is required only when sasl.oauthbearer.expected.audience is set. When it is unset, a validly-signed token that omits the aud claim is accepted. (A token that carries an aud claim is\n  still rejected when no expected audience is configured.)\n\nBoth options default to unset and have been documented as optional (Importance LOW) since KIP-768 (3.1.0).\n\nImpact:\n  The exposure is realizable mainly in deployments whose JWKS endpoint serves signing keys for more than one issuer (a multi-tenant identity provider, an aggregated/shared JWKS, or shared signing keys): a token\n  minted for a different issuer, or a token lacking an audience, may be accepted to authenticate — potentially as a higher-privileged principal. In a single-issuer deployment where the provider controls the iss\n  claim and issues audience-scoped tokens, exposure is limited. The signature trust anchor remains enforced in all cases. This is distinct from CVE-2026-33557 (acceptance of unsigned/unvalidated tokens); here the\n  signature is validated and only the issuer/audience checks are not enforced by default.\n\nMitigation (all affected versions, no upgrade required):\n  Set both sasl.oauthbearer.expected.issuer (to your provider's exact iss value) and sasl.oauthbearer.expected.audience (to the audience(s) your brokers expect), per listener. Brokers that set these perform\n  exact-match issuer/audience checks and are not affected. Both have been available since 3.1.0.\n\nResolution:\n  - From Apache Kafka 4.4.0: a broker configured with a JWKS endpoint fails to start unless both sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience are set (secure by default). Deployments that\n  intentionally do not restrict the issuer and/or audience may opt out per axis with sasl.oauthbearer.allow.unverified.issuer=true and/or sasl.oauthbearer.allow.unverified.audience=true.","summary":"Apache Kafka brokers using the bundled JWKS-backed SASL/OAUTHBEARER validator verify the token signature but do not check the issuer or audience claims unless sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience are set. A token signed by another issuer sharing the same JWKS, or a token without an aud claim, can be accepted and may authenticate as a higher-privileged principal.","severity":"medium","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["kafka","managed-kafka","oauth","package","sasl","self-hosted"],"not_affected_if":["default-config-already-set","no-oauthbearer","single-issuer-audience-scoped"],"how_to_check":"- Inspect broker configs for `sasl.oauthbearer.expected.issuer` and `sasl.oauthbearer.expected.audience` on each OAUTHBEARER listener; if either is missing, the broker is exposed.\n- Check the broker version with `kafka-broker-api-versions.sh --version` or the Kafka distribution; affected versions are those with the bundled JWKS validator from 3.1.0 onward.\n- Check whether the JWKS endpoint serves keys for more than one issuer (multi-tenant IdP, shared JWKS).","fix":"- Set `sasl.oauthbearer.expected.issuer` to the provider's exact iss value and `sasl.oauthbearer.expected.audience` to the audience brokers expect, per listener; this works on all affected versions.\n- Upgrade to Apache Kafka `4.4.0` or later, where a JWKS-configured broker refuses to start unless both options are set.\n- Only if intentionally unrestricted, opt out per axis with `sasl.oauthbearer.allow.unverified.issuer=true` and/or `sasl.oauthbearer.allow.unverified.audience=true`.","status":"open","published_at":"2026-10-28T17:17:37+00:00","first_seen_at":"2026-10-28T17:34:08+00:00","triaged_at":"2026-10-28T17:35:17+00:00","updated_at":"2026-10-28T17:35:17+00:00","category":"package","confidence":0.8,"triage":{"at":"2026-10-28T17:35:17Z","model":"claude-haiku-5-5","output":{"fix":"- Set `sasl.oauthbearer.expected.issuer` to the provider's exact iss value and `sasl.oauthbearer.expected.audience` to the audience brokers expect, per listener; this works on all affected versions.\n- Upgrade to Apache Kafka `4.4.0` or later, where a JWKS-configured broker refuses to start unless both options are set.\n- Only if intentionally unrestricted, opt out per axis with `sasl.oauthbearer.allow.unverified.issuer=true` and/or `sasl.oauthbearer.allow.unverified.audience=true`.","reach":"network","title":"Kafka: OAUTHBEARER skips issuer and audience checks unless configured","impact":"high","summary":"Apache Kafka brokers using the bundled JWKS-backed SASL/OAUTHBEARER validator verify the token signature but do not check the issuer or audience claims unless sasl.oauthbearer.expected.issuer and sasl.oauthbearer.expected.audience are set. A token signed by another issuer sharing the same JWKS, or a token without an aud claim, can be accepted and may authenticate as a higher-privileged principal.","category":"package","nicknames":[],"who_cares":["kafka","oauth","sasl","self-hosted","managed-kafka"],"confidence":0.8,"needs_auth":"no","worst_case":"Attacker authenticates with a foreign or audience-less token as a privileged principal.","analyst_note":"Confirm the affected version range and whether the NVD entry or Apache advisory lists exact fixed versions beyond 4.4.0.","how_to_check":"- Inspect broker configs for `sasl.oauthbearer.expected.issuer` and `sasl.oauthbearer.expected.audience` on each OAUTHBEARER listener; if either is missing, the broker is exposed.