SUNRPC: Reject krb5 v2 wrap tokens with oversized ec field

Overview

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.

Affected

• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < ac9efc29346b6942e71f0e24a3178a9121330fab
• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < b5746095b59f4d11f39f08751d36bbdfd1e5deac
• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < 9fc6d6e6db49ab55208d5dd1c424c1d0d9aa7296
• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < 4abb44a29bb515df6f49123848765c70fc9b5021
• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < dfcd81ab45613d45996fbf483d4ef54b0ec90ab9
• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < 1f9856af065b6158271687e9788407b9573fd16f
• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < 880effc943ed82d6811f272ed3307c222b40f4d6
• Linux >= cf4c024b908353fcc48309374d39e3399d67dfd1 < ad484748eec0a66eac0f13ab53b3fbedb7333c91
• Linux 3.13

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, … · updated 2026-09-18 · vex