CVE-2021-3521 (published 2022-08-22T15:15Z)

There is a flaw in RPM\'s signature functionality. OpenPGP subkeys are associated with a primary key via a "binding signature." RPM does not check the binding signature of subkeys prior to importing them. If an attacker is able to add or socially engineer another party to add a malicious subkey to a legitimate public key, RPM could wrongly trust a malicious signature. The greatest impact of this flaw is to data integrity. To exploit this flaw, an attacker must either compromise an RPM repository or convince an administrator to install an untrusted RPM or public key. It is strongly recommended to only use RPMs and public keys from trusted sources.

CVSS v3 base score: 4.7, MEDIUM (CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N)

Weakness information:
cwe_id: CWE-347
cwe_name: Improper Verification of Cryptographic Signature
description: The product does not verify, or incorrectly verifies, the cryptographic signature for data.
extended_description: 
            
            
         
         

Fix information:
repo_url: https://github.com/rpm-software-management/rpm
commit_details: repo_url: https://github.com/rpm-software-management/rpm
author: Panu Matilainen
author_date: 2021-09-30 09:59:30+03:00
msg: Validate and require subkey binding signatures on PGP public keys

All subkeys must be followed by a binding signature by the primary key
as per the OpenPGP RFC, enforce the presence and validity in the parser.

The implementation is as kludgey as they come to work around our
simple-minded parser structure without touching API, to maximise
backportability. Store all the raw packets internally as we decode them
to be able to access previous elements at will, needed to validate ordering
and access the actual data. Add testcases for manipulated keys whose
import previously would succeed.

Depends on the two previous commits:
7b399fcb8f52566e6f3b4327197a85facd08db91 and
236b802a4aa48711823a191d1b7f753c82a89ec5

Fixes CVE-2021-3521.
repository: repo_url: https://github.com/rpm-software-management/rpm
repo_name: rpm-software-management/rpm
description: The RPM package manager
repo_language: C
owner: rpm-software-management
file_changes: filename: rpmpgp.c
change_type: ModificationType.MODIFY
diff: @@ -1062,37 +1062,121 @@ static pgpDigParams pgpDigParamsNew(uint8_t tag)
     return digp;
 }
 
+static int hashKey(DIGEST_CTX hash, const struct pgpPkt *pkt, int exptag)
+{
+    int rc = -1;
+    if (pkt->tag == exptag) {
+	uint8_t head[] = {
+	    0x99,
+	    (pkt->blen >> 8),
+	    (pkt->blen     ),
+	};
+
+	rpmDigestUpdate(hash, head, 3);
+	rpmDigestUpdate(hash, pkt->body, pkt->blen);
+	rc = 0;
+    }
+    return rc;
+}
+
+static int pgpVerifySelf(pgpDigParams key, pgpDigParams selfsig,
+			const struct pgpPkt *all, int i)
+{
+    int rc = -1;
+    DIGEST_CTX hash = NULL;
+
+    switch (selfsig->sigtype) {
+    case PGPSIGTYPE_SUBKEY_BINDING:
+	hash = rpmDigestInit(selfsig->hash_algo, 0);
+	if (hash) {
+	    rc = hashKey(hash, &all[0], PGPTAG_PUBLIC_KEY);
+	    if (!rc)
+		rc = hashKey(hash, &all[i-1], PGPTAG_PUBLIC_SUBKEY);
+	}
+	break;
+    default:
+	/* ignore types we can't handle */
+	rc = 0;
+	break;
+    }
+
+    if (hash && rc == 0)
+	rc = pgpVerifySignature(key, selfsig, hash);
+
+    rpmDigestFinal(hash, NULL, NULL, 0);
+
+    return rc;
+}
+
 int pgpPrtParams(const uint8_t * pkts, size_t pktlen, unsigned int pkttype,
 		 pgpDigParams * ret)
 {
     const uint8_t *p = pkts;
     const uint8_t *pend = pkts + pktlen;
     pgpDigParams digp = NULL;
-    struct pgpPkt pkt;
+    pgpDigParams selfsig = NULL;
+    int i = 0;
+    int alloced = 16; /* plenty for normal cases */
+    struct pgpPkt *all = xmalloc(alloced * sizeof(*all));
     int rc = -1; /* assume failure */
+    int expect = 0;
+    int prevtag = 0;
 
     while (p < pend) {
-	if (decodePkt(p, (pend - p), &pkt))
+	struct pgpPkt *pkt = &all[i];
+	if (decodePkt(p, (pend - p), pkt))
 	    break;
 
 	if (digp == NULL) {
-	    if (pkttype && pkt.tag != pkttype) {
+	    if (pkttype && pkt->tag != pkttype) {
 		break;
 	    } else {
-		digp = pgpDigParamsNew(pkt.tag);
+		digp = pgpDigParamsNew(pkt->tag);
 	    }
 	}
 
-	if (pgpPrtPkt(&pkt, digp))
+	if (expect) {
+	    if (pkt->tag != expect)
+		break;
+	    selfsig = pgpDigParamsNew(pkt->tag);
+	}
+
+	if (pgpPrtPkt(pkt, selfsig ? selfsig : digp))
 	    break;
 
