CVE-2026-89550

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

SUNRPC: svcauth_gss: enforce krb5 token minimum length

svcauth_gss_unwrap_priv() validates only an upper bound on the

wire-supplied opaque length before handing the buffer to

gss_unwrap():

if (len > xdr_stream_remaining(xdr))

goto unwrap_failed;

offset = xdr_stream_pos(xdr);

...

maj_stat = gss_unwrap(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

gss_krb5_unwrap_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 _rotate_left()'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 <= GSS_KRB5_TOK_HDR_LEN)

goto unwrap_failed;