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.
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
CVSS Score: 9.1
AV:N - gss_krb5_verify_mic_v2() is reached from nfsd RPCSEC_GSS on TCP/2049 (svcauth_gss_accept → svcauth_gss_verify_header/unwrap_integ) and from the NFS client processing a remote server's RPCSEC_GSS verifier or krb5i checksum, so a remote peer triggers it with crafted SUNRPC traffic.
AC:L - A single RPCSEC_GSS Call or Reply with a MIC/checksum shorter than GSS_KRB5_TOK_HDR_LEN plus cksum_len (callers accept as little as 4 bytes) deterministically over-reads; no race or attacker-uncontrollable layout is required. Kerberized NFS is a standard enterprise deployment.
PR:N - svcauth_gss_verify_header() is the MIC check itself and runs before credentials are verified; the only gate is an rsc-cache lookup on a cleartext, monotonically increasing 64-bit GSS handle that an unauthenticated client can sniff or enumerate. A malicious NFS server needs no privileges on the client.
UI:N - On nfsd the over-read is triggered solely by an inbound RPCSEC_GSS message; on the client it fires from the RPC receive path once an NFS/RPCSEC_GSS mount exists. No separate victim action is required at attack time.
S:U - The slab over-read and any resulting oops stay inside the host kernel SUNRPC/GSS code. This is ordinary same-host kernel impact, not a VM escape, IOMMU bypass, or other changed-scope boundary.
C:H - gss_unwrap_resp_integ() kmallocs only the attacker-chosen checksum length, then gss_krb5_verify_mic_v2() reads GSS_KRB5_TOK_HDR_LEN+cksum_len (28-40) bytes via gss_krb5_mic_build_sg()/sg_pcopy_to_buffer, an out-of-bounds read of adjacent slab well beyond a few bytes.
I:N - The defect is a read-only over-read consumed as a MIC comparison; rfc3961_verify_mic copies those bytes into a local buffer and memcmp()s them. There is no out-of-bounds write, use-after-free, or control-flow hijack primitive.
A:H - Reading 28-40 bytes past a small kmalloc object or past an RPC receive buffer near a page boundary can oops or panic on KASAN, redzone, or unmapped next-page kernels, and the attacker can repeat the crafted RPC to deny NFS service.
| Attack Vector |
Network |
Scope |
Unchanged |
| Attack Complexity |
Low |
Confidentiality Impact |
High |
| Privileges Required |
None |
Integrity Impact |
None |
| User Interaction |
None |
Availability Impact |
High |
AV:N - gss_krb5_verify_mic_v2() is reached from nfsd RPCSEC_GSS on TCP/2049 (svcauth_gss_accept → svcauth_gss_verify_header/unwrap_integ) and from the NFS client processing a remote server's RPCSEC_GSS verifier or krb5i checksum, so a remote peer triggers it with crafted SUNRPC traffic.
AC:L - A single RPCSEC_GSS Call or Reply with a MIC/checksum shorter than GSS_KRB5_TOK_HDR_LEN plus cksum_len (callers accept as little as 4 bytes) deterministically over-reads; no race or attacker-uncontrollable layout is required. Kerberized NFS is a standard enterprise deployment.
PR:N - svcauth_gss_verify_header() is the MIC check itself and runs before credentials are verified; the only gate is an rsc-cache lookup on a cleartext, monotonically increasing 64-bit GSS handle that an unauthenticated client can sniff or enumerate. A malicious NFS server needs no privileges on the client.
UI:N - On nfsd the over-read is triggered solely by an inbound RPCSEC_GSS message; on the client it fires from the RPC receive path once an NFS/RPCSEC_GSS mount exists. No separate victim action is required at attack time.
S:U - The slab over-read and any resulting oops stay inside the host kernel SUNRPC/GSS code. This is ordinary same-host kernel impact, not a VM escape, IOMMU bypass, or other changed-scope boundary.
C:H - gss_unwrap_resp_integ() kmallocs only the attacker-chosen checksum length, then gss_krb5_verify_mic_v2() reads GSS_KRB5_TOK_HDR_LEN+cksum_len (28-40) bytes via gss_krb5_mic_build_sg()/sg_pcopy_to_buffer, an out-of-bounds read of adjacent slab well beyond a few bytes.
I:N - The defect is a read-only over-read consumed as a MIC comparison; rfc3961_verify_mic copies those bytes into a local buffer and memcmp()s them. There is no out-of-bounds write, use-after-free, or control-flow hijack primitive.
A:H - Reading 28-40 bytes past a small kmalloc object or past an RPC receive buffer near a page boundary can oops or panic on KASAN, redzone, or unmapped next-page kernels, and the attacker can repeat the crafted RPC to deny NFS service.
CVSS 3.1