VulnerabilityPublished 29 Oct 2026

CVE-2026-96640: Aider · Pillow PDF append accepts malformed shared page-tree DAG causing resource exhaustion

Affects aiderSource GCVE Database

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Authoritative record:CVE-2026-96640· High 8.2

Not known to be exploited · CVSS 8.2 High.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

8.2High

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

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 4.0	8.2

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: Present
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: High
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 21 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 29 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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), 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

Affected software

Product	Affected	Fixed in
Homebrewaider`pkg:brew/aider`Other threats	Affected >=0.86.1_1	Fixed in–

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

Description

### Summary Pillow's documented PDF append path can spend disproportionate CPU time on a compact malformed PDF page tree. An attacker who can supply an existing PDF to an application that appends an image with `Image.save(..., format="PDF", append=True)` can provide a malformed `/Pages` tree where each level repeats the same child `/Pages` reference in `/Kids`. Pillow treats the graph as a tree and expands each repeated reference in `PdfParser.linearize_page_tree()` without tracking visited or duplicate indirect references. In the bounded proof below, a `1410`-byte PDF expands to `8192` page references from one distinct page object and takes about `9.6` seconds through the append path. A comparable linear control with the same depth stays at one page reference and completes in about `0.02` seconds. This is in scope for Pillow's security policy because it is reproducible in `main`, affects the latest public release tested (`12.3.0`), and is not just an unexpected Python exception. The vulnerable operation accepts attacker-controlled PDF bytes through Pillow's PDF parser and causes deterministic resource consumption before normal completion. ### Details #### Affected versions - Package: `Pillow` - Ecosystem: `pip` / PyPI - Affected component: `src/PIL/PdfParser.py` page-tree traversal reached from `src/PIL/PdfImagePlugin.py` PDF append handling - Runtime-tested affected release: `12.3.0` - Runtime-tested affected development version: `13.0.0.dev0` - Tested commit: `b39b9649312fbc381060e8c7c7f9f0de4cc63daa` - Current source artifact hash: `sha256(git archive --format=tar HEAD) = 2ccefca23d432456765be8d3c34564c65ba016b0c9da1e44ca389762f7eda054` - Release `12.3.0` commit: `bb1d8e8ab8d29048624d96e3ee53cecf7c13d13d` - Release `12.3.0` source artifact hash: `sha256(git archive --format=tar 12.3.0) = 94bed33fd20123938dbc0153d295250bb4e3c09251b6d4a34f7595e25d409c81` - Source-level affected: `5.1.0`, `12.2.0`, `12.3.0`, and `HEAD` - Likely affected range: `>= 5.1.0` - Patched versions: none known Evidence fact strings: - Runtime-tested affected: 12.3.0 - Runtime-tested affected: 13.0.0.dev0 at b39b9649312fbc381060e8c7c7f9f0de4cc63daa - Source-level affected: 5.1.0 - Source-level affected: 12.2.0 - Source-level affected: HEAD - Likely affected range: >= 5.1.0, no fixed version known - Pre-boundary note: 5.0.0 has PdfImagePlugin.py but no src/PIL/PdfParser.py `5.0.0` contains `src/PIL/PdfImagePlugin.py` but does not contain `src/PIL/PdfParser.py`. `PdfParser.py` was introduced by `6207b44ab1ff4a91d8ddc7579619876d0bb191a4` (`2018-01-18 issue #2959: support appending to existing PDFs`), which shipped in `5.1.0`. #### Technical details The reachable path is: 1. A service or application receives an attacker-influenced existing PDF and appends an image using Pillow's public PDF save path: ```python Image.new("L", (1, 1), 0).save(existing_pdf_file, format="PDF", append=True) ``` 2. `src/PIL/PdfImagePlugin.py` routes `append=True` to `PdfParser`: ```python is_appending = im.encoderinfo.get("append", False) filename_str = filename.decode() if isinstance(filename, bytes) else filename if is_appending: existing_pdf = PdfParser.PdfParser(f=fp, filename=filename_str, mode="r+b") else: existing_pdf = PdfParser.PdfParser(f=fp, filename=filename_str, mode="w+b") ``` 3. `src/PIL/PdfParser.py` parses the existing PDF and linearizes the page tree: ```python self.page_tree_root = self.read_indirect(self.pages_ref) self.pages = self.linearize_page_tree(self.page_tree_root) self.orig_pages = self.pages[:] ``` 4. `linearize_page_tree()` recursively follows every `/Kids` entry and extends the page list. It does not track page-tree node identity, duplicate references, or active traversal references: ```python def linearize_page_tree( self, node: PdfDict | None = None ) -> list[IndirectReference]: page_node = node if node is not None else self.page_tree_root check_format_condition( page_node[b"Type"] == b"Pages", "/Type of page tree node is not /Pages" ) pages = [] for kid in page_node[b"Kids"]: kid_object = self.read_indirect(kid) if kid_object[b"Type"] == b"Page": pages.append(kid) else: pages.extend(self.linearize_page_tree(node=kid_object)) return pages ``` 5. During append, `rewrite_pages()` then iterates the expanded `orig_pages` list and scans/replaces entries in `self.pages`, adding more cost to the already expanded duplicate-reference list. The missing invariant is: ```text The PDF page tree should be treated as an acyclic tree of unique indirect references before Pillow builds pages/orig_pages or rewrites pages. ``` The proof intentionally uses a malformed PDF page-tree DAG: each `/Pages` node repeats the same next `/Pages` child reference twice. Pillow accepts this graph and expands it as if the repeated reference represent

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