UBUNTU-CVE-2026-89550: Divide by Zero

In the Linux kernel, the following vulnerability has been resolved: SUNRPC: svcauthgss: enforce krb5 token minimum length svcauthgssunwrappriv() validates only an upper bound on the wire-supplied opaque length before handing the buffer to gssunwrap(): if (len > xdrstreamremaining(xdr)) goto unwrapfailed; 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 gssunwrap(). For a krb5 v2 context that lands in gsskrb5unwrapv2(), which reads the 16-byte RFC 4121 token header fields at ptr+4 and ptr+6 and then calls rotateleft() 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, rotateleft() 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 unwrapfailed;