CVE-2026-89537
CriticalCVSS 9.1Exploitation Probability (EPSS)
Low risk9th percentile - higher than 9% of all known CVEs
Summary
The Linux kernel has a vulnerability in the SUNRPC function gss_krb5_verify_mic_v2(), which fails to check the minimum length of a MIC token before reading its contents. A remote attacker (e.g., a malicious NFS server) can supply a short token, causing an out-of-bounds read of the allocated buffer. The issue was fixed by adding a length guard.
Risk Assessment
The flaw allows a remote out-of-bounds read in the kernel, potentially leading to memory disclosure or a system crash (denial of service). It affects systems using Kerberos with NFS/RPC.
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-89537. If updating is not possible, consider restricting access to NFS/RPC services from untrusted networks.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2 gss_krb5_verify_mic_v2() reads the token ID at ptr[0..1], the flags byte at ptr[2], and padding at ptr[3..7], then passes ptr + GSS_KRB5_TOK_HDR_LEN and cksum_len to gss_krb5_mic_build_sg(). None of these accesses check read_token->len first. The minimum safe token size is GSS_KRB5_TOK_HDR_LEN (16) plus ctx->krb5e->cksum_len (12-24, depending on the enctype). All callers accept shorter tokens from the wire: - gss_unwrap_resp_integ() enforces only an upper bound (offset + len <= rcv_buf->len) before allocating mic.data = kmalloc(len) and passing it to gss_verify_mic(). A malicious NFS server can therefore supply a short checksum opaque, producing a small slab allocation that the Kerberos MIC verifier reads past. - gss_validate() enforces only len <= RPC_MAX_AUTH_SIZE (400) before passing the wire-supplied length to gss_validate_seqno_mic(), which constructs a mic xdr_netobj and calls gss_verify_mic(). - svcauth_gss_verify_header() enforces only checksum.len >= XDR_UNIT (4 bytes) before dispatching to gss_verify_mic(). - svcauth_gss_unwrap_integ() checks only that the checksum fits in gsd->gsd_scratch. Add a length guard at the top of gss_krb5_verify_mic_v2(), before any ptr[] access or scatterlist construction. Well-formed MIC tokens from gss_krb5_get_mic_v2() already have exactly GSS_KRB5_TOK_HDR_LEN + cksum_len bytes, so valid traffic is unaffected.