{"id":"RUSTSEC-2025-0173","summary":"`PamHandle::get_user` can cause an authentication bypass","details":"In affected versions, `PamHandle::get_user` can return `Err(PAM_SUCCESS)` after it gets the username successfully.\nPAM modules that return this \"error\" code from `sm_authenticate` or `sm_acct_mgmt` early-return a success code to PAM without doing any further verification.\nThis means a user could log in when they shouldn't be able to (e.g., if the module is `sufficient`).\n\nThe `pam_try!` macro of this crate early-returns the code in this exact way:\n\n```rust\nfn sm_authenticate(pamh: &mut PamHandle, args: Vec\u003c&CStr\u003e, flags: PamFlag) -\u003e PamResultCode {\n    let user = pam_try!(pamh.get_user(None)); // Can return PAM_SUCCESS here.\n    // If it does, the module does not get to this line.\n}\n```\n\nThe root cause is an [incorrect FFI declaration][ffi-decl] for [`pam_get_user(3)`][pam-get-user].\nThis C function writes a pointer to the username through its `user` argument (of type `const char **user`).\nThis crate's binding declared this argument as a shared reference (`&*mut c_char`).\nWriting through a shared reference is [undefined behavior][ub-immutable].\n\nThus, the compiler can assume that the local pointer, whose address is passed as `user`, is still null after the call.\nAfter the call, the affected code [returns `Ok`][null-check] only if the return code is `PAM_SUCCESS` *and* this pointer is not null.\nOtherwise, it returns the return code as an error.\nIf the compiler assumes that the pointer is null, `get_user` returns an error even when the return code is `PAM_SUCCESS`.\nRelease builds have been [observed to do this][observed-irl].\nDebug builds usually work correctly, so tests in debug mode can miss the problem.\n\nVersion 0.1.2 changes the declaration to `&mut *const c_char`.\nThe fix is in commit [401c961](https://github.com/lvkv/pam-rs/commit/401c961800c4dde356464b6a7d455af844a3954a).\n\nIf a module cannot upgrade, do not return the error code from `get_user`.\nReturn a failure code instead, for example `pam_try!(pamh.get_user(None), PamResultCode::PAM_AUTH_ERR)`.\nThis makes the module deny access instead of allowing it, but the undefined behavior remains.\n\n[pam-get-user]: https://github.com/linux-pam/linux-pam/blob/28fce1fd55b003fec90f34e5bc785e9439b216f2/libpam/include/security/pam_modules.h#L30-L31\n[ffi-decl]: https://github.com/lvkv/pam-rs/blob/3d9318b5b812d9142595928deec23e5843a60995/pam/src/module.rs#L49-L53\n[ub-immutable]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html#r-undefined.immutable\n[null-check]: https://github.com/lvkv/pam-rs/blob/3d9318b5b812d9142595928deec23e5843a60995/pam/src/module.rs#L196-L202\n[observed-irl]: https://github.com/lvkv/pam-rs/issues/16#issuecomment-3693665220","modified":"2026-10-26T14:00:04.237415395Z","published":"2025-06-02T12:00:00Z","database_specific":{"license":"CC0-1.0"},"references":[{"type":"PACKAGE","url":"https://crates.io/crates/pam-bindings"},{"type":"ADVISORY","url":"https://rustsec.org/advisories/RUSTSEC-2025-0173.html"},{"type":"REPORT","url":"https://github.com/lvkv/pam-rs/issues/16"}],"affected":[{"package":{"name":"pam-bindings","ecosystem":"crates.io","purl":"pkg:cargo/pam-bindings"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.0.0-0"},{"fixed":"0.1.2"}]}],"ecosystem_specific":{"affected_functions":null,"affects":{"arch":[],"os":[],"functions":[]}},"database_specific":{"cvss":null,"source":"https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2025-0173.json","informational":null,"categories":["privilege-escalation"]}}],"schema_version":"1.9.0"}