Here's the conversion of CMS to opaque ASN1_STRING.

I decided to add two sanity checks on *rsz. I don't think these can
actually be hit: the required OID should ensure non-zero length and
since we don't allow files larger than MAX_FILE_SIZE, the upper bound
should also be impossible even if indefinite length encoding is used.

Anyway, it seems like the cautious thing to do: it avoids a malloc(0)
and spending time on parsing a preposterously large eContent or, worse,
hitting a malloc failure due to malicious remote data.

Index: cms.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/cms.c,v
diff -u -p -r1.58 cms.c
--- cms.c	19 Nov 2025 23:21:56 -0000	1.58
+++ cms.c	2 Dec 2025 15:32:14 -0000
@@ -37,21 +37,26 @@ cms_extract_econtent(const char *fn, CMS
 	if (res == NULL || rsz == NULL)
 		return 1;
 
-	if ((os = CMS_get0_content(cms)) == NULL || *os == NULL) {
+	if ((os = CMS_get0_content(cms)) == NULL || *os == NULL ||
+	    (*rsz = ASN1_STRING_length(*os)) == 0) {
 		warnx("%s: RFC 6488 section 2.1.4: "
 		    "eContent: zero-length content", fn);
 		return 0;
 	}
 
+	if (*rsz > MAX_FILE_SIZE) {
+		warnx("%s: overlong eContent of length %zu", fn, *rsz);
+		return 0;
+	}
+
 	/*
 	 * The eContent in os is owned by the cms object and it has to outlive
 	 * it for further processing by the signedObject handlers. Since there
 	 * is no convenient API for this purpose, duplicate it by hand.
 	 */
-	if ((*res = malloc((*os)->length)) == NULL)
+	if ((*res = malloc(*rsz)) == NULL)
 		err(1, NULL);
-	memcpy(*res, (*os)->data, (*os)->length);
-	*rsz = (*os)->length;
+	memcpy(*res, ASN1_STRING_get0_data(*os), *rsz);
 
 	return 1;
 }