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;