The application must enable the remember_me strategy and run its per-request plug in the browser pipeline, with require_token_presence_for_authentication? at its default of false. Enabling that setting makes the guard read the key the session actually holds, which closes the issue.
The attacker also needs a position from which to set a cookie in the victim's browser, such as cookie tossing from a sibling subdomain, an HTTP host without HSTS, or a shared or kiosk browser.
"},{"base64":false,"type":"text/markdown","value":"The application must enable the `remember_me` strategy and run its per-request plug in the browser pipeline, with `require_token_presence_for_authentication?` at its default of `false`. Enabling that setting makes the guard read the key the session actually holds, which closes the issue.\n\nThe attacker also needs a position from which to set a cookie in the victim's browser, such as cookie tossing from a sibling subdomain, an HTTP host without HSTS, or a shared or kiosk browser."}],"value":"The application must enable the remember_me strategy and run its per-request plug in the browser pipeline, with require_token_presence_for_authentication? at its default of false. Enabling that setting makes the guard read the key the session actually holds, which closes the issue.\n\nThe attacker also needs a position from which to set a cookie in the victim's browser, such as cookie tossing from a sibling subdomain, an HTTP host without HSTS, or a shared or kiosk browser."}],"cpeApplicability":[{"nodes":[{"cpeMatch":[{"criteria":"cpe:2.3:a:team-alembic:ash_authentication:*:*:*:*:*:*:*:*","versionEndExcluding":"4.15.0","versionStartIncluding":"4.10.0","vulnerable":true},{"criteria":"cpe:2.3:a:team-alembic:ash_authentication:*:*:*:*:*:*:*:*","versionEndExcluding":"5.0.0-rc.14","versionStartIncluding":"5.0.0-rc.0","vulnerable":true}],"negate":false,"operator":"OR"}],"operator":"AND"}],"credits":[{"lang":"en","type":"reporter","value":"Peter Ullrich"},{"lang":"en","type":"remediation developer","value":"James Harton"},{"lang":"en","type":"coordinator","value":"Jonatan Männchen / EEF"}],"dateAssigned":"2026-09-17T14:23:56.000Z","descriptions":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"Authentication Bypass by Spoofing vulnerability in team-alembic ash_authentication allows an attacker who can plant a remember-me cookie in a victim's browser to replace that victim's authenticated session with one for the attacker's own account.
\nAshAuthentication.Plug.Helpers.sign_in_using_remember_me/3 skips re-authenticating an already-signed-in visitor by checking the session for \"<subject_name>_token\", but store_in_session/2 writes that key only when require_token_presence_for_authentication? is enabled and otherwise writes the bare subject name. At the default setting the guard therefore reads a key that is never written, its already-signed-in branch is unreachable, and the remember-me sign-in runs on every request through the per-request browser pipeline plug. A planted remember-me cookie is consequently honoured even for a visitor holding a live authenticated session, so whatever the victim enters afterwards lands in data the attacker controls. The read path in authenticate_resource_from_session/4 selects the key correctly, so the guard and the reader disagree about which key holds the session.
This issue affects ash_authentication: from 4.10.0 before 4.15.0 and from 5.0.0-rc.0 before 5.0.0-rc.14.
"},{"base64":false,"type":"text/markdown","value":"Authentication Bypass by Spoofing vulnerability in team-alembic ash_authentication allows an attacker who can plant a remember-me cookie in a victim's browser to replace that victim's authenticated session with one for the attacker's own account.\n\n`AshAuthentication.Plug.Helpers.sign_in_using_remember_me/3` skips re-authenticating an already-signed-in visitor by checking the session for `\"An attacker who can set a remember-me cookie in a victim's browser silently replaces the victim's authenticated session with one for the attacker's own account, so data the victim subsequently enters is written to records the attacker controls. The victim is given no indication their identity changed.
