CVE-2026-53500

Thumbor treats ALLOWED_SOURCES string patterns as unescaped regex, allowing hostname bypass via wildcard dot

import re
from thumbor.config import Config
from thumbor.context import Context
from thumbor.loaders import http_loader as loader

config = Config()
config.ALLOWED_SOURCES = ["s.glbimg.com"]   # typical user config
ctx = Context(None, config, None)

# These should be blocked, both return True due to the unescaped dot
print(loader.validate(ctx, "http://sXglbimgYcom/secret.jpg"))  # True ← bypass
print(loader.validate(ctx, "http://sAglbimg.com/secret.jpg"))  # True ← bypass

# Legitimate origin, correctly allowed
print(loader.validate(ctx, "http://s.glbimg.com/logo.jpg"))    # True ← correct

for pattern in context.config.ALLOWED_SOURCES:
    if isinstance(pattern, Pattern):
        match = url
    else:
        pattern = f"^{pattern}$"   # <-- dots not escaped, act as regex wildcard
        match = res.hostname

    if re.match(pattern, match):
        return True

An attacker who can influence the image source URL passed to Thumbor can fetch images from arbitrary hosts, bypassing the ALLOWED_SOURCES allowlist.

Preconditions:

• ALLOWED_SOURCES contains at least one plain-string entry (the common case; all official documentation examples use plain strings).
• The attacker can supply or influence the image URL, true whenever ALLOW_UNSAFE_URL = True (the default), or when the application forwards user input to a signed URL endpoint.

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

CVE-2026-53500 has a CVSS score of 8.2 (High). 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 (7.8.0); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Apply re.escape() to plain-string patterns before compiling them, so every
character is matched literally:

else:
    pattern = f"^{re.escape(pattern)}$"   # dots and other metacharacters are now literal
    match = res.hostname

This is a one-call addition with no breaking change for correctly written configurations. Users who need real regular-expression behaviour should supply a compiled pattern (re.compile(r"s\.glbimg\.com")), which is already handled by the existing isinstance(pattern, Pattern) branch and is unaffected by this change.

The ALLOWED_SOURCES docstring in config.py was also updated to document the two-mode behaviour explicitly.

1. What is CVE-2026-53500? CVE-2026-53500 is a high-severity inefficient regular expression (ReDoS) vulnerability in thumbor (pip), affecting versions <= 7.7.7. It is fixed in 7.8.0. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
2. How severe is CVE-2026-53500? CVE-2026-53500 has a CVSS score of 8.2 (High). 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.
3. Which versions of thumbor are affected by CVE-2026-53500? thumbor (pip) versions <= 7.7.7 is affected.
4. Is there a fix for CVE-2026-53500? Yes. CVE-2026-53500 is fixed in 7.8.0. Upgrade to this version or later.
5. Is CVE-2026-53500 exploitable, and should I be worried? Whether CVE-2026-53500 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
6. What actually determines whether CVE-2026-53500 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.
7. How do I fix CVE-2026-53500? Upgrade thumbor to 7.8.0 or later.

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