CVE-2026-89550

In the Linux kernel, the following vulnerability has been resolved:

SUNRPC: svcauth_gss: enforce krb5 token minimum length

svcauthgssunwrap_priv() validates only an upper bound on the

wire-supplied opaque length before handing the buffer to

gss_unwrap():

    if (len > xdrstreamremaining(xdr))

            goto unwrap_failed;

    offset = xdrstreampos(xdr);

    ...

    majstat = gssunwrap(ctx, offset, offset + len, buf);

The wire value len flows unchanged as the upper bound into the

krb5 unwrap path, so a len in [0, 16] passes this check and is

handed to gss_unwrap(). For a krb5 v2 context that lands in

gsskrb5unwrap_v2(), which reads the 16-byte RFC 4121 token

header fields at ptr+4 and ptr+6 and then calls rotate_left()

before any integrity check. With a sub-header length the header

reads run past the token, and rotateleft()'s shift %= buf->len

path can divide by zero when buf->len has been driven to zero by

the truncated token. A header-only token (len == 16) is equally

invalid: with a non-zero RRC field and the opaque blob ending at

the XDR buffer boundary, rotate_left() builds a zero-length

subbuffer, reaching the same division.

Reject the token at the server entry point before it reaches the

krb5 unwrap core. A valid sealed RFC 4121 token must contain

the 16-byte header plus at least some encrypted payload.

Fix by adding a minimum-length check immediately after the

existing upper-bound check:

    if (len <= GSSKRB5TOKHDRLEN)

            goto unwrap_failed;