CVE-2026-89538

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

SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

gss krb5 unwrap v2() sets buf->len to a logical length, which can be much smaller than head[0].iov len (the allocated receive-page capacity). It then calls xdr buf trim() with a trim length derived from the 16-bit "extra count" (ec) field in the Kerberos v2 token header.

The ec field is authenticated by the post-decrypt memcmp() against the encrypted header copy, so a randomly-mutated value is rejected. However, any peer holding a valid GSS context can legitimately encrypt a token whose ec exceeds the plaintext length. Per RFC 4121, such a token is structurally malformed.

Although xdr buf trim() now clamps the buf->len subtraction to avoid unsigned underflow, the buffer is still left in a semantically invalid state (zero length, inconsistent iov lengths) when ec is oversized.

Reject these tokens before calling xdr buf trim(), giving callers a well-defined GSS S DEFECTIVE TOKEN error and keeping the xdr buf internally consistent. The wrapped blob begins at a nonzero offset -- both callers pass len as offset + opaque len -- so buf->len still counts the offset bytes that precede the blob. Compare the trim length against the remaining wrapped segment, buf->len - offset, rather than the whole buffer; comparing against buf->len alone leaves an offset-wide window in which an oversized ec passes the test and xdr buf trim() cuts into the bytes ahead of the blob.