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Prior to commit d67f4185, the Gorilla WebSocket library (github.com/gorilla/websocket) used the `math/rand` package to generate mask keys for WebSocket frames. The WebSocket protocol (RFC 6455) requires that masking keys be unpredictable to prevent attackers from injecting malicious content into WebSocket connections. The `math/rand` package is a pseudo-random number generator (PRNG) intended for general-purpose tasks like simulations, not for security-sensitive operations.
The core issue is that `math/rand` produces a deterministic sequence of numbers. Its output is entirely dependent on its initial seed. In many Go applications, this seed is often set using `time.Now().UnixNano()` or a similar predictable value. If an attacker can guess or determine this seed (by, for example, observing the application’s start time or other network artifacts), they can regenerate the exact same sequence of “random” numbers used to create the mask keys.
With the ability to predict the mask key, an attacker can intercept a WebSocket frame, strip the original mask, and replace it with their own payload. They can then re-apply the mask using the predicted key. Because the key is predictable, the server or client will successfully unmask the frame, and the attacker’s injected data will be processed as if it were legitimate. This effectively allows for WebSocket frame content injection, potentially leading to data corruption, command injection, or other attacks depending on the application’s logic.
The fix for this vulnerability was implemented in commit `d67f4185` and released in version v1.5.3. The solution was to replace the weak `math/rand` with crypto/rand, a cryptographically secure pseudo-random number generator (CSPRNG) that draws entropy from the operating system. This change ensures that the generated mask keys are truly unpredictable.
DailyCVE Form:
Platform: Gorilla WebSocket
Version: < v1.5.3
Vulnerability: Weak PRNG
Severity: High
date: 2026-08-24
Prediction: Already Patched
What Undercode Say:
Check your go.mod for the vulnerable version grep "github.com/gorilla/websocket" go.mod Example output showing a vulnerable version github.com/gorilla/websocket v1.5.2 Update to the patched version go get github.com/gorilla/[email protected]
Exploit: (Educational Purposes!)
A proof-of-concept for predicting mask keys is not provided here to prevent misuse. The vulnerability stems from math/rand‘s deterministic nature when seeded with a known value. An attacker would need to guess or obtain the seed, then regenerate the mask key sequence.
Protection:
Upgrade to Gorilla WebSocket version v1.5.3 or later.
Impact:
Successful exploitation allows an attacker to inject arbitrary content into WebSocket frames, potentially leading to data manipulation, command injection, or other severe application-specific attacks.
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Sources:
Reported By: github.com
Extra Source Hub:
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