"},{"base64":false,"type":"text/markdown","value":"An attacker who can set a remember-me cookie in a victim's browser silently replaces the victim's authenticated session with one for the attacker's own account, so data the victim subsequently enters is written to records the attacker controls. The victim is given no indication their identity changed."}],"value":"An attacker who can set a remember-me cookie in a victim's browser silently replaces the victim's authenticated session with one for the attacker's own account, so data the victim subsequently enters is written to records the attacker controls. The victim is given no indication their identity changed."}]}],"metrics":[{"cvssV4_0":{"Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","Safety":"NOT_DEFINED","attackComplexity":"LOW","attackRequirements":"PRESENT","attackVector":"NETWORK","baseScore":9.1,"baseSeverity":"CRITICAL","privilegesRequired":"NONE","providerUrgency":"NOT_DEFINED","subAvailabilityImpact":"NONE","subConfidentialityImpact":"NONE","subIntegrityImpact":"NONE","userInteraction":"NONE","valueDensity":"NOT_DEFINED","vectorString":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N","version":"4.0","vulnAvailabilityImpact":"NONE","vulnConfidentialityImpact":"HIGH","vulnIntegrityImpact":"HIGH","vulnerabilityResponseEffort":"NOT_DEFINED"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"cweId":"CWE-290","description":"CWE-290 Authentication Bypass by Spoofing","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-09-17T21:57:50.146Z","orgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","shortName":"EEF"},"references":[{"name":"GitHub Advisory","tags":["related","vendor-advisory"],"url":"https://github.com/team-alembic/ash_authentication/security/advisories/GHSA-hh34-374j-pfr5"},{"name":"EEF CNA record for CVE-2026-76949","tags":["related"],"url":"https://cna.erlef.org/cves/CVE-2026-76949.html"},{"name":"OSV record EEF-CVE-2026-76949","tags":["related"],"url":"https://osv.dev/vulnerability/EEF-CVE-2026-76949"},{"name":"Introducing commit 3d3de31 in team-alembic/ash_authentication","tags":["related"],"url":"https://github.com/team-alembic/ash_authentication/commit/3d3de314692558d06ec13945f857f00514e95a8c"},{"name":"Fix commit b9568a5 in team-alembic/ash_authentication","tags":["patch"],"url":"https://github.com/team-alembic/ash_authentication/commit/b9568a53b438247238b1c5a4f49c8d84650f4bba"},{"name":"Fix commit 9a34136 in team-alembic/ash_authentication","tags":["patch"],"url":"https://github.com/team-alembic/ash_authentication/commit/9a34136b844abe179da1075067cf2835c64dcc12"}],"source":{"discovery":"EXTERNAL"},"title":"Remember-me sign-in guard reads a session key that is never written in ash_authentication, allowing session replacement","workarounds":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"Set require_token_presence_for_authentication? true on the authenticated resource's token configuration. store_in_session/2 then writes the key the guard reads, so the already-signed-in branch fires and the remember-me sign-in no longer runs against a live session. Note this changes session storage semantics for the whole application and requires a token resource.
Otherwise remove the remember-me plug from the browser pipeline, which disables remember-me auto-login entirely.
"},{"base64":false,"type":"text/markdown","value":"Set `require_token_presence_for_authentication? true` on the authenticated resource's token configuration. `store_in_session/2` then writes the key the guard reads, so the already-signed-in branch fires and the remember-me sign-in no longer runs against a live session. Note this changes session storage semantics for the whole application and requires a token resource.\n\nOtherwise remove the remember-me plug from the browser pipeline, which disables remember-me auto-login entirely."}],"value":"Set require_token_presence_for_authentication? true on the authenticated resource's token configuration. store_in_session/2 then writes the key the guard reads, so the already-signed-in branch fires and the remember-me sign-in no longer runs against a live session. Note this changes session storage semantics for the whole application and requires a token resource.\n\nOtherwise remove the remember-me plug from the browser pipeline, which disables remember-me auto-login entirely."}]}},"cveMetadata":{"assignerOrgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","assignerShortName":"EEF","cveId":"CVE-2026-76949","datePublished":"2026-09-17T21:57:50.146Z","dateReserved":"2026-09-17T05:30:01.744Z","dateUpdated":"2026-09-17T21:57:50.146Z","state":"PUBLISHED"},"dataType":"CVE_RECORD","dataVersion":"5.2"}