-	p += (pkt.body - pkt.head) + pkt.blen;
+	if (selfsig) {
+	    /* subkeys must be followed by binding signature */
+	    if (prevtag == PGPTAG_PUBLIC_SUBKEY) {
+		if (selfsig->sigtype != PGPSIGTYPE_SUBKEY_BINDING)
+		    break;
+	    }
+
+	    int xx = pgpVerifySelf(digp, selfsig, all, i);
+
+	    selfsig = pgpDigParamsFree(selfsig);
+	    if (xx)
+		break;
+	    expect = 0;
+	}
+
+	if (pkt->tag == PGPTAG_PUBLIC_SUBKEY)
+	    expect = PGPTAG_SIGNATURE;
+	prevtag = pkt->tag;
+
+	i++;
+	p += (pkt->body - pkt->head) + pkt->blen;
 	if (pkttype == PGPTAG_SIGNATURE)
 	    break;
+
+	if (alloced <= i) {
+	    alloced *= 2;
+	    all = xrealloc(all, alloced * sizeof(*all));
+	}
     }
 
-    rc = (digp && (p == pend)) ? 0 : -1;
+    rc = (digp && (p == pend) && expect == 0) ? 0 : -1;
 
+    free(all);
     if (ret && rc == 0) {
 	*ret = digp;
     } else {

programming_language: C
filename: Makefile.am
change_type: ModificationType.MODIFY
diff: @@ -108,6 +108,9 @@ EXTRA_DIST += data/SPECS/hello-config-buildid.spec
 EXTRA_DIST += data/SPECS/hello-cd.spec
 EXTRA_DIST += data/keys/rpm.org-rsa-2048-test.pub
 EXTRA_DIST += data/keys/rpm.org-rsa-2048-test.secret
+EXTRA_DIST += data/keys/CVE-2021-3521-badbind.asc
+EXTRA_DIST += data/keys/CVE-2021-3521-nosubsig.asc
+EXTRA_DIST += data/keys/CVE-2021-3521-nosubsig-last.asc
 EXTRA_DIST += data/macros.testfile
 EXTRA_DIST += data/macros.debug
 EXTRA_DIST += data/SOURCES/foo.c

programming_language: Shell
filename: CVE-2021-3521-badbind.asc
change_type: ModificationType.ADD
diff: @@ -0,0 +1,25 @@
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+Version: rpm-4.17.90 (NSS-3)
+
+mQENBFjmORgBCAC7TMEk6wnjSs8Dr4yqSScWdU2pjcqrkTxuzdWvowcIUPZI0w/g
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+L0OGwZSr9m5s17VXxfspgU5ugc8kBJfzABEBAAE=
+=WCfs
+-----END PGP PUBLIC KEY BLOCK-----
+

programming_language: CoffeeScript
filename: CVE-2021-3521-nosubsig-last.asc
change_type: ModificationType.ADD
diff: @@ -0,0 +1,25 @@
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+Version: rpm-4.17.90 (NSS-3)
+
+mQENBFjmORgBCAC7TMEk6wnjSs8Dr4yqSScWdU2pjcqrkTxuzdWvowcIUPZI0w/g
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+L0OGwZSr9m5s17VXxfspgU5ugc8kBJfzABEBAAE=
+=WCfs
+-----END PGP PUBLIC KEY BLOCK-----
+

programming_language: CoffeeScript
filename: CVE-2021-3521-nosubsig.asc
change_type: ModificationType.ADD
diff: @@ -0,0 +1,37 @@
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+Version: rpm-4.17.90 (NSS-3)
+
+mQENBFjmORgBCAC7TMEk6wnjSs8Dr4yqSScWdU2pjcqrkTxuzdWvowcIUPZI0w/g
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+E4XX4jtDmdZPreZALsiB
+=rRop
+-----END PGP PUBLIC KEY BLOCK-----
+

programming_language: TeX
filename: rpmsigdig.at
change_type: ModificationType.MODIFY
diff: @@ -240,6 +240,34 @@ gpg(185e6146f00650f8) = 4:185e6146f00650f8-58e63918
 [])
 AT_CLEANUP
 
+AT_SETUP([rpmkeys --import invalid keys])
+AT_KEYWORDS([rpmkeys import])
+RPMDB_INIT
+
+AT_CHECK([
+runroot rpmkeys --import /data/keys/CVE-2021-3521-badbind.asc
+],
+[1],
+[],
+[error: /data/keys/CVE-2021-3521-badbind.asc: key 1 import failed.]
+)
+AT_CHECK([
+runroot rpmkeys --import /data/keys/CVE-2021-3521-nosubsig.asc
+],
+[1],
+[],
+[error: /data/keys/CVE-2021-3521-nosubsig.asc: key 1 import failed.]
+)
+
+AT_CHECK([
+runroot rpmkeys --import /data/keys/CVE-2021-3521-nosubsig-last.asc
+],
+[1],
+[],
+[error: /data/keys/CVE-2021-3521-nosubsig-last.asc: key 1 import failed.]
+)
+AT_CLEANUP
+
 # ------------------------------
 # Test pre-built package verification
 AT_SETUP([rpmkeys -K <signed> 1])

programming_language: Shell