VulnerabilityPublished 6 Oct 2026Updated 27 Oct 2026

CVE-2026-104874: Reference leak in CIMultiDict/MultiDict items-view union and subtraction

Affects pferdSource GCVE Database

Authoritative record:CVE-2026-104874· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 3.9.4_1 when convenient.

Data as of 27 Oct 2026, 23:13 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 3.9.4_1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 2 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade pferd to 3.9.4_1.

Affected software

Product	Affected	Fixed in
Homebrewpferd`pkg:brew/pferd`Other threats	Affected >=3.8.3_1 <3.9.4_1	Fixed in 3.9.4_1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## Description A reference leak in the items-view union and subtraction operators of aio-libs/multidict 6.7.0 through 6.9.0 (C extension) lets a remote client drive unbounded, unreclaimable memory growth by having each operand element leak one key-identity object and one value object. The reflected-union path (`operand | d.items()`, `multidict_itemsview_or2_impl`) and the subtraction path (`d.items() - operand`, `multidict_itemsview_sub1_impl`) parse each element into new strong references but release only the tuple wrapper, never the identity and value. Servers in the aio-libs stack build these views over attacker-supplied HTTP headers and query strings, so the operand size is under remote control. Forced garbage collection does not recover the leaked objects, so resident memory rises monotonically until the process is killed. --- ## Root Cause `_multidict_itemsview_parse_item()` returns **new** references: a fresh identity via `md_calc_identity()` and a fresh value via `Py_NewRef()`. The `or2_impl` first parse loop requests both but clears only `arg`: ```c // views.h:565 — or2_impl first parse loop while ((st = PyIter_NextItem(iter, &arg)) > 0) { int tmp = _multidict_itemsview_parse_item( self, arg, &identity, NULL, &value); // identity + value: new refs if (tmp < 0) goto fail; else if (tmp > 0) { if (_set_add(tmp_set, identity, value) < 0) goto fail; } Py_CLEAR(arg); // :575 clears arg ONLY } ``` `_set_add()` builds its own tuple with `PyTuple_Pack` and `Py_DECREF`s it, so it never borrows the loop's `identity`/`value`. On the next iteration those variables are overwritten, so the previous references are lost permanently. The `sub1_impl` first loop (`:686-696`) has the identical omission. This is an editing slip, not an ownership contract: every sibling path clears all three references per iteration, `and1_impl` (`:304-306`), `and2_impl` (`:389-391`), `or1_impl` (`:506-508`), and the second (`md_next`) loops of both functions (`:605-607`, `:726-728`). PR #1413 added a `Py_DECREF(tpl)` to the *second* loop of `or2`/`sub1`, fixing a different temporary-tuple leak; it never touched the first parse loop, so this leak remains on `master`. `views.h` is byte-identical between `v6.6.0` and `v6.7.0`, yet 6.6.4 does not leak and 6.7.0 does. The regression is behavioural, introduced by a key-identity ownership change in `hashtable.h` across that boundary that made `md_calc_identity()` return a fresh strong reference the unchanged parse loop never releases. --- ## Reproduction Environment | Item | Value | |------|-------| | **Runtime** | CPython 3.14.6 | | **multidict** | 6.9.0 (PyPI binary wheel, C extension) | | **OS** | macOS (darwin 25.6.0, arm64) | --- ## Proof of Concept ### POC Source Code #### poc_refcount.py — leak proof with intersection control ```python import sys, gc from multidict import CIMultiDict def probe(name, run_op): d = CIMultiDict(); d["seed"] = "x" value = object() # unique sentinel N = 100000 operand = [("k%d" % i, value) for i in range(N)] gc.collect(); before = sys.getrefcount(value) run_op(d, operand) gc.collect(); after = sys.getrefcount(value) print(f"[{name}] leaked strong refs = {after - before}") probe("or2 operand | items", lambda d, o: o | d.items()) # reflected union probe("sub1 items - operand", lambda d, o: d.items() - o) # subtraction probe("and2 items & operand", lambda d, o: d.items() & o) # CONTROL: clears, expect 0 ``` #### poc_dispatch.py — maps which paths leak ```python import sys, gc from multidict import CIMultiDict def probe(name, fn): d = CIMultiDict(); d["seed"] = "x" v = object(); N = 50000 operand = [("k%d" % i, v) for i in range(N)] gc.collect(); before = sys.getrefcount(v) fn(d, operand); gc.collect(); after = sys.getrefcount(v) print(f"{name:35s} leaked={after - before}") probe("operand | d.items() (or2)", lambda d, o: o | d.items()) probe("d.items() | operand (or1)", lambda d, o: d.items() | o) probe("d.items() - operand (sub1)", lambda d, o: d.items() - o) probe("operand - d.items() (rsub)", lambda d, o: set(o) - d.items()) probe("d.items() & operand (and)", lambda d, o: d.items() & o) ``` #### poc_rss.py — availability consequence ```python import sys, gc, resource from multidict import CIMultiDict def rss_mb(): r = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss return r / (1024 * 1024) if sys.platform == "darwin" else r / 1024 d = CIMultiDict(); d["seed"] = "x" gc.collect(); print(f"start RSS = {rss_mb():.1f} MB") for i in range(200): operand = [(f"k{i}_{j}", f"v{i}_{j}") for j in range(50000)] _ = operand | d.items() del operand gc.collect() # prove GC cannot reclaim the leak if (i + 1) % 40 == 0: print(f"after {(i+1)*50000:>9,} elements: RSS = {rss_mb():.1f} MB") ``` ### Execution Steps 1. `python3 -m venv venv` (CP

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