Summary
Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature
HMAC validation bypass via multiple .replace() calls when removing URL signature
Thumbor’s HMAC validation can be bypassed due to the use of Python’s .replace() when removing the signature from the URL before validation. Since .replace() removes all occurrences of the substring, an attacker can insert the same signature multiple times in the URL and manipulate the final URL used for validation.
This allows crafting URLs where the validated string differs from the actual requested resource, enabling loading images from unintended domains or paths.
Details
Thumbor signs URLs using HMAC-SHA1 to prevent abuse such as loading arbitrary external images or invoking filters without authorization.
During request validation, Thumbor removes the signature from the request URL before recalculating the HMAC. The relevant code:
url_signature = self.context.request.hash
if url_signature:
signer = self.context.modules.url_signer(
self.context.server.security_key
)
try:
quoted_hash = quote(self.context.request.hash)
except KeyError:
self._error(400, f"Invalid hash: {self.context.request.hash}")
return
url_to_validate = url.replace(
f"/{self.context.request.hash}/", ""
).replace(f"/{quoted_hash}/", "")
valid = signer.validate(
unquote(url_signature).encode(), url_to_validate
)
The issue is that .replace() removes every occurrence of the substring in the URL, not just the first one.
Because the signature itself appears in the URL, an attacker can inject additional copies of the signature elsewhere in the path. When Thumbor performs .replace(), these extra occurrences are also removed, resulting in a different url_to_validate than the original request.
Example
Valid request:
/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.com/v1/.../image.jpg
By injecting the same hash inside the URL:
/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.co/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/m/v1/.../image.jpg
After .replace() removes all occurrences of the hash, the URL used for validation differs from the effective request path. This allows manipulation of the upstream host or path.
Further manipulation is possible due to the second .replace() for the URL-encoded hash (%3D), enabling more precise path manipulation.
Example:
/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.com/ddoyYVYUbDf6Po/ddoyYVYUbDf6Po_dzOrBhCDrXLc%3D/ddoyYVYUbDf6Po/v1/...
Root Cause
Use of .replace() without limiting the number of replacements when removing the signature from the URL.
Since the signature is part of the request path, using a global replacement allows attackers to place additional occurrences of the same substring to influence the validated string.
Impact
This behavior may allow attackers to:
- Bypass HMAC URL validation
- Load images from arbitrary domains
- Abuse Thumbor deployments as an open proxy / image fetcher
- Circumvent domain restrictions intended by the signed URL mechanism
CVE-2026-53501 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.
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.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Remove only the first occurrence of the signature or explicitly parse the URL components instead of performing global string replacement.
Example:
url.replace(f"/{self.context.request.hash}/", "", 1)
Alternatively, reconstruct the unsigned URL deterministically from the parsed request components.
Frequently Asked Questions
- What is CVE-2026-53501? CVE-2026-53501 is a high-severity security vulnerability in thumbor (pip), affecting versions <= 7.7.7. It is fixed in 7.8.0.
- How severe is CVE-2026-53501? CVE-2026-53501 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.
- Which versions of thumbor are affected by CVE-2026-53501? thumbor (pip) versions <= 7.7.7 is affected.
- Is there a fix for CVE-2026-53501? Yes. CVE-2026-53501 is fixed in 7.8.0. Upgrade to this version or later.
- Is CVE-2026-53501 exploitable, and should I be worried? Whether CVE-2026-53501 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-2026-53501 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-2026-53501? Upgrade
thumborto 7.8.0 or later.