{"id":"CVE-2026-89542","summary":"SUNRPC: harden gss_krb5_unwrap_v2 against short tokens","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nSUNRPC: harden gss_krb5_unwrap_v2 against short tokens\n\ngss_krb5_unwrap_v2() reads the EC and RRC header fields at ptr+4 and\nptr+6 before validating that the token is at least GSS_KRB5_TOK_HDR_LEN\n(16) bytes long, and its rotate_left() helper passes buf-\u003elen - base\nto xdr_buf_subsegment() without verifying that base \u003c= buf-\u003elen. When\na caller hands in a sub-16-byte token, or a token whose declared len\nleaves base past the end of the buffer, three distinct failures follow:\n\n    gss_krb5_unwrap_v2(offset, len, buf)\n      ptr = buf-\u003ehead[0].iov_base + offset\n      ec  = *(ptr + 4)              /* OOB read on short head */\n      rrc = *(ptr + 6)              /* OOB read on short head */\n      rotate_left(offset + 16, buf, rrc)\n        xdr_buf_subsegment(buf, &subbuf,\n                           base, buf-\u003elen - base)   /* u32 wrap when base \u003e len */\n        _rotate_left(&subbuf, shift)\n          shift %= buf-\u003elen         /* divide-by-zero when base == len */\n\nAfter decryption, the cleanup arithmetic has the same shape:\n\n    movelen = min_t(unsigned int, buf-\u003ehead[0].iov_len, len);\n    movelen -= offset + GSS_KRB5_TOK_HDR_LEN + headskip;\n    BUG_ON(offset + GSS_KRB5_TOK_HDR_LEN + headskip + movelen \u003e\n                                            buf-\u003ehead[0].iov_len);\n\nThe BUG_ON re-adds the value just subtracted, so it reduces to\nmin(A, B) \u003e A and is permanently false; it cannot catch the unsigned\nunderflow of movelen, which then drives a ~UINT_MAX-byte memmove().\n\nAdd four defense-in-depth guards inside the unwrap core so it is safe\nregardless of what its callers validate:\n\n  - reject tokens with len - offset \u003c GSS_KRB5_TOK_HDR_LEN before\n    touching ptr+4/ptr+6;\n  - bail from rotate_left() when buf-\u003elen \u003c= base, covering both the\n    underflow and zero-length cases;\n  - return early from _rotate_left() when buf-\u003elen is zero, so the\n    shift %= buf-\u003elen modulo cannot fault;\n  - replace the dead BUG_ON with a live check that returns\n    GSS_S_DEFECTIVE_TOKEN before the movelen subtraction.","modified":"2026-09-13T03:47:19.060185762Z","published":"2026-09-11T19:44:18.931Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89542.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/6959297aaa9572783d620a226d73c3fb94494888"},{"type":"WEB","url":"https://git.kernel.org/stable/c/806584a4b67a7233870c33e5b8f872e76dd02988"},{"type":"WEB","url":"https://git.kernel.org/stable/c/a7894e10572d53eb10109b8d07459cc8d3435811"},{"type":"WEB","url":"https://git.kernel.org/stable/c/dddcb0f4b7e27fac16a78ea9a1c8ec2e8a241087"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89542.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-89542"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"de9c17eb4a912c9028f7b470eb80815144883b26"},{"fixed":"dddcb0f4b7e27fac16a78ea9a1c8ec2e8a241087"},{"fixed":"806584a4b67a7233870c33e5b8f872e76dd02988"},{"fixed":"a7894e10572d53eb10109b8d07459cc8d3435811"},{"fixed":"6959297aaa9572783d620a226d73c3fb94494888"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89542.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.6.35"},{"fixed":"6.12.109"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.50"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.2.4"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89542.json"}}],"schema_version":"1.9.0"}