vulnerability / NVI-00012920 / Revision 3
Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature
GitHub lists this reviewed advisory for thumbor. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
8.2 high
GitHub source severity; exploitability and local consequence require separate evidence.
As of · GitHub Advisory Database
0.352%
FIRST EPSS model v2026.06.15; exploitation probability over 30 days, percentile 0.29032. Missing scores are unknown.
As of · FIRST EPSS
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
thumbor
source listed<= 7.7.7
# HMAC validation bypass via multiple `.replace()` calls when removing URL signature ## Summary 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: ```python 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
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. FIRST EPSS
A fixed release is not deployment evidence.
Check current vendor support and compatibility. After making a change, verify the deployed build and relevant runtime conditions. Keep the evidence, owner and time. An updated lockfile alone does not prove what is running.
04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
Each assertion retains its source. Mirrored records may repeat one underlying publisher; additional links are not independent corroboration.
- GHSA-mw3h-qjxj-6xg9 ↗
GitHub Advisory Database · vendor advisory
Observed . GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.
- CVE-2026-53501 ↗
FIRST EPSS · official catalog
Observed . FIRST EPSS; probability of exploitation in the next 30 days, not local compromise. Dataset date/model retained; FIRST services terms apply.
Public revision history
Only published revisions appear here. Routine fetches do not create a material-change event.
- Revision 3 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 2 ·
Dated EPSS estimate updated; this is not a new disclosure or proof of exploitation.
- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.