\n- Check the broker version with `kafka-broker-api-versions.sh --version` or the Kafka distribution; affected versions are those with the bundled JWKS validator from 3.1.0 onward.\n- Check whether the JWKS endpoint serves keys for more than one issuer (multi-tenant IdP, shared JWKS).","needs_analyst":true,"not_affected_if":["default-config-already-set","single-issuer-audience-scoped","no-oauthbearer"],"who_is_affected":"Kafka clusters that authenticate clients with SASL/OAUTHBEARER through the bundled JWKS-backed validator and leave the expected issuer or expected audience unset. Exposure is highest when the JWKS endpoint serves signing keys for multiple issuers, such as a multi-tenant identity provider or a shared JWKS. Single-issuer deployments where the provider controls iss and issues audience-scoped tokens face limited exposure, and clusters not using OAUTHBEARER can skip it.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000909,"input_tokens":4041,"output_tokens":1009,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-110952':12A,15A '-2026':11A,14A,300C '-33557':301C '-768':205C '3.1.0':206C,368C '4.4.0':373C 'accept':75B,140C,173C,254C,302C 'affect':323C,362C 'aggregated/shared':233C 'alway':100C 'anchor':289C 'and/or':402C,412C 'anoth':60B 'apach':26B,371C 'aud':71B,149C,170C,179C 'audienc':6A,48B,138C,148C,187C,251C,280C,342C,403C 'audience-scop':279C 'authent':78B,89C,256C 'avail':366C 'axi':408C 'back':34B,98C 'broker':28B,87C,345C,349C,375C 'bundl':31B,95C 'carri':135C,177C 'case':294C 'check':7A,44B,108C,315C,358C 'claim':49B,72B,118C,150C,171C,180C,276C 'client':90C 'configur':9A,114C,189C,376C 'control':273C 'cve':10A,13A,299C 'default':192C,320C,393C 'deploy':214C,269C,394C 'differ':244C 'distinct':297C 'document':198C 'endpoint':217C,380C 'enforc':291C,318C 'exact':335C,355C 'exact-match':354C 'expect':137C,186C,346C 'explicit':113C 'exposur':209C,283C 'fail':381C 'higher':82B,261C 'higher-privileg':81B,260C 'host':25B 'ident':230C 'impact':207C 'import':201C 'intent':396C 'iss':117C,275C,336C 'issu':278C 'issuer':4A,46B,61B,116C,144C,225C,245C,268C,401C 'issuer/audience':314C,357C 'jwks':33B,65B,97C,216C,234C,379C 'jwks-back':32B,96C 'jwt':103C 'kafka':1A,16B,19B,27B,86C,372C 'key':220C,238C 'kip':204C 'lack':249C 'limit':285C 'listen':348C 'low':202C 'main':212C 'manag':18B 'managed-kafka':17B 'match':356C 'may':77B,252C,404C 'mint':145C,241C 'mitig':321C 'multi':228C 'multi-ten':227C 'oauth':20B 'oauthbear':2A 'omit':168C 'one':224C 'opt':405C 'option':191C,200C 'packag':21B 'per':347C,407C 'perform':111C,353C 'potenti':257C 'princip':84B,263C 'privileg':83B,262C 'provid':231C,272C,333C 'realiz':211C 'regardless':141C 'reject':183C 'remain':290C 'requir':152C,327C 'resolut':369C 'restrict':399C 'sasl':22B 'sasl.oauthbearer.allow.unverified.audience':413C 'sasl.oauthbearer.allow.unverified.issuer':410C 'sasl.oauthbearer.expected.audience':53B,155C,339C,388C 'sasl.oauthbearer.expected.issuer':51B,123C,330C,386C 'sasl/oauthbearer':35B,92C 'scope':281C 'secur':391C 'self':24B 'self-host':23B 'serv':218C 'set':55B,125C,157C,328C,351C,390C 'share':62B,236C 'sign':58B,133C,165C,219C,237C 'signatur':40B,104C,287C,308C 'sinc':203C,367C 'singl':267C 'single-issu':266C 'skip':3A 'start':383C 'still':182C 'tenant':229C 'token':39B,57B,68B,134C,166C,175C,240C,248C,282C,305C 'true':411C,414C 'trust':288C 'two':106C 'unless':8A,50B,384C 'unset':129C,161C,194C 'unsigned/unvalidated':304C 'upgrad':326C 'use':29B,93C 'valid':36B,99C,132C,164C,310C 'validly-sign':131C,163C 'valu':337C 'verifi':37B,101C,120C 'version':324C 'whose':215C 'without':69B","nicknames":[],"mitigations":[],"who_affected":"Kafka clusters that authenticate clients with SASL/OAUTHBEARER through the bundled JWKS-backed validator and leave the expected issuer or expected audience unset. Exposure is highest when the JWKS endpoint serves signing keys for multiple issuers, such as a multi-tenant identity provider or a shared JWKS. Single-issuer deployments where the provider controls iss and issues audience-scoped tokens face limited exposure, and clusters not using OAUTHBEARER can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-28T17:17:37+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-110952","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-110952","severity":null,"published_at":"2026-10-28T17:17:37+00:00","modified_at":"2026-10-28T17:17:37+00:00","first_seen_at":"2026-10-28T17:34:08+00:00","fetched_at":"2026-10-28T17:34:08